Record-breaking La Niña events - An analysis of the La Niña life cycle and the impacts and significance of the 2010-11 and 2011-12 La Niña events ...

 
Record-breaking La Niña events - An analysis of the La Niña life cycle and the impacts and significance of the 2010-11 and 2011-12 La Niña events ...
Record-breaking La Niña events
An analysis of the La Niña life cycle and the impacts and significance
of the 2010–11 and 2011–12 La Niña events in Australia
Record-breaking La Niña events - An analysis of the La Niña life cycle and the impacts and significance of the 2010-11 and 2011-12 La Niña events ...
Contents

Introduction

At a glance: the impact of these La Niña
events in Australia                                                1

Background                                                              Story

What are El Niño and La Niña events?                               2    From El Niño to La Niña                                                3

When do El Niño and La Niña events occur?                          4    From one La Niña to the next                                           5

The three phases of El Niño–Southern Oscillation                   6

The Southern Oscillation Index                                     8    Record high Southern Oscillation Index values                          9

El Niño–Southern Oscillation impacts – rainfall                    10   Record rainfall and widespread flooding across Australia 11

El Niño–Southern Oscillation impacts – temperature                 12   Record sea surface temperatures                                       14

                                                                        Warmer nights and cooler days                                         15

Other El Niño–Southern Oscillation impacts                         16   Tropical cyclone activity during 2010–11 and 2011–12                  17

The Indian Ocean Dipole                                            18   Negative Indian Ocean Dipole increases rain                           19

The Southern Annular Mode                                          18   Positive Southern Annular Mode and why                                19
                                                                        Western Australia missed out on rain

                                                                        A timeline of events                                                  20

Stay informed

References and acknowledgements                                    24

Published by the Bureau of Meteorology, July 2012
                                                                          About this publication
GPO Box 1289
Melbourne VIC 3001                                                        La Niña events greatly influence Australia’s climate.
Tel: (03) 9669 4000                                                       The 2010–12 La Niñas were two of the most significant events
Website: www.bom.gov.au                                                   in Australia’s recorded meteorological history. This publication
ISBN: 978 0 642 70621 8                                                   explores these extraordinary events and their effect on the
                                                                          weather and climate of Australia during 2010–12.
© Commonwealth of Australia 2012

This work is copyright. Apart from any use as permitted under             It provides an overview of how La Niña events occur and the
the Copyright Act 1968, no part may be reproduced without prior           extreme impacts that these events can have on Australia’s climate.
written permission from the Bureau of Meteorology. Requests and
                                                                          The publication is structured according to two interrelated,
inquiries concerning reproduction and rights should be addressed
                                                                          parallel streams to assist the reader – ‘background’ and ‘story’.
to the Publishing Unit, Bureau of Meteorology, GPO Box 1289,
                                                                          The ‘background’ stream explains some of the major factors that
Melbourne 3001. Requests for reproduction of material from
                                                                          drive La Niña events and how they influence Australia’s climate.
the Bureau website should be addressed to AMDISS, Bureau
                                                                          The ‘story’ stream illustrates the significance and widespread
of Meteorology, at the same address.
                                                                          impacts of the 2010–11 and 2011–12 La Niña events.
Cover photograph: Steve Davis
                                                                          Unless otherwise indicated, all temperature and rainfall anomalies
A heavy rain shower crosses Cleveland Bay, off Townsville,                (i.e. departures from average) in this publication are calculated
Queensland, as the first storms of the Wet Season brew                    with respect to the 1961–1990 average, as recommended by the
(Bureau of Meteorology Weather Calendar, April 2011)                      United Nations World Meteorological Organization.
Record-breaking La Niña events - An analysis of the La Niña life cycle and the impacts and significance of the 2010-11 and 2011-12 La Niña events ...
Introduction

At a glance: the impact of these La Niña
events in Australia

The successive La Niña events spanning 2010–12 were associated with
record rainfall over much of Australia and some of the biggest floods
in living memory. This followed years of severe drought in many parts
of the country, and while it brought relief to many Australians, it also
brought devastation to others.

Some facts about the 2010–11 and 2011–12 La Niña events

2010–11 La Niña
The 2010–11 La Niña event was           In October and December 2010,
one of the strongest on record,         and February and March 2011,
comparable in strength with             the Southern Oscillation Index
the La Niña events of 1917–18,          values (a measure of a La Niña’s
1955–56 and 1975–76                     strength) were the highest
                                        recorded for each month since
                                        records commenced in 1876

2011 was Australia’s coolest            2010 was Australia’s third-wettest
year in a decade (2001–2011)            calendar year on record

The Murray–Darling Basin                2011 was Australia’s second-wettest
experienced its wettest calendar        calendar year (with the wettest
year on record in 2010 and              year since national rainfall records
Western Australia experienced           began in 1900 being 1974 – also
its wettest year on record in 2011      a La Niña year)

Ocean temperatures to the               April 2010 to March 2012
north of Australia were                 was Australia's wettest
highest on record in 2010               two-year period on record

Widespread flooding                     See pages 20–23 for a full
occured in many parts                   timeline of the La Niña events
of Australia associated
                                                                           1
with the record rainfalls
Record-breaking La Niña events - An analysis of the La Niña life cycle and the impacts and significance of the 2010-11 and 2011-12 La Niña events ...
Background

What are El Niño and La Niña events?

                                        El Niño and La Niña events are a natural part of the global climate
           The El Niño–Southern
                                        system. They occur when the Pacific Ocean and the atmosphere
           Oscillation (ENSO)
           is described in more
                                        above it change from their neutral (‘normal’) state for several seasons.
           detail on pages 6–7.         El Niño events are associated with a warming of the central and
                                        eastern tropical Pacific, while La Niña events are the reverse, with
                                        a sustained cooling of these same areas.
    Coupled                      ?
                                        These changes in the Pacific Ocean                  Even in a neutral state, temperatures
    Because ENSO involves               and its overlying atmosphere occur in a             in the Pacific Ocean vary from east to
    interaction between the ocean       cycle known as the El Niño–Southern                 west – for example, the western Pacific
    and the atmosphere – both of        Oscillation (ENSO). The atmosphere                  ‘warm pool’ in the tropical Pacific has
    which play a role in reinforcing    and ocean interact, reinforcing each                some of the warmest large-scale ocean
    changes in each other – it is       other and creating a ‘feedback loop’                temperatures in the world. During
    known as a coupled ocean–           which amplifies small changes in the                an ENSO event, ocean temperatures
    atmosphere phenomenon.              state of the ocean into an ENSO event.              become warmer than usual or cooler
                                        When it is clear that the ocean and                 than usual at different locations, which
                                        atmosphere are fully coupled an ENSO                are reflected in ocean temperature
    Thermocline                  ?      event is considered established.                    gradients. The most important driver
                                                                                            of ENSO is these temperature gradients
    Thermocline comes from the                                                              across the Pacific, both at the surface
    Greek for ‘heat slope’ and is the                                                       and below the surface, particularly
    name for the region separating                                                          at the thermocline.
    warm, well-mixed surface water
    from cool, deep ocean water.
    Typically water temperatures                                                                                                 60°N

    above the thermocline are more
    than 25°C while those below the                                                                                              40°N
    thermocline are 15°C or less.

