Simultaneous Atlantic-Pacific blocking and the Northern Annular Mode

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QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY
Q. J. R. Meteorol. Soc. 134: 1635–1646 (2008)
Published online 6 October 2008 in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/qj.310

Simultaneous Atlantic–Pacific blocking and the Northern
Annular Mode
Tim Woollings and Brian Hoskins
Department of Meteorology, University of Reading, UK

ABSTRACT: A synoptic situation termed ‘high-latitude blocking’ (HLB) is shown to occur frequently in both the Atlantic
and Pacific sectors, and to result in flow anomalies very similar to those associated with the negative phase of the Northern
Annular Mode (NAM) in the respective sector. There is a weak but significant link between the occurrence of HLB in
the two sectors, with Atlantic HLB tending to lead Pacific HLB by 1–3 days. This link arises from rare events in which
both sectors are almost simultaneously affected by a large-scale wave-breaking event which distorts the polar trough over
Northern Canada. In several cases the tropospheric wave-breaking occurs in tandem with a large-scale disturbance of the
stratospheric polar vortex.
There is, therefore, a physical link between the Atlantic and Pacific sectors, but analysis suggests that this does not contribute
to determining the pattern of the NAM, as conventionally defined from monthly mean data. However, an alternative version
of the NAM, derived directly from daily data, does appear to reflect this physical link. These conflicting results highlight
the sensitivity of the NAM to the period over which data are averaged. Copyright c 2008 Royal Meteorological Society

KEY WORDS Rossby wave-breaking; westward propagation; stratosphere
Received 22 November 2007; Revised 14 July 2008; Accepted 21 July 2008

1. Introduction patterns are associated with a large amount of variance
on decadal time-scales (Hurrell, 1995; Thompson and
There is still much debate over the nature of the dominant Wallace, 2001), but they could act to focus attention on
pattern of atmospheric variability in the Northern Hemi- quite different dynamical processes (Wallace, 2000).
sphere. The traditional view sees the pattern in mean sea Recently, Woollings et al. (2008, hereafter W08) sug-
level pressure (MSLP) as consisting of just two opposing gested a new interpretation of the NAO as representing
centres of action located near Iceland and the Azores, in variations in the occurrence of high-latitude blocking
a configuration known as the North Atlantic Oscillation (HLB) over the Atlantic. HLB occurs when Rossby waves
or NAO (e.g. Wanner et al., 2001). However, Thompson break in the upper troposphere, in a manner similar to the
and Wallace (1998, 2000) suggested that the NAO is just wave-breaking often seen in the stratosphere (McIntyre
the Atlantic part of a hemispheric pattern which is known and Palmer, 1983). In this way, HLB is similar to mid-
as the Northern Annular Mode (NAM), and features three latitude blocking, the difference being that HLB features
centres of action over the polar region, the Azores and blocking-like anticyclones lying poleward of the jet axes,
the North Pacific. which tend to divert the jet and storm track to the south
The NAM emerges robustly as the leading empirical rather than to ‘block’ them. In this new interpretation, the
orthogonal function (EOF) of monthly mean MSLP negative phase of the NAO is associated with an increased
or geopotential height, but is not identified by other occurrence of HLB, while the positive phase simply rep-
techniques such as correlation analysis. In fact, as shown resents decreased occurrence. However, note that other
by Deser (2000), correlations between the two midlatitude theories attribute the positive phase of the NAO to the
centres are very weak. The debate is therefore focused on occurrence of anticyclonic wave-breaking in the subtrop-
whether there is any physical link between variations at ics (e.g. Abatzoglou and Magnusdottir, 2006).
these two centres, or whether the Pacific centre is simply This clearly suggests a local, rather than annular,
an artifact of EOF analysis (Ambaum et al. 2001; Wallace view of Northern Hemisphere variability. However, as
and Thompson, 2002). EOF analysis has, for example, shown by W08 there is a link between HLB in the
been shown to identify annular structures even in artificial Atlantic and Pacific sectors, with Atlantic HLB leading
datasets constructed out of independent variations at Pacific HLB by a few days. The direction of influence
different longitudes (Dommenget and Latif, 2002; Gerber is therefore to the west, or upstream. The westward
and Vallis, 2005). The issue is important because both propagation of planetary-scale patterns has been noted in
observations by, for example, Branstator (1987), Kushnir
*Correspondence to: Tim Woollings, Department of Meteorology, (1987), Rinne and Järvenoja (1995) and Lau and Nath
Earley Gate, Reading RG6 6BB, UK. E-mail: t.j.woollings@rdg.ac.uk (1999), and in models by Franzke et al. (2000) and