                                                                                                                                           Tropic of Cancer
                                                                                                                                 20°N

    Central and Eastern
                                         Equator

                                                                                                                                 Equator
    Pacific Ocean                                                                                                                          Tropic of Capricorn

     Warmer water        El Niño                                                                                                 20°S

     Cooler water        La Niña
                                                                                                                                 40°S
     Neither warm        Neutral
     nor cool                                      100°E     140°E            180°E            140°W          100°W       60°W
                                                                             dateline
                                                                             Date  Line

                                          Pacific Ocean – even in neutral state the Western Pacific is warm

                                            Why are they called El Niño and La Niña?

                                            The term El Niño translates from Spanish as ‘the boy-child’. Peruvian fishermen
                                            originally used the term to describe the appearance, around Christmas, of a
                                            warm ocean current off the South American coast. It is now the commonly
                                            accepted term to describe the warming of the central and eastern tropical
                                            Pacific Ocean. La Niña translates as ‘girl-child’ and is the opposite ENSO phase
                                            to El Niño.

2
Record-breaking La Niña events - An analysis of the La Niña life cycle and the impacts and significance of the 2010-11 and 2011-12 La Niña events ...
Story

From El Niño to La Niña

The 2009–10 El Niño                        Monthly sea surface temperature anomalies (differences
commenced in May 2009,                     from normal) in the Pacific Ocean indicate where the ocean
reaching its peak in late                  is warmer than usual (red) and cooler than usual (blue).
December 2009 before
breaking down in the first                                                                                                            4.0 °C
quarter of 2010. The Pacific                                                                                                          3.0
Ocean returned to neutral                                                                                                             2.0
by late April 2010, but                                                                                                               1.0
                                                                                                                                      0.5
continued to cool rapidly
                                                                                                                                     –0.5
during autumn.
                                                                                                                                     –1.0
As early as April 2010, a number of                                                                                                  –2.0
                                                 Australia                                                                           –3.0
climate models from meteorological
agencies around the world suggested                                                                                                  –4.0 °C
a La Niña event could commence
                                           December 2009 – peak of the 2009–10 El Niño; warmer than normal sea surface temperatures
later in 2010. Subsequently, the first
                                           in the central and eastern Pacific
observed signals of a potential La Niña
became apparent in the tropical Pacific
Ocean during the following month.
                                                                                                                                      4.0 °C
As sea surface temperatures                                                                                                           3.0
approached values (or ‘thresholds’)                                                                                                   2.0
associated with a La Niña in July                                                                                                     1.0
2010, and as long-range outlooks                                                                                                      0.5
became more consistent, the Bureau                                                                                                   –0.5
of Meteorology announced that a                                                                                                      –1.0
La Niña event was more likely than                                                                                                   –2.0
not to persist for the rest of the year.         Australia                                                                           –3.0
By October 2010, and with the                                                                                                        –4.0 °C
event showing parallels to the La Niña
                                           May 2010 – between the end of El Niño and the start of the 2010–11 La Niña; relatively neutral
events of the early 1970s, seasonal        state of the ocean
outlooks were increasingly suggesting
wet conditions for northern and
eastern Australia. As a result, the
Bureau began briefing key federal                                                                                                     4.0 °C
and state government agencies of                                                                                                      3.0
increased flood and tropical cyclone                                                                                                  2.0
risk, and decreased bushfire potential,                                                                                               1.0
over the summer period.                                                                                                               0.5
                                                                                                                                     –0.5
The La Niña strengthened further
                                                                                                                                     –1.0
during spring and into summer,
                                                                                                                                     –2.0
peaking around January 2011, before
                                                 Australia                                                                           –3.0
weakening during autumn 2011. The
                                                                                                                                     –4.0 °C
2010–11 La Niña drew to a close in
May 2011, with both Pacific Ocean          January 2011 – peak of the 2010–11 La Niña; cooler than normal sea surface temperatures
and atmospheric indicators returning       in the central and eastern Pacific
to neutral levels by mid-year.

                                                                                                                                               3
Record-breaking La Niña events - An analysis of the La Niña life cycle and the impacts and significance of the 2010-11 and 2011-12 La Niña events ...
Background

When do El Niño and La Niña events occur?

                                          The tropical Pacific Ocean and atmosphere swings, or oscillates,
    Climate models                 ?      between warm, cool and neutral phases on a timescale of a few years.

    Climate models come                   A typical El Niño or La Niña event may      In general, El Niño events tend to
    in two forms:                         show its first signs of development         only last for a single cycle (i.e. one
                                          during the southern hemisphere              year from autumn to autumn), but
    1 Statistical climate models are
                                          autumn and strengthen over winter           it is not uncommon for multi-year
      based on what has happened
                                          and spring. It will normally start to       La Niña events to occur. For example,
      in the past – that is, they use
                                          decay in the mid to late southern           the 1998–2001 La Niña affected three
      historical patterns to estimate
      what is likely to happen in
                                          summer, and finally dissipate in the        consecutive years from autumn 1998
      the future.                         subsequent autumn. ENSO events              to autumn 2001.
                                          typically decay during autumn, as this
    2 Dynamical climate models are        is the time of year when the tropical
      based on physics – that is, they    Pacific Ocean naturally evens out the
      model the physical processes        temperature difference between the
      driving the current climate         east and west. This annual weakening
      situation forward in time to
                                          of the temperature gradient across the
      predict what is likely to happen.
                                          Pacific also means the weather patterns
    Data are collected from a wide        which help reinforce a La Niña or El Niño
    variety of sources – including        ease, allowing ENSO to return to neutral.
    satellites, buoys (moored,
    drifting and expendable), sea
    level analysis and meteorological
    surface observations – all drawn
    from a network of national and
    international observing systems.

                                            Watching out for events

                                            The first signs of an emerging El Niño or La Niña event are often
                                            observed in the ocean. The Bureau of Meteorology monitors and
                                            reports on a range of ENSO indicators, including:

                                            • short-term bursts of tropical           • atmospheric air pressure
                                              rainfall activity                       • cloudiness – measuring the amount
                                            • water temperatures at the sea             of cloud in tropical regions
                                              surface and at depth                    • the strength of the trade winds
                                            • ocean heat content –                      and winds higher in the atmosphere
                                              measuring the amount of                 • ocean currents.
                                              energy stored in the ocean
                                            • the Southern Oscillation Index
                                              (see page 8)

           The Southern                     These climate indicators provide information about current ENSO conditions,
           Oscillation Index (SOI)          and are inputs into climate models that are used to predict conditions for the
           is described in more             months ahead.
           detail on page 8.

4
Record-breaking La Niña events - An analysis of the La Niña life cycle and the impacts and significance of the 2010-11 and 2011-12 La Niña events ...
Story

From one La Niña to the next

The central Pacific began to             Monthly sea surface temperature anomalies (difference
cool again during winter 2011,           from normal) in the Pacific Ocean indicate where the ocean
and from September 2011                  is warmer than usual (red) and cooler than usual (blue).
models and observations
indicated a re-emergence                                                                                                          4.0 °C
of the La Niña during spring                                                                                                      3.0
was likely.                                                                                                                       2.0
                                                                                                                                  1.0
The 2011–12 La Niña was relatively                                                                                                0.5
late forming, with most indicators                                                                                              –0.5
only reaching La Niña thresholds by                                                                                             –1.0
mid-October 2011. This event was                                                                                                –2.0
not forecast to be as strong as the            Australia                                                                        –3.0
2010–11 La Niña, as reflected in                                                                                                –4.0 °C
seasonal climate outlooks issued at
the time. Outlooks indicated a wetter    September 2011 – cooling in the central Pacific as the La Niña re-forms
than average spring and early summer
for much of northern and eastern
Australia, but were not as dramatic
                                                                                                                                  4.0 °C
as they were during the previous year.
                                                                                                                                  3.0
The La Niña consolidated over spring                                                                                              2.0
2011, with many indicators only                                                                                                   1.0
strengthening slightly during October                                                                                             0.5
and early November. The event                                                                                                   –0.5
reached its peak during December                                                                                                –1.0
2011 when climate models forecast a                                                                                             –2.0
                                               Australia
return to a neutral ENSO phase during                                                                                           –3.0
autumn 2012. The 2011–12 La Niña                                                                                                –4.0 °C
gradually declined over late summer,
with some atmospheric indicators         December 2011 – peak of the 2011–12 La Niña; cooler than normal central and eastern Pacific,
                                         but not as cool as at the 2010–11 La Niña peak
continuing to show a La Niña signal
during March 2012, while oceanic
indicators were generally faster to
return to neutral levels. The 2011–12                                                                                             4.0 °C
La Niña concluded in late March 2012.                                                                                             3.0
                                                                                                                                  2.0
                                                                                                                                  1.0
  Want more?                                                                                                                      0.5
  The Bureau publishes a regular                                                                                                –0.5
  ENSO Wrap-Up and Model                                                                                                        –1.0
  Summary on its website                                                                                                        –2.0
                                               Australia                                                                        –3.0
  http://www.bom.gov.au/
  climate/enso/                                                                                                                 –4.0 °C

  These reports contain information      March 2012 – 2011–12 La Nina has declined; ocean temperatures in the Pacific approach neutral
  about current conditions across
  the tropical Pacific Ocean, as
  well as a summary of predictions
  for ENSO conditions several
  months ahead from a number
  of international dynamical climate
  models, including the Bureau’s
  own model, POAMA (Predictive
  Ocean Atmosphere Model for
  Australia).
                                                                                                                                           5
Record-breaking La Niña events - An analysis of the La Niña life cycle and the impacts and significance of the 2010-11 and 2011-12 La Niña events ...
Background

The three phases of El Niño–Southern Oscillation

                                       The neutral phase
    Convection                   ?
                                       In the neutral state (neither El Niño   Warm sea surface temperatures in
    Convection is the process
                                       nor La Niña) trade winds blow east      the western Pacific pump heat and
    generally associated with warm
                                       to west across the surface of the       moisture into the atmosphere above.
    rising air and the formation
    of cloud.
                                       tropical Pacific Ocean, bringing        In a process known as atmospheric
                                       warm moist air and warmer               convection, this warm air rises high
                                       surface waters towards the western      into the atmosphere and, if the air
                                       Pacific and keeping the central         is moist enough, causes towering
    Walker Circulation           ?     Pacific Ocean relatively cool. The      cumulonimbus clouds and rain. This
                                       thermocline is deeper in the west       now-drier air then travels east before
    The Walker Circulation             than the east.                          descending over the cooler eastern
    is named after Sir Gilbert                                                 tropical Pacific. The pattern of air rising
    Walker, the scientist who first
                                                                               in the west and falling in the east with
    recognised a semi-regular
                                                                               westward moving air at the surface is
    pattern of high and low rainfall
                                                                               referred to as the Walker Circulation.
    (and hence feast and famine)
    over India.

                                         Neutral
    Upwelling                    ?
    Upwelling is a vertical
    motion of water. When wind
    or currents displace water
    at the surface of the ocean,
    water from deeper in the ocean
    is drawn up to replace the
    displaced water. Deep water
    is generally cooler and richer
    in nutrients than surface water
    so upwelling is important in
    supporting productivity in the
    oceans of the world.

                                       El Niño

                                       During an El Niño event,                Sea surface temperatures around
                                       trade winds weaken or may               northern Australia are cooler than normal
                                       even reverse, allowing the area         and the focus of convection migrates
                                       of warmer than normal water             away from Australia eastward towards
                                       to move into the central and            the central tropical Pacific Ocean. This
                                       eastern tropical Pacific Ocean.         results in increased rainfall for nations
                                                                               such as Kiribati and Peru, but less rainfall
                                       These warmer than normal ocean
                                                                               over Australia. The greatest impacts are
                                       temperatures are associated with a
                                                                               usually felt over inland eastern Australia,
                                       deepening of the thermocline in the
                                                                               while effects for regions such as
                                       central to eastern Pacific. A weaker
                                                                               southwest Western Australia and coastal
                                       upwelling of cooler ocean waters
                                                                               New South Wales can vary from event
                                       from below also contributes to warmer
                                                                               to event, and in western Tasmania the
                                       sea surface temperatures.
6                                                                              effects are generally weak.
Record-breaking La Niña events - An analysis of the La Niña life cycle and the impacts and significance of the 2010-11 and 2011-12 La Niña events ...
La Niña
                                                                                    Below the surface
During a La Niña event, the Walker        Convection and hence cloudiness over
Circulation intensifies with greater      the region north of Australia increases   ENSO events are typically led
convection over the western Pacific       as stronger winds provide more            and sustained by changes in
                                          moisture to the overlying atmosphere      the amount of heat held in the
and stronger trade winds.
                                                                                    waters below the surface of the
                                          and the Walker Circulation intensifies.
As the trade winds strengthen, the                                                  tropical Pacific Ocean.
                                          This strengthens the Australian
pool of warmer water is confined to the
                                          monsoon and, if the conditions are        The deeper ocean is important
far western tropical Pacific, resulting
in warmer than usual sea surface          right, directs increased humidity and     in gauging the strength, and
temperatures in the region north of       rainfall inland over Australia. La Niña   hence potential longevity, of
Australia. Sea surface temperatures       events are associated with increased      an event. These large stores
across the central and eastern tropical   rainfall over much of northern and        of heat (El Niño) or lack of heat
Pacific Ocean become cooler than          eastern Australia. Parts of northern      (La Niña) act like a flywheel and
usual and the thermocline moves closer    and central Australia tend to feel the    ensure that an event will not
to the surface – cool waters from the     impacts of La Niña more than they         dissipate rapidly. For example,
deep ocean are drawn to the surface as    feel the impacts of El Niño.              during the 1997–98 El Niño
upwelling strengthens.                                                              event – which many consider the
                                                                                    El Niño of the century – surface
  La Niña                                                                           temperatures were around
                                                                                    3.5°C warmer than normal in
                                                                                    the eastern tropical Pacific, but
                                                                                    temperatures at
                                                                                    150 m below the surface were
                                                                                    up to 8°C above average.
                                                                                    Conversely, during the 2010–11
                                                                                    La Niña, eastern Pacific Ocean
                                                                                    surface temperatures were up
                                                                                    to 2 °C cooler than normal, but
                                                                                    the subsurface temperatures
                                                                                    were almost 7 °C below average.