1636 T. J. WOOLLINGS AND B. J. HOSKINS

Doblas-Reyes et al. (2001). These patterns often feature minus the southern box becomes larger than zero, a
blocking-like anomalies over both ocean basins. Lejenäs reversal is defined. Temporal and spatial scales are then
and Madden (1992) showed that blocking episodes do, in applied to ensure that the events identified are large-scale,
fact, often occur in association with the ridges of these quasi-stationary and persistent (lasting at least five days),
westward propagating planetary scale wave patterns, and and these are then termed episodes. (Berrisford et al.,
Luo (2005) presented evidence that westward propagating 2007, or W08 provide more details.) This is referred to in
patterns such as these can contribute to the onset of general as a wave-breaking index, and identifies events
blocking-like features in simple models. These studies in midlatitudes classed as blocking, and events on the
show that westward-travelling Rossby waves are clearly poleward side of the storm tracks termed HLB.
important, but here we suggest a different mechanism As shown by W08, in the Northern Hemisphere
which also acts to connect the Atlantic and Pacific. this identifies two dominant regions of frequent HLB
There is evidence for a dynamical process giving rise occurrence in the Atlantic and Pacific sectors, and there
to almost simultaneous occurrence of HLB over both appears to be a significant link between the two sectors.
the Atlantic and the Pacific. This results in NAO-like In the following two subsections we first identify all
anomalies over both basins, as seen in the NAM pattern, simultaneous Atlantic–Pacific HLB events and then, since
and so it could provide justification for the annular in many cases the two events occur simultaneously by
mode perspective. In this paper the link between Atlantic coincidence, we proceed to identify those events which
and Pacific HLB is further demonstrated, showing that we believe to be dynamically linked.
simultaneous Atlantic and Pacific events occur more often
than would be expected by chance. The contribution of 2.1. Simultaneous HLB
this simultaneous occurrence to the NAM pattern is then
assessed. The Atlantic–Pacific HLB link can be seen by correlating
the occurrence of wave-breaking throughout the Northern
Hemisphere with its occurrence over the North Atlantic.
2. The Atlantic–Pacific HLB link At each spatial grid point a daily time series is constructed
which equals one if a wave-breaking episode occurs, and
HLB episodes are identified in 44 complete winters zero if not. Similarly, a time series of Atlantic wave-
(December–February (DJF) 1957–1958 to 2000–2001) breaking is defined to be equal to one if a wave-breaking
from the ERA-40 re-analysis using the the 2D index episode occurs anywhere within the region 30–70 ◦ W,
described by Berrisford et al. (2007). This index identifies 50–60 ◦ N and zero otherwise. This is the same region in
blocking episodes via the associated wave-breaking, by which wave-breaking was related to the NAO by W08.
searching for a reversal in the meridional contrast of These time series are then correlated at various lags, and
potential temperature θ on the dynamical tropopause (the the resulting correlation maps are shown in Figure 1.
PV2 surface). At each point, θ PV2 is averaged over two Significance is estimated using a t-test with an effective
boxes of 5◦ longitude by 15◦ latitude, to the north and sample size derived from the lag–1 autocorrelations of
south of the point. When the value of the northern box each series, as in W08. These maps show a significant

Figure 1. Maps of the correlation of wave-breaking episode occurrence throughout the Northern Hemisphere with the occurrence over the North
Atlantic. Lags from −8 to +8 days with respect to the Atlantic series are shown, with the maps ordered in the clockwise direction. Contours
are drawn at 0.05 intervals, with negative contours dashed and the zero contour omitted, and the 95% significance level is marked by the bold
contour. The Pacific region used is shaded in the lag 4 panel.