                                                                                    Such large changes in the
                                                                                    deeper ocean ensure that the
                                                                                    surface waters stay warm or
                                                                                    cool even when the atmosphere
                                                                                    above might try to push the
                                                                                    system back towards neutral,
                                                                                    and so can sustain or lead an
  El Niño                                                                           ENSO event.

                                                                                                                        7
Record-breaking La Niña events - An analysis of the La Niña life cycle and the impacts and significance of the 2010-11 and 2011-12 La Niña events ...
Background

The Southern Oscillation Index

The Southern Oscillation
Index (SOI) is a measure of
                                       La Niña events over time
the intensity or strength of the
Walker Circulation. It is one
of the key atmospheric indices         As La Niña events recur on a two to seven-year cycle, there
for gauging the strength of            have been many over the last century, varying in strength
El Niño and La Niña events             and impacts. The SOI and sea surface temperatures can be
and their potential impacts            used to compare the intensity of La Niña events.
on the Australian region.              (See graph below for more details.)

The SOI measures the difference        Atmospheric and oceanic intensity of La Niña events since 1900. Intensity
in surface air pressure between        ranked by SOI values for atmosphere, while oceanic intensity is ranked by
Tahiti and Darwin. The index is best   sea surface temperature indicators (only available reliably since mid-century).
represented by monthly (or longer)     Some multi-year events have two or three La Niña peaks.
averages as daily or weekly SOI
values can fluctuate markedly due
to short-lived, day-to-day weather
patterns, particularly if a tropical
cyclone is present.

Sustained positive SOI values above
about +8 indicate a La Niña event
while sustained negative values
below about –8 indicate an El Niño.

8
Story

Record high Southern Oscillation Index values

During the 2010–12 La Niña events, record and near-record
high Southern Oscillation Index values occurred in many individual months
and multi-month periods.

The surface air pressures near Tahiti               typical weather patterns over
were consistently high throughout                   the South Pacific, which relate to                                   The El Niño–Southern
both the 2010–11 and 2011–12 La Niña                the strong trade winds experienced                                   Oscillation (ENSO)
events, and they were particularly high             over the same period and an                                          is described in more
during the first event. These high SOI              enhanced Walker Circulation.                                         detail on pages 6–7.
values reflect large changes in the

  Monthly SOI values for the 2010–11 and 2011–12 La Niña events – numerous record and near-record values for that particular month are indicated

Comparison to past La Niña events
The 2010–11 La Niña was one of the strongest on record,                                                   event, bringing significantly above-average
comparable in strength to the events of 1917–18, 1955–56 and                                              rainfall across the north of Australia, although
                                                                                                          most parts of southeastern Australia
1975–76. Widespread impacts were experienced across a large
                                                                                                          received below-average rainfall, consistent
part of Australia, including record rainfall and severe flooding.
                                                                                                          with persistent drought conditions in place
While the intensity of atmospheric indicators       The previous two La Niña episodes                     for the region since at least 2000.
during the event was exceptional, tropical          were also multi-year events, lasting from             The 1998–2001 La Niña persisted for
Pacific Ocean indicators did not reach record       September 2007 to March 2009 and from                 three years with three distinct peaks over
cool levels. This may have been partly due          May 1998 to March 2001, respectively.                 the summers of 1998–99, 1999–2000,
to the general warming trend in the Pacific                                                               and 2000–01. The event was generally
                                                    The 2007–08 and 2008–09 La Niña events
Ocean, which has warmed around 0.5°C                                                                      moderate in strength, with widespread
                                                    were weak to moderate, with relatively
since 1950.                                                                                               above-average rainfall and flooding,
                                                    minor impacts across Australia. Although
The 2011–12 La Niña was a weaker                    the 2007–08 event brought the typical                 particularly in New South Wales and
event, but still of moderate strength by            heavy rainfall to most of northern Australia          Queensland. While much of the north
both atmospheric and oceanic measures.              and the eastern tropics, the southern                 received record high falls, parts of the
Despite flooding in a number of areas during        half of the Murray–Darling Basin did not              southeast and Tasmania missed out on the
summer, the impact of this second event             receive the above-average rainfall and                heavy rainfall, and hence did not get the relief
upon Australia's climate was generally less         cool temperatures typical of past La Niña             from the dry conditions that had started
significant than during the previous event.         events. The 2008–09 La Niña was a short               around late 1996.

                                                                                                                                                           9
Background

El Niño–Southern Oscillation impacts – rainfall

The complex interactions
between the ocean,                       Rainfall patterns during La Niña and El Niño events
atmosphere and adjacent
landmasses across the Pacific            Australian rainfall data for 13 of the strongest ‘classic’ or ‘canonical’
mean that ENSO events have               events (having the typical autumn to autumn pattern of evolution and
impacts on weather in areas              decay) since 1900 have been combined to form a composite of average
outside the tropical Pacific             impacts of La Niña and El Niño events upon rainfall across Australia.
region. El Niño and La Niña
                                         Each map shows mean rainfall deciles, where green to blue tones indicate
events are associated with               above-average to very much above-average rainfall totals and yellow to red
distinct climatic conditions             tones indicate below-average to very much below-average rainfall. Note that
around the Pacific.                      the rainfall patterns can vary significantly from one event to the next.

La Niña events are associated
with greater convection over the
                                         El Niño                                    La Niña
warmer ocean to Australia’s north.
Typically this leads to higher than      El Niño is typically associated            La Niña is typically associated
average rainfall across much of          with reduced rainfall in northern          with increased rainfall in northern
Australia, particularly inland eastern   and eastern Australia                      and eastern Australia
and northern regions, sometimes
causing floods.

During El Niño events, the ocean
near Australia is cooler than usual,
bringing lower than average
winter–spring rainfall over eastern
and northern Australia. Although
most major Australian droughts                                           Deciles                                       Deciles
have been associated with El Niño
events, widespread drought is             1   2    3   4   5/6   7   8   9   10      1    2   3    4   5/6   7    8   9    10

certainly not guaranteed when
                                         Winter/spring rainfall – below average     Winter/spring rainfall – above average
an El Niño is present.                   across eastern Australia                   across most of eastern and northern Australia

  Deciles                        ?
  Deciles are calculated by
  (1) taking all available data
  (say, annual Australian rainfall
  from 1901 to 2000), (2) ordering
  them from lowest to highest,
  and (3) dividing them into                                             Deciles                                       Deciles

  10 separate groups of equal
  size. Each group is called a            1   2    3   4   5/6   7   8   9   10      1    2   3    4   5/6   7    8   9    10

  decile – the lowest 10 per cent        Summer rainfall – mostly near              Summer rainfall – above average
  of historical values is decile 1,      average                                    in eastern and northern Australia
  the next lowest 10 per cent will
  be decile 2, and so on, up to the
                                         The onset years for the 13 strongest      The onset years for the 13 strongest
  highest 10 per cent of historical      'classic' El Niño events used are 1905,   'classic' La Niña events used are 1906,
  values which lie in decile 10. This    1914, 1940, 1941, 1946, 1965, 1972,       1910, 1916, 1917, 1950, 1955, 1956, 1971,
  creates a scale against which we       1977, 1982, 1991, 1994, 1997 and 2002.    1973, 1975, 1988, 1998 and 2010.
  can rank the amount or intensity
  of a measurement or event.