ATLANTIC–PACIFIC BLOCKING AND THE NORTHERN ANNULAR MODE 1637

increase of wave-breaking occurrence over the Pacific is realised every 7 days. Thus two samples of 230 and
from lags of –4 to 8 days. The largest correlations are 228 days were chosen randomly from a pool of 566 days,
seen with the Atlantic leading the Pacific, as in W08, and and the occurrence of simultaneous days multiplied by 7.
there is a general impression of a westward propagation of The observed value of 732 corresponds to the top 1.7%
wave-breaking activity with time. Lagged correlations of of the resulting distribution, so the result is significant at
hemispheric streamfunction with Atlantic blocking, and the 2% level by this test.
also of hemispheric blocking with Pacific blocking have Composites of θ PV2 and MSLP anomalies for these
also been performed (not shown) and these exhibit the 732 days of simultaneous HLB are shown in Figure 2.
same features. In Figure 1 there is also a decrease in The characteristic signature of wave-breaking is the
blocking activity over southern Europe, and this feature overturning of θ PV2 contours, so that the usual negative
extends eastwards with time. This is a signature of the meridional gradient is reversed. In both sectors a cyclonic
reduction of southern European blocking associated with overturning is evident, and this acts to deform the
negative NAO periods. (W08 provides more discussion.) stationary trough which lies just upstream of each basin.
Based on the lag 4 correlation map, where the The wave-breaking leads to anticyclonic anomalies to the
largest and most coherent correlations are seen, a region north, and cyclonic anomalies to the south, which extend
160◦ E–160 ◦ W, 45–50 ◦ N is chosen to represent the down to the surface. The Atlantic and Pacific surface
Pacific events. As in the Atlantic, a Pacific wave-breaking anomalies are very similar to those associated with the
episode is said to occur if a wave-breaking episode is NAM, though the Pacific HLB results in a distinct high
identified at any point in this region. Note that this lati- pressure centre over Alaska, which interestingly leads
tude marks the reversal of the θ PV2 gradient, so in both to an overall pattern similar to the rotated geopotential
regions the anticyclone lies on the poleward side of the height EOF of Figure 2d in Christiansen (2002). The θ PV2
storm tracks. While these events are very similar to clas- composite is also reminiscent of the NAM (e.g. Figure 9
sical blocking episodes, they do not ‘block’ the westerly of Feldstein and Franzke, 2006).
winds and storms, and so are referred to as HLB. There are 110 Pacific onset days, defined as the
HLB episodes are relatively common in both regions. first Pacific HLB day after five non-HLB days. Out
Out of a total 3960 DJF days, 1608 days feature an of these there are 53 Pacific onset days for which a
Atlantic HLB episode and 1595 a Pacific episode. Given HLB episode exists in the Atlantic on the day before
this occurrence, if Atlantic and Pacific events were Pacific onset. By chance this would only be expected
completely independent, simultaneous occurrence would for 1608 × 110/3960 = 45 Pacific onsets, and the Monte
be expected on roughly 1608 × 1595/39602 ≈ 16% of Carlo method suggests this is significant at the 3% level.
all days, i.e. about 648 days. In fact, simultaneous HLB is (This was done by drawing samples of 1608/7 = 230
seen on 732 days (about 18% of days), so this does appear and 110 from a total of 3960/7 = 566 days, without
to happen slightly more often than would be expected by subsequent rescaling, as the series of onset days has no
chance. The significance of this has been assessed using autocorrelation.) Note that the last of the events occurs
a Monte Carlo method. Randomly choosing two samples too close to the end of the data period to perform lagged
of 1608 and 1595 days gives a distribution of the number analysis, so this event is neglected.
of simultaneous days with a mean of 648 and a standard As further evidence of the westward propagation of
deviation of 15 (using 1000 realisations). This suggests HLB activity, the evolution of the 52 cases is shown
that the result is highly significant, but does not account in Figure 3 as a Hovmöller diagram of the occurrence
for the autocorrelation in the data, manifested in the form of wave-breaking in the latitude band 45–55 ◦ N. The
of the clustering of blocking days into events. An attempt occurrence is given by the fraction of cases out of the
has been made to account for this by scaling the number 52 for which wave-breaking occurs, and is shown as an
of days by 7, effectively assuming an independent sample anomaly from the mean occurrence at a given location.

Figure 2. Composites of (a) θ PV2 (contour interval 5 K) and (b) MSLP anomalies (contours at ±1, ±3, ±5 hPa, with negative contours dashed)
for days exhibiting simultaneous HLB in both Atlantic and Pacific regions.

1638 T. J. WOOLLINGS AND B. J. HOSKINS

−10 55

Number of events
−5

45
0

40
5
day

35
−10 −5 0 5 10
10 Day

Figure 4. The number of cases of Atlantic HLB observed, as a function
of time from the onset day of Pacific HLB (day 0).
15

onset for the events to count as ‘linked’. This criterion
20
identifies 12 linked events, which have 164 days of
simultaneous HLB associated with them (about 4% of
all days). This may be a slight overestimate since some
25
0 100 200 300 of these events could have occurred by chance. These 12
longitude events are listed in Table I.
Figure 3. Hovmöller plot of anomalies of wave-breaking occurrence in These 12 events comprise only those simultaneous
the band 45–55◦ N for the 52 Pacific onset days preceded by an Atlantic disturbances which qualify as HLB episodes. Given
HLB. The occurrence is the fraction of the cases which exhibit wave-
breaking in this band at a given longitude. Day 0 is the Pacific onset the literature described in the introduction, there are
day, and the contour interval is 0.1, with negative contours dotted and presumably many instances of simultaneous ridges, or
the zero contour omitted. westward-travelling wave patterns, which do not qualify,
and so are not considered here. For example, the striking
Note that Atlantic and Pacific episodes occur at slightly
different latitudes, so the latitude band used here is a
Table I. Details of the 12 linked simultaneous HLB events
compromise between the two. This Hovmöller shows a identified in section 2.2, including subjective judgements on the
large occurrence of Atlantic HLB in the period a few similarity of each event to the synoptic evolution in Figure 5.
days before Pacific onset, as would be expected given
the choice of cases plotted. There is also a weak signal of Pacific onset Synoptic Sudden Comments
increased wave-breaking propagating further west, across pattern warming
Asia and Europe and back to the Atlantic. The signal √ √
takes around 20–25 days to circuit the globe, in line with 4 Feb 1958 √
the time-scales suggested by Branstator (1987), Kushnir 26 Dec 1962 √
(1987) and Lau and Nath (1999) for westward-travelling 3 Jan 1965
planetary-scale patterns, but it does not appear to have 20 Dec 1966 Atlantic event
a constant speed. The continued propagation around to weak and not
Europe is remarkable, and does seem to be linked to the clearly linked to
simultaneous Atlantic–Pacific events. If all Pacific onsets Pacific.
are used to create the Hovmöller, this feature is much 6 Feb 1969 Atlantic and
weaker. Pacific events
appear
2.2. Linked simultaneous HLB √ independent.
27 Feb 1970 √ √
In many cases the simultaneous occurrence of HLB 12 Jan 1971 √
events is presumably a coincidence. An attempt is now 7 Jan 1973
made to identify the Atlantic and Pacific events which 17 Dec 1973 √ √ Weak event
lead to the correlations between the two sectors, and 9 Jan 1977 √ √
so must be linked in some way. Figure 4 shows the 25 Dec 1984 √
occurrence of Atlantic HLB seen as a function of time 27 Feb 1996
with respect to the Pacific onset days. There is a large
increase in Atlantic HLB in the few days immediately A tick in the third column indicates that the central date of a
stratospheric sudden warming lies within one week of the Pacific onset
before Pacific onset. This motivates the criterion that the date, using the central dates derived from ERA-40 by Charlton and
Atlantic onset should occur 1–3 days before the Pacific Polvani (2007).