10
Story

Record rainfall and widespread flooding across Australia

In 2010, Australia experienced its third-wettest year since national
                                                                                                                    Widespread flooding
rainfall records began in 1900, with second place taken by 2011.
Averaged across Australia, both years experienced rainfall well above                                               The record-breaking rainfall
the long-term average of 465 mm – 703 mm in 2010 and 708 mm in 2011.                                                during the 2010–11 La Niña led
                                                                                                                    to widespread flooding in many
Only 1974, dominated by one of the                     as a whole.                                                  regions between September
strongest La Niña events on record, was                                                                             2010 and March 2011. As well as
wetter with 760 mm. 2010 was also the                  While the 2010–11 La Niña event was
                                                                                                                    the severe flooding in southeast
wettest year on record for the Murray–                 costly in an economic and social sense,
                                                                                                                    Queensland, large areas of
Darling Basin and Queensland, while                    it relieved one of the longest and most
                                                                                                                    northern and western Victoria,
2011 was the wettest year on record for                severe droughts across the Murray–
                                                                                                                    New South Wales, northwestern
Western Australia.                                     Darling Basin in recorded history. Heavy
                                                                                                                    Western Australia and eastern
                                                       rain provided a significant boost
During the 2010–11 La Niña, most                       to water storages in Queensland,
                                                                                                                    Tasmania were subject to
of mainland Australia experienced                      New South Wales and South Australia.                         significant flooding. There were
significantly higher than average rainfall             Nationally, water held in major publicly                     also some highly unseasonable
over the nine months from July 2010                    owned storages rose by more than                             rain events in the tropics during
to March 2011. Parts of Tasmania also                  20 per cent between May 2010 and                             what is typically its dry season.
received heavy rainfall while southwest                May 2011.
Western Australia missed out,                                                                                       Flooding was also widespread
experiencing its driest year on record. A              During the 2011–12 La Niña, rainfall                         during the 2011–12 La Niña.
number of new Australian rainfall records              was above average for most of mainland                       Much of inland southern and
were set: wettest September, December                  Australia for the six months from October                    far northern Queensland, most
and March on record and second-wettest                 2011 to March 2012, but not as much                          of New South Wales, northern
October and February. May to October                   above average as for the 2010–11 event.                      Victoria, and central Australia
2010 was the wettest ‘dry’ season on                   Nevertheless, several rainfall records                       experienced flooding at least
record in northern Australia, and July to              were set: second-wettest November and                        once between late November
December 2010 was the wettest second                   spring on record for Western Australia,                      2011 and March 2012.
half of the year on record for Australia as            and second-wettest March for New
a whole.                                               South Wales.

Several seasonal records were also set:
wettest spring on record for Australia,                Combined, the two events yielded                                       See the timeline
and all States except Victoria and                     Australia’s wettest 24-month period                                    on pages 20–23 for
Tasmania; wettest summer on record for                 on record (April 2010 to March 2012),                                  more details on the
Victoria; and second-wettest summer on                 and wettest two-calendar-year period                                   widespread flooding
record for Western Australia and Australia             (2010–2012). The record rainfall of                                    across Australia.
                                                       1411mm in 2010–2011 beat the previous

Heavy rainfall during both La Niña events
2010–11 La Niña event                                  2011–12 La Niña                                        The two events combined
Extreme rainfall                                       Well above-average rainfall                            Record-breaking rainfall

          1     2–3     4–7     8–9     10   Deciles             1     2–3     4–7     8–9     10   Deciles             1     2–3     4–7     8–9     10   Deciles

Lowest Very    Below Average Above Very      Highest   Lowest Very    Below Average Above Very      Highest   Lowest Very    Below Average Above Very      Highest
on     much    average       average much    on        on     much    average       average much    on        on     much    average       average much    on
record below                         above   record    record below                         above   record    record below                         above   record
       average                       average                  average                       average                  average                       average

July 2010 to March 2011 rainfall deciles               October 2011 to March 2012 rainfall deciles            July 2010 to March 2012 rainfall deciles
(based on climatology of gridded monthly               (based on climatology of gridded monthly               (based on climatology of gridded monthly
rainfall analyses from 1900)                           rainfall analyses from 1900)                           rainfall analyses from 1900)
                                                                                                                                                                 11
Background

El Niño–Southern Oscillation impacts – temperature

                                       The increased cloudiness and rainfall associated with La Niña periods
 Evaporative cooling             ?     typically reduces daytime temperatures and keeps nights warmer,
                                       particularly over northern and eastern Australia. In the north of
 Evaporation of surface water,
                                       Australia, the monsoon is typically enhanced, which can lead to
 such as lakes, or moisture in
 the soil, cools the environment
                                       both cooler days and nights during the summer monsoon season as
 surrounding it. This happens          the higher rainfall allows for increased evaporative cooling, and
 because changing a substance          increased onshore winds provide additional cooling in the same way
 from a liquid phase to a gaseous      a sea-breeze brings relief from a hot summer day.
 phase (evaporation) requires
 energy. The energy required
 for a phase change is known as
 latent heat, and can be provided
 by absorption of solar radiation
 or by drawing heat energy from
 the air or other substances in
 contact with the liquid.

     Temperature patterns              El Niño
     during ENSO events

     Average impacts of
     La Niña and El Niño
     events on maximum
     (daytime) and minimum
     (night-time) temperatures
     across Australia are shown                                                  Deciles                                                Deciles
     below in composite maps
     combining temperature               1   2     3    4   5/6    7    8    9       10      1    2    3    4    5/6    7     8    9        10
     data from 12 of the strongest
                                       Winter/spring daytime temperatures –                Summer daytime temperatures –
     ‘classic’ events.                 above average across southern Australia             slightly above average for most of eastern
                                                                                           and northern Australia
     The maps show mean
     maximum and minimum
     temperature deciles, where        La Niña
     blue tones indicate below-
     average temperatures and
     orange to red tones indicate
     above-average temperatures.
     Note that the temperature
     patterns can vary significantly
     from one event to the next.

                                                                                 Deciles                                                Deciles

                                         1   2     3    4   5/6    7    8    9       10      1    2    3    4    5/6    7     8    9        10

                                       Winter/spring daytime temperatures –                Summer daytime temperatures –
                                       below average across southern Australia             below average across most of northern
                                                                                           and eastern Australia

12
In contrast, during El Niño events,                  The temperature effect of El Niño
reduced cloudiness means daytime                     events are felt most strongly during                         During La Niña
temperatures are typically warmer                    winter and spring, while the effects of                      increased cloudiness and
than normal, exacerbating the effect of              La Niña events tend to have the greatest                     rainfall can lead to cooler days.
lower than normal rainfall by increasing             impact between October and March.
evaporation. Reduced cloudiness                      The effect of La Niña events also tends                      During El Niño
                                                                                                                  reduced cloudiness can
also means that nights can be cool,                  to be stronger than that for El Niño;
                                                                                                                  lead to warmer days.
sometimes leading to widespread and                  temperatures are generally further below
severe frosts; Australia’s lowest recorded           average during La Niña events than they
temperature, –23.0 ˚C, was observed                  are above average during El Niño events.
at Charlotte Pass on 29 June 1994,
during the 1994–95 El Niño event.