ATLANTIC–PACIFIC BLOCKING AND THE NORTHERN ANNULAR MODE 1639

event described by Branstator (1987) is not included in circulation over the period, which is not characterised
this list. by a meridional dipole over the Pacific (e.g. Thompson
It is interesting that most of the linked events occur et al., 2000). The trend in events does seem to be a robust
before 1980 (Table I). If this represents a real trend, then feature of the data, as it is supported by the EOF analysis
it is a fairly dramatic one. In general, re-analysis data of section 5.
are less trustworthy in the earlier, pre-satellite, period.
However, the events described here occur largely over
the North Atlantic, Canada and Alaska (e.g. Figure 5), 3. Synoptic description of the linked simultaneous
where observational coverage over this period was better HLB events
than in many other regions. Some trend may be expected The correlations presented here confirm that there is
given that there is a very close link between HLB and the a significant tendency for westward propagation of
NAO, which exhibited a strong positive trend from the high-latitude blocking activity. As described by Naka-
1960s to the 1990s. If the ERA-40 period is split into two mura and Wallace (1993), the upstream shift of block-
halves (winters 1957–1958 to 1978–1979 and 1979–1980 ing often occurs via the eddy-straining mechanism of
to 2000–2001), there are 934 Atlantic HLB days in the Shutts (1983), whereby a block results in diffluent flow
first half, but only 674 in the second half; a decrease upstream, leading to meridionally elongated eddies and
of 28%. In the same periods there were respectively subsequent wave-breaking just upstream of the initial
131 and 33 days of linked simultaneous events; a much block. However, the tendency for westward propaga-
larger decrease of 75%. Note that while this decrease is tion is also evident across the North American, and
dramatic, it does not account for all of the reduction in even Eurasian, continents, where synoptic eddy activity is
Atlantic HLB. There is a much weaker change in Pacific weak. This instead favours the suggestion by Lejenäs and
HLB days, with 772 in the first half and 823 in the second Madden (1992) that low-frequency planetary wave pat-
half. This is consistent with the general linear trend of terns propagate to the west and act to encourage blocking

Figure 5. Daily evolution of θ PV2 for the simultaneous HLB in February 1996, showing the 00 UTC fields each day from 24 February to 3
March.

1640 T. J. WOOLLINGS AND B. J. HOSKINS

in various regions around the globe as they pass. How- and the situation is now very similar to the composite
ever, in the events studied here there is a delay of only a of all simultaneous HLB days in Figure 2(a). The 12
few days between Atlantic and Pacific HLB onsets. This linked events have been examined individually, and out
time-scale is shorter than that suggested for the travelling of these 9 are subjectively considered to broadly follow
wave patterns. this evolution (Table I).
Some insight into the reason for this short time-scale In this synoptic interpretation then, the western Atlantic
is given by the synoptic evolution of the linked simul- and eastern Pacific are almost simultaneously affected by
taneous HLB events, an example of which is shown in the distortion of the polar trough. This appears distinct
Figure 5. In the first frame a large trough of polar air (in from the two mechanisms previously discussed, namely
blue) extends south over western North America. Over the westward propagation of long Rossby waves and the
the next few days this trough is deformed and splits, upstream propagation of a blocking anomaly under eddy
with some of the polar air moving west towards the straining. It appears that all three of these mechanisms
Pacific, and some moving east towards the Atlantic. The are evident in atmospheric data, and are likely combined
trough is effectively pinched from both sides by high-θ in analyses which search for travelling patterns, such as
air. This is especially clear in the map for 28 February, complex EOF analysis.
when the high-θ air masses lie over Alaska and south-
ern Greenland. The low-θ air of the trough lies to the
south of these air masses, so that the meridional gradient 4. Stratospheric connection
is reversed giving the signature of wave-breaking. The
same polar air mass is therefore involved in two simulta- There is increasing evidence that disturbances in the
neous wave-breaking events: a cyclonic overturning over stratosphere are linked to variations of the NAO/NAM
the east coast of North America, and an anticyclonic over- at the surface, so this motivates us to look for any
turning over the west coast. (Tyrlis and Hoskins, 2008, stratospheric involvement in the linked simultaneous
discuss cyclonic and anticyclonic breaking, and why they HLB events. If there is stratospheric involvement, then
dominate over the upstream and downstream storm track these events may contribute to the observed stratosphere–
regions respectively.) The distorted polar trough forms troposphere connection associated with the NAM.
both the upstream limb of the Atlantic event and the Baldwin and Dunkerton (2001) used a NAM index
downstream limb of the Pacific event. On 29 February calculated at all levels throughout the atmosphere to
a subsequent breaking begins in the Pacific just upstream demonstrate stratospheric involvement in the NAM, by
of the original anticyclonic event, in line with the eddy compositing the index with respect to strong disturbances
straining theory. The signature of this second breaking is in the stratosphere. Here we use the same data (available
clear in the map for 1 March, with a cyclonic overturning from http://www.nwra.com/resumes/baldwin/) to make
of the contours just east of the Kamchatka Peninsula. This composites around the Pacific onset day of the 12 linked
second breaking shifts the blocking pattern upstream, so events. The resulting composite, shown in Figure 7, con-
that a HLB is seen over the central Pacific a few days firms that there are disturbances in the stratosphere asso-
after that over the Atlantic. ciated with the linked events. The decrease of the NAM
This evolution is visible in composites of the upper- index seems to occur more or less simultaneously in the
tropospheric streamfunction for the 12 linked events, stratosphere and troposphere, without the characteristic
shown in Figure 6, though the features are somewhat lead of the stratosphere shown by Baldwin and Dunker-
smeared when compared to Figure 5. On day –2 the ton. The 12 events have been examined individually, and
trough over Canada is beginning to be pinched from there are strong differences from event to event. In cases
both the Atlantic and Pacific sides. The pattern amplifies where the stratosphere is most strongly involved, there
over the next few days, with the anticyclonic anoma- is generally an element of downward propagation, but
lies over southern Alaska and Greenland developing the composite suggests that this is not generally the case.
simultaneously. By day 2 the Pacific anomaly is shifting Also apparent in Figure 7 is the presence of tropospheric
upstream as the cyclonic wave-breaking develops there, anomalies over the month before onset. Note, however,