                                                                                                              The onset years for the 12 strongest
                                           Deciles                                                  Deciles   'classic' El Niño events used are
                                                                                                              1914, 1940, 1941, 1946, 1965, 1972,
                                                                                                              1977, 1982, 1991, 1994, 1997 and 2002.
  1    2    3    4    5/6    7    8    9      10       1    2    3    4    5/6    7    8        9       10

Winter/spring minimum temperatures –                 Summer minimum temperatures –
above average for the southwest, below average       above average across southern Australia
for the northeast and parts of the east

                                                                                                              The onset years for the 12 strongest
                                           Deciles                                                  Deciles   'classic' La Niña events used are 1910,
                                                                                                              1916, 1917, 1950, 1955, 1956, 1971,
  1    2    3    4    5/6    7    8    9      10       1    2    3    4    5/6    7    8    9          10
                                                                                                              1973, 1975, 1988, 1998 and 2010.

Winter/spring minimum temperatures –                 Summer minimum temperatures –
above average for much of northern Australia,        below average for northern Australia,
below average for most of the southwest              above average for parts of the southeast

                                                                                                                                                        13
Record sea surface temperatures

Following the 2009–10 El Niño, 2010 was globally (both land and ocean)
                                                                                          –2.0 –1.5 –1.0 –0.5 0.0 0.5 1.0 1.5 2.0 °C
the warmest year on record, marginally warmer than 1998 (which
immediately followed the strongest El Niño event of that century).

Sea surface temperatures north of         with the Australian monsoon arriving                                                               10°N
Australia were also at record-breaking    earlier and being stronger than
highs – October, November and             normal during the 2010–11 northern
December 2010 tropical sea surface        wet season. Ultimately, the increase                                                               10°S
temperatures north of Australia broke     in monsoonal activity and evaporated
previous records by large margins and,    water added to the potential for
in contrast to temperatures over land,    high rainfall over northern and
                                                                                                                                             30°S
ocean temperatures around Australia       eastern Australia.
were the highest on record during 2010.
                                          Sea surface temperatures around the
This contributed to the strength of the   northern coasts of Australia were also           70°E          110°E      150°E        170°W

2010–11 La Niña and its impacts on        above average during the 2011–2012                      90°E           130°E   170°E           150°W

Australia. The very high sea surface      La Niña event, particularly between
temperatures contributed to an increase   December 2011 and February 2012,               Sea surface temperature anomalies (°C)
in evaporation and high (and at times     though were not at the record-breaking         in the Australian region, for the period
record) humidity levels over Australia.   levels seen in 2010. This would suggest        May 2010 to April 2011
The increase in humidity was associated   a weaker influence on Australian rainfall.

     Cool down under, warm globally

     2011 was Australia’s coolest year in a decade (2001–2011). Eight                  Global temperatures are also
     of the last nine years with sustained La Niña conditions recorded                 influenced by La Niña and El Niño
                                                                                       events due to the exchange of heat
     a cooler than normal Australian average temperature, except 2008,
                                                                                       between the atmosphere and oceans.
     which was Australia's warmest year on record commencing with                      The end of an El Niño is typically
     a La Niña.                                                                        associated with higher than average
                                                                                       global air temperatures. However, years
                                                                                       commencing with a La Niña in place are
                                                                                       cooler than average.

                                                                                       The World Meteorological Organization
                                                                                       ranked 2011 as the equal-tenth-
                                                                                       warmest year on record. 2011 was
                                                                                       the warmest La Niña year on record
                                                                                       globally, considerably warmer than the
                                                                                       most recent moderate-to-strong La Niña
                                                                                       years (2008, 2000, and 1989).

                                                                                       The ten-year average for 2002–2011
                                                                                       was the equal-warmest ten-year period
                                                                                       on record both for Australia and globally.

                                                                                       Australian (top) and global (bottom) annual
                                                                                       mean temperature anomalies (difference
                                                                                       from normal) – in this instance La Niña years
                                                                                       are defined as those where central Pacific
                                                                                       sea surface temperature anomalies were
                                                                                       below –1 °C for a sustained period leading
                                                                                       into the start of the year

14
Story

Warmer nights and cooler days

Record high sea surface temperatures in the tropics from June
to December 2010 also contributed to high minimum temperatures                                                  2010–11 La Niña
in this region, as sea surface temperatures moderate minimum                                                    The 2010–11 La Niña was
air temperatures over coastal and island regions.                                                               associated with much cooler
                                                                                                                than average daytime
For instance, at Horn and Coconut                      High rainfall and associated cloud                       temperatures and much
islands in the Torres Strait, the previous             cover generally kept Australian                          warmer than average
Queensland August record high                          maximum temperatures below average
                                                                                                                night-time temperatures.
minimum temperature of 25.4 °C was                     during both La Niña events. Conversely,
surpassed on 24 separate occasions                     increased cloud cover and damp soils                     2011–12 La Niña
during 2010, peaking at 26.8 °C at                     meant less heat was lost during the
Horn Island on 19 August. Similarly,                   nights, resulting in above-average                       The 2011–12 La Niña was
record warm ocean waters off Western                   minimum temperatures for most of                         associated with cooler than
Australia's west coast led to the hottest              Australia during the 2010–11 event,                      average daytime temperatures
year on record for southwest Australia.                and for southern Australia during the                    in some areas, but the deviation
                                                       2011–12 event.                                           from normal was not as marked.

The maps below show mean maximum (daytime) and minimum (night-
time) temperature deciles, where blue tones indicate below-average
temperatures and orange tones indicate above-average temperatures.

2010–11 La Niña                                        2011–12 La Niña

                                                                                                              2010

                                                                                                              Record warm ocean waters off
                                                                                                              the west coast of Western Australia
                                                                                                              was associated with the hottest year
                                                                                                              on record for southwest Australia.

          1      2–3    4–7     8–9     10   Deciles             1      2–3    4–7     8–9     10   Deciles

Lowest Very    Below Average Above Very      Highest   Lowest Very    Below Average Above Very      Highest
on     much    average       average much on           on     much    average       average much on
record below                         above record      record below                         above record
       average                       average                  average                       average

Daytime temperatures (July 2010 to March               Daytime temperatures (October 2011 to
2011) – below to very much below average               March 2012) – below average in some parts
across most of inland Australia                        of inland Australia, above average in some
                                                       coastal regions

          1      2–3    4–7     8–9     10   Deciles             1      2–3    4–7     8–9     10   Deciles

Lowest Very    Below Average Above Very      Highest   Lowest Very    Below Average Above Very      Highest
on     much    average       average much on           on     much    average       average much on
record below                         above record      record below                         above record
       average                       average                  average                       average

Night-time temperatures (July 2010 to                  Night-time temperatures (October 2011 to March
March 2011) – above average to highest                 2012) – above average across parts of southern
on record for large parts of the country               Australia and the north, below average inland
                                                                                                                                                   15
Background

Other El Niño–Southern Oscillation impacts

                                        Winds and cyclones                           Cloudiness
Other La Niña impacts
                                        Measurements of winds, both from             Changes in the location of convection
• stronger easterly trade winds         tracking weather balloons and from           over the Pacific mean that the location
• increased risk of tropical cyclones   satellite observations, also tell us about   and level of cloudiness can be used
  around Australia                      ENSO. When trade winds blowing from          to track the intensity and phase of
• increased cloudiness over             the east increase at the surface, and        the Walker Circulation, much as the
  Australia.                            winds around 10 km above the ground          SOI tracks changes in atmospheric
                                        strengthen in the opposite direction, it     pressure. Radiation (‘heat’) released
                                        is clear that the Walker Circulation has     into space acts as a good proxy (an
Other El Niño impacts                   strengthened – typical of La Niña. If the    indirect measure) for cloudiness as
                                        Walker Circulation shifts into an El Niño    cloud tops are generally far cooler
• weakened easterly trade winds
                                        phase, the surface easterlies weaken,        than the earth’s surface, and hence
• reduced risk of tropical cyclones
                                        or even reverse, and the westerly winds      more long-wave radiation is released
  around Australia
                                        aloft do likewise.                           from cloud-free regions, and less in
• reduced cloudiness over Australia.                                                 cloudy regions. This radiation can
                                        With a stronger Walker Circulation
                                                                                     be easily and accurately detected by
                                        comes a shift westward in the typical
                                                                                     satellites. Cloudiness along the equator
                                        area of the Pacific affected by tropical
                                                                                     – particularly near the Date Line –
                                        cyclones. Hence the only wet seasons
                                                                                     typically increases during an El Niño and
                                        in which Queensland has experienced
                                                                                     decreases during a La Niña. Likewise,
                                        multiple tropical cyclones crossing its
                                                                                     rainfall for equatorial countries near
                                        coast have been during La Niña periods.
                                                                                     the Date Line, such as Kiribati, is lower
                                                                                     during La Niña and higher during El Niño
                                                                                     – the opposite of Australian impacts.

  Tropical depression           ?         No two events are the same
  A tropical depression is a low
  pressure system of clouds and           Each El Niño and La Niña event is different from region to region, and
  thunderstorms with a defined            event to event. Variations in the timing, location and magnitude of ocean
  circulation but does not usually        temperature anomalies and wind patterns cause differences both in the
  have an eye or the spiral shape         strength and extent of climate impacts.
  associated with cyclones or
  more powerful storms.                   In addition, a chaotic ‘weather’ factor is involved in any event. For instance,
                                          flooding in western Victoria during the 2010–11 La Niña event originated from
                                          two tropical cyclones (Anthony and Yasi) which decayed over central Australia
         The Indian Ocean                 before moving south. While both were ‘random’ weather events, more cyclones
         Dipole (IOD) and the             and tropical depressions occur in the Australian region and more cross the
         Southern Annular                 eastern coast during La Niña events. ENSO also interacts with other drivers
                                          of climate variability, such as the Indian Ocean Dipole (IOD) and the Southern
         Mode (SAM) are
                                          Annular Mode (SAM), which increases the likelihood of each event differing
         described in more
                                          from the last.
         detail on pages 18–19.

16
Story

Tropical cyclone activity during 2010–11 and 2011–12

Tropical cyclone activity in the 2010–11 season was, overall, near
normal, with 10 tropical cyclones in the Australian region, while the
2011–12 season was below average, with 7 tropical cyclones. Usually,
during La Niña events, tropical cyclone numbers around Australia’s
north are higher than the long-term average (11) over the November
to April tropical cyclone season.

However, five of the tropical cyclones         landfall in Queensland since 1918
during 2010–11 were in the severe              (when two powerful tropical cyclones
category, which is above average. At           hit Innisfail and Mackay, also at the tail
least 29 systems developed into tropical       end of a La Niña), crossed the coast
depressions, which is one level below          between Cairns and Townsville on
a tropical cyclone. This is well above         3 February 2011.
the number of tropical depressions                                                                  Tropical cyclone Yasi as it approached the
                                               Tropical cyclone Carlos brought
observed in any tropical cyclone season                                                             Queensland coast just south of Cairns,
                                               very heavy rainfall to the greater Darwin            captured by the Moderate Resolution Imaging
in the Australian region since at least
                                               area between 15 and 17 February 2011,                Spectroradiometer (MODIS) on NASA’s Aqua
the mid-1990s (20 tropical depressions
                                               breaking numerous records, including                 satellite, 2 February 2011
developed in 2011–12, also above
                                               record high daily rainfall at Darwin
average). In addition, three tropical
                                               Airport with 367.6 mm on 16 February
cyclones ( Tasha, Yasi, and Anthony)                                                                strongest cyclone to cross the Western
                                               and a record three-day total of
crossed the Queensland coast in the                                                                 Australian coast since Laurence in 2009,
                                               684.8 mm. The average February
2010–11 season, and two ex-tropical                                                                 and brought significant rainfall to a
                                               monthly rainfall at Darwin Airport
cyclones ( Grant and Jasmine) in                                                                    broad area of western and central
                                               is 376.1 mm.
2011–12. Historically, multiple landfalls of                                                        Western Australia.
severe tropical cyclones between Port          Tropical cyclone Grant brought very
                                                                                                    A tropical depression made landfall
Douglas and Ballina on the Queensland          heavy falls to the Top End over 25 and
                                                                                                    in the Gascoyne region of Western
coast have only occurred in a single           26 December 2011, resulting in flooding
                                                                                                    Australia, and while it never reached
season during La Niña years. It                north of Katherine, which caused
                                                                                                    official tropical cyclone status it resulted
remains unclear why both seasons saw           significant infrastructure damage,
                                                                                                    in 207.8 mm of rainfall at Carnarvon on
numerous tropical depressions but an           cutting roads and rail links, as well as
                                                                                                    17 December 2010 and 255 mm for the
unusually small number of these                derailing a freight train.
                                                                                                    event. Carnarvon’s average December
eventually formed into tropical cyclones.
                                               Tropical cyclone Lua made landfall                   rainfall is 5.6 mm and its annual average
Severe tropical cyclone Yasi, possibly         on the Pilbara coast as a severe tropical            total rainfall is 231 mm.
the most powerful cyclone to make              cyclone on 17 March 2012. It was the

  Cloud cover                                                                                                                               55   Wm -2

                                                                                                                                            45
  During the peak of the 2010–11
                                                                                                                                            35
  La Niña event, cloudiness over
                                                                                                                                            25
  Australia increased. This increase
                                                                                                                                            15
  in cloud can be correlated with
                                                                                                                                             5
  the increased rainfall (as seen
                                                                                                                                            –5
  on page 10), reduced daytime
                                                                                                                                           –15
  temperatures and increased night-
                                                                                                                                           –25
  time temperatures (pages 12–13)
                                                                                                                                           –35
  observed during the La Niña event.
                                                                                                                                           –45
                                                                                                                                           –55   Wm -2

                                               Outgoing long-wave radiation (OLR) anomalies over the greater Australasian region for summer
                                               2010–11. Negative anomalies (blue and purple colours) indicate increased cloudiness and less
                                               heat lost to space, while positive anomalies (yellow and red colours) indicate reduced cloudiness.
                                               The widespread cloud over the Australia–Indonesia region during the summer is clearly shown.