Figure 6. Composites of 250 hPa streamfunction for the 12 linked simultaneous HLB events, with respect to the Pacific onset day (day 0). The
full field is contoured in bold at –14, –12, –10 and −8 × 107 s−1 , and also the anomalies are contoured every 5 × 106 s−1 , with negative contours
dashed and the zero contour omitted.

ATLANTIC–PACIFIC BLOCKING AND THE NORTHERN ANNULAR MODE 1641

NAM index composite
101

p (hpa)
102

103
−20 0 20 40 60 80
Lag (days)

Figure 7. Baldwin and Dunkerton’s NAM index as a function of height and time, composited over the 12 linked simultaneous HLB events, with
the Pacific onset day as day 0. Contours are drawn every 0.25 standard deviations, with the zero contour omitted and values less than –0.25
shaded.

that by definition the period before onset will be free of by Charlton and Polvani (2007), and these are indicated
HLB in both basins, so would be expected to project onto in Table I. Interestingly, all four of these are classed as
the positive phase of the NAM, as seen. split vortex, rather than displacement warmings, at least
As further evidence of stratospheric involvement, and in the ERA-40 data. Note that 11% of all winter days
also of the large case-to-case variability, Figure 8 shows lie within one week of a warming, so from a sample of
the polar vortex on the Pacific onset day of each of the 12 12 days only 1.3 days would be expected to coincide
events. In several cases the vortex is deformed, split or with a warming by chance. In Monte Carlo resamplings
displaced from the pole. Four of the events occur within (as used in section 2.1 but without scaling) the observed
a week of a stratospheric sudden warming, as defined occurrence of four events corresponds to the top 0.5%

4/2/1958 26/12/1962 3/1/1965 20/12/1966

6/2/1969 27/2/1970 12/1/1971 7/1/1973

17/12/1973 9/1/1977 25/12/1984 27/2/1996

Figure 8. Potential vorticity on the 850 K isentropic surface on the Pacific onset day of each of the linked simultaneous HLB events in Table I.
Contours are drawn every 200 PVU from 400 PVU upwards.

1642 T. J. WOOLLINGS AND B. J. HOSKINS

of the distribution. Note that the dates given by Charlton conventional NAM, though the Pacific centre is weaker,
and Polvani (2007) for sudden warmings correspond to suggesting that on the daily time-scale the two midlatitude
the central time of the event, whereas for simultaneous centres do not co-vary to the extent suggested by the first
blocking the date is the onset date. For this reason the EOF of low-frequency data. This pattern will be referred
dates are not comparable and it is not possible to infer a to as the daily NAM. The second EOF is a wavenumber-
lead or lag association from the dates. one pattern with one centre over northern Asia and the
The tropospheric and stratospheric events do, therefore, other over the eastern Pacific, and so describes much of
appear to be connected. There are potentially influences the Pacific variance not contained in the first EOF.
in both directions, so that a two-way coupling is possible. The EOF analysis is then repeated with the 732
Firstly, the dramatic tropospheric events can be expected simultaneous HLB days removed from the dataset. Note
to have an impact on the stratosphere through anomalous that since there are similar numbers of Atlantic and
upward propagation of wave activity. Secondly, the Pacific HLB days in total, a similar fraction of HLB days
stratospheric flow in several cases features anomalously are removed for each sector. The resulting leading EOF,
high potential vorticity (PV) values over North America, shown in Figure 9(b), is very similar over the Atlantic,
either in the form of a trough (e.g. in 1966) or associated but there is no signal over the Pacific, so that the overall
with a displacement of the vortex in that direction (e.g. pattern closely resembles the NAO. This suggests that
in 1984). In these cases the tropospheric circulation the Pacific centre of action in the daily NAM arises
associated with the stratospheric PV could act to enhance because of the simultaneous occurrence of HLB in both
the polar trough over Canada, which is a key component sectors. A Monte Carlo test suggests that the absence of
of the simultaneous wave-breaking. a Pacific centre here is highly significant. (100 datasets
To summarise, there is a tendency for stratospheric were formed by randomly removing 732 days, and in the
variations to accompany the tropospheric linked HLB resulting EOFs the Pacific centre is never smaller than
events, but there is much case-to-case variability. In some 1.5 hPa/std.)
cases the stratosphere appears to lead the troposphere,
while in others it appears to lag, so that there is no
5.2. Linked simultaneous HLB events
consistent lead or lag between the tropospheric and
stratospheric events in this, admittedly small sample. Given the abundance of HLB events in both sectors,
many of the simultaneous events occur by chance. The
EOF analysis is repeated after removing just the 164
5. Simultaneous HLB and the NAM simultaneous HLB days associated with the 12 linked
5.1. Simultaneous HLB events events in Table I. This results in a leading EOF with a
much weakened Pacific centre of action (Figure 9(c)), so
In this section, the contribution of simultaneous Atlantic– these few days (just 4% of the total) do appear to be
Pacific HLB to the NAM pattern is assessed. In order to responsible for a relatively large fraction of the Pacific
do this, a version of the NAM is constructed directly signal. The significance of this is assessed using a Monte
from daily data (as recommended by Feldstein, 2000, Carlo method. 100 different datasets were formed by
for teleconnection patterns), firstly using all winter days, randomly removing just 164 of the 732 simultaneous
and then with simultaneous HLB days removed. MSLP HLB days, leaving datasets of 3796 days. The first EOF
anomalies for all 3960 DJF days were calculated by was then calculated for each dataset, and the magnitude
subtracting a smooth seasonal cycle, which was obtained of the Pacific centre in each was obtained. Figure 10
by averaging over the 44 years and then filtering with a shows the resulting distribution, with a cross marking
discrete cosine transform, retaining only the mean and the magnitude of the Pacific centre obtained when the
the first two harmonics. The first EOF of this daily linked simultaneous HLB events are removed (as in
data, which explains 10% of the variance, is shown Figure 9(c)). The 164 days associated with the linked
in Figure 9(a). The pattern is clearly similar to the events are therefore highly significant in contributing to

Figure 9. (a) First EOF of daily MSLP anomalies north of 20◦ N, plotted by regressing MSLP onto the principal component time series. The
contour interval is 1 hPa per standard deviation of the time series, with negative contours dashed and the zero contour omitted. (b) is as (a), but
with the 732 simultaneous HLB days removed. (c) is as (a), but only removing the 164 linked simultaneous HLB days.