                                                                                                                                                         17
Background

The Indian Ocean Dipole

                                           The Indian Ocean Dipole (IOD) is defined by the difference in sea
 The Indian Ocean Dipole
                                           surface temperature between two areas (or poles, hence a dipole)
                                           – a western pole in the Arabian Sea (western Indian Ocean) and an
     Positive event
                                           eastern pole in the eastern Indian Ocean south of Indonesia. The IOD
     • Warmer sea surface
                                           affects the climate of Australia and other countries that surround
       temperatures in the western
       Indian Ocean relative to the east   the Indian Ocean Basin, and is a significant contributor to rainfall
     • Easterly wind anomalies             variability in this region.
       across the Indian Ocean and
       less cloudiness to Australia’s
       northwest                           Like ENSO, the change in temperature          throughflow (the flow of warm tropical
     • Less rainfall over southern
                                           gradients across the Indian Ocean results     ocean water from the Pacific into the
       Australia and the Top End.          in changes in the preferred regions of        Indian Ocean). Hence, positive IOD
                                           rising and descending moisture and air.       events are often associated with
     Negative event
                                                                                         El Niño and negative events with
     • Cooler sea surface temperatures
                                           In scientific terms, the IOD is a coupled
                                                                                         La Niña. When the IOD and ENSO
       in the western Indian Ocean         ocean and atmosphere phenomenon,
                                                                                         are in phase the impacts of El Niño and
       relative to the east                similar to ENSO but in the equatorial
                                                                                         La Niña events are often most extreme
     • Winds become more westerly,         Indian Ocean. It is thought that the IOD
       bringing increased cloudiness
                                                                                         over Australia, while when they are
                                           has a link with ENSO events through
       to Australia’s northwest                                                          out of phase the impacts of El Niño
                                           an extension of the Walker Circulation
     • More rainfall in the Top End                                                      and La Niña events can be diminished.
                                           to the west and associated Indonesian
       and southern Australia.

The Southern Annular Mode

                                           The Southern Annular Mode (SAM), also known as the Antarctic
 Southern Annular Mode
                                           Oscillation, describes the north–south movement of the westerly wind
                                           belt that circles Antarctica, dominating the middle to higher latitudes
     Positive phase
                                           of the southern hemisphere.
     • Band of westerly winds
       contracts toward Antarctica
     • Higher pressures over
                                           The changing position of the westerly         During autumn and winter, a positive
       southern Australia                  wind belt influences the strength and         SAM value can mean cold fronts and
     • Can relate to stable, dry           position of cold fronts and mid-latitude      storms are farther south, and hence
       conditions.                         storm systems, and is an important            southern Australia generally misses
                                           driver of rainfall variability in southern    out on rainfall. However, in spring
     Negative phase
                                           Australia.                                    and summer, a strong positive SAM
     • Band of westerly winds                                                            can mean that southern Australia is
       expands towards the equator         In a positive SAM event, the belt
                                                                                         influenced by the northern half of high
     • More (or stronger) low              of strong westerly winds contracts
                                                                                         pressure systems, and hence there are
       pressure systems over               towards Antarctica. This results in
                                                                                         more easterly winds bringing moist air
       southern Australia                  weaker than normal westerly winds
                                                                                         from the Tasman Sea. This increased
     • Can mean increased storms           and higher pressures over southern
                                                                                         moisture can turn to rain as the winds hit
       and rain.                           Australia, restricting the penetration
                                                                                         the coast and the Great Dividing Range.
                                           of cold fronts inland.
                                                                                         In recent years, a high positive SAM has
                                           Conversely, a negative SAM event
                                                                                         dominated during autumn–winter, and
                                           reflects an expansion of the belt of strong
                                                                                         has been a significant contributor to the
                                           westerly winds towards the equator. This
                                                                                         'big dry' observed in southern Australia
                                           shift in the westerly winds results in more
                                                                                         from 1997 to 2010.
                                           (or stronger) storms and low pressure
18                                         systems over southern Australia.
Story

Negative Indian Ocean Dipole increases rain

The IOD was negative from late August to late November 2010
                                                                                         –2.0 –1.5 –1.0 –0.5 0.0 0.5 1.0 1.5 2.0 °C
encouraging clouds to form over the eastern Indian Ocean and
enhancing the flow of moisture over Australia from the northwest.

                                                                                                                                            10°N
The 2010 IOD event was characterised          across Australia which may partially
by much warmer than normal ocean              explain the strong rainfall response.
surface temperatures in the eastern           However, during the 2010–11 La Niña
                                                                                                                                            10°S
Indian Ocean, and slightly above-             a strong positive Southern Annular
average ocean surface temperatures            Mode (SAM) partly offset at least some
in the west. This differed from the           of the effect of the negative IOD across
                                                                                                                                            30°S
classical pattern for a negative IOD,         southern, and particularly southwestern,
which has warmer than usual water             Australia (see SAM below).
in the east and cooler than usual water
                                              There was a weakly positive IOD             70°E          110°E      150°E        170°W
in the west. At least part of the unusual
                                              event during spring 2011, coinciding               90°E           130°E   170°E           150°W
behaviour of the IOD can be linked
                                              with the 2011–12 La Niña. The positive
to global warming which is leading to
                                              IOD may have partially moderated the
a rapid warming of the Indian Ocean                                                      October 2010 monthly sea surface temperature
                                              effect of the La Niña in southeastern      anomalies (˚C) in the Indian Ocean show
both in the east and west.
                                              Australia during these months, reducing    much warmer than usual waters in the eastern
The combination of a negative IOD             rainfall from what might otherwise have    end of the dipole near Australia. (Anomalies
and La Niña conditions has historically       been observed.                             are calculated with respect to the Reynolds
increased the likelihood of heavy rainfall                                               climatology for 1971–2000 from the National
                                                                                         Climatic Data Center.)

Positive Southern Annular Mode and
why Western Australia missed out on rain

The SAM was positive from March 2010 until February 2011. Record-
high positive monthly values were observed in 2010 for June, July and                        SAM event
November, while August and October were the second-highest on
record. The SAM appears to have a weak but discernible relationship                          The strong positive SAM event
                                                                                             during 2010 is likely to have
with La Niña and El Niño in summer. Hence the positive SAM values
                                                                                             contributed to the dry conditions
in late 2010 may well have been enhanced by the La Niña event.
                                                                                             in southwest Western Australia.

For southwest Western Australia, 2010         The persistently strong SAM during
was the driest year on record, continuing     2010 meant that high-pressure
and worsening the long drying trend           anomalies dominated parts of southern
that has affected the southwest since         Australia for most of the southern
the late 1960s. At least partly as a          hemisphere winter–spring. The
result of climate change, there has           dominance of the SAM over southern
been a change in the dominant weather         Australia is likely to have contributed
systems over the region, including a          to dry conditions in southwest Western
weakening of storms and southwards            Australia during winter and spring.
movement of storm tracks and cold
fronts, leading to a reduction in rainfall.

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