ATLANTIC–PACIFIC BLOCKING AND THE NORTHERN ANNULAR MODE 1643

25 the lower end of this range, but this number is probably a
conservative estimate. This criterion aims to ensure that
20 the spatial correlation of the EOF pattern with that given
by an infinitely sampled dataset is greater than 0.975.
15 For comparison the area-weighted correlation between the
patterns in Figure 9(a) and (c) is 0.92. (Both patterns
10 are weighted by the square root of the cosine of the
latitude prior to calculating the correlation.) Despite being
5
very well separated, the EOF pattern is quite sensitive to
a small part of the data, though note that the fraction
0
1.2 1.4 1.6 1.8 2 2.2 of variance associated with the EOF does not change
Pacific Centre (hPa/std) significantly when these days are removed. In general,
more caution may be needed when using EOF analysis
Figure 10. Histogram of the amplitude of the Pacific centre of the first
daily EOF in 100 datasets formed by randomly removing only 164 of than is suggested by North et al.
the 732 simultaneous HLB days. The circle marks the first EOF of the Since the NAM is usually defined using monthly mean
full dataset, and the cross that of the dataset formed by removing the data, the contribution of the linked events to the monthly
164 simultaneous HLB days associated with the linked events.
NAM is also assessed. Here the monthly NAM is defined
as the first EOF of monthly mean MSLP anomalies, and
the Pacific centre of the daily NAM. Since most of the is shown in Figure 11(a). When monthly means are taken,
linked events occurred before 1980, this result has also Atlantic and Pacific HLB events which are separated by
been tested by calculating the first daily EOF over the a few days appear contemporaneously. To estimate the
winters from 1979–1980 to 2001–2002. This EOF (not effect of the linked simultaneous HLB events, it is there-
shown) is very similar to that with the linked events fore necessary to identify days associated with the linked
removed, adding confidence to this result, and also to events when either sector features HLB. To do this the
the trend in the number of linked events. Pacific onsets occurring 1–3 days after an Atlantic onset
The sensitivity of these results to the choice of the are identified as before. Then all subsequent Pacific HLB
Atlantic and Pacific regions defined in section 2.1 has days until the Pacific decay date (i.e. the last Pacific HLB
also been tested. As shown in Figure 1, Atlantic and day before five non-HLB days) are counted, as are all
Pacific HLB events are correlated across wide bands preceding Atlantic HLB days back to the Atlantic onset
date. This identifies 217 days associated with the linked
of longitude but narrow bands of latitude. The results
events when either sector features a HLB. These 217
are therefore sensitive to the latitudes used, since the
days are then removed from the dataset, and replaced
regions have to coincide with the regions of significant
randomly by other days (of the same calendar month)
correlations. The sensitivity to longitude has been tested
from the remaining 3743, before taking monthly means.
by shifting the longitudinal boundaries of both regions
This has been repeated many times to test the sensitivity
by 10 ◦ in each direction. In these tests the NOSIM EOF
to the random selection of days, but the resulting EOFs
in Figure 9 is virtually unchanged. The Pacific centre
are almost identical. The pattern of the first EOF is shown
of the NO LINKED EOF does vary a little, but only in
in Figure 11(b), and shows a negligible weakening of the
one out of 8 tests is the centre stronger than 1.65 hPa/std
Pacific centre compared to that in the full dataset. The
(cf. Figure 10), so the results are fairly robust to small
linked events do not, therefore, appear to contribute sig-
changes in the regions used.
nificantly to determining the pattern of the monthly NAM.
It is somewhat concerning that the EOF pattern is so
sensitive to just 4% of the data, especially since this
first EOF is separated from the second according to the
6. Concluding remarks
criteria√of North et al. (1982). North et al. derive the error
λ = 2/N λ on an eigenvalue λ, where N is the the Simultaneous Atlantic–Pacific HLB does occur more
number of independent samples. The criterion states that often than would be expected by chance, because in a
two EOFs are separated if λ1 − λ1 > λ2 + λ2 . Given number of cases both basins are almost simultaneously
that λ corresponds to one standard deviation, this does affected by a large-scale distortion of the polar trough
not appear to be an overly strict criteria. However, the over Canada. In these events this polar trough forms
leading EOF of MSLP used here is very well separated the upstream limb of a cyclonic wave-breaking over the
from the second. Assuming an independent sample every Atlantic and also the downstream limb of an anticyclonic
7 days, so that N = 3960/7, gives λ1 = 0.103, λ1 = wave-breaking over the Pacific. A subsequent cyclonic
0.006, λ2 = 0.077 and λ2 = 0.005, so that in fact wave-breaking then shifts the Pacific blocking anomaly
λ1 − 2λ1 > λ2 + 2λ2 . Quadrelli et al. (2005) estimate upstream into the central basin, where the HLB onset
the number of independent samples needed for well- occurs a couple of days after that in the Atlantic. These
separated eigenvectors, or EOF patterns. Using their linked events are quite rare, so the Atlantic–Pacific
Figure 2, and the values of λ1 and λ2 above, suggests that connection is weak. This finding is in contrast with
500–1000 independent samples would be needed. The several previous studies which reported no significant
number of independent samples assumed here (566) is at tendency for simultaneous occurrence of blocking events

1644 T. J. WOOLLINGS AND B. J. HOSKINS

Figure 11. (a) First EOF of monthly mean MSLP anomalies. The contour interval is 1 hPa per standard deviation of the principal component
timeseries, with negative contours dashed and the zero contour omitted. (b) is as (a), but with the 217 days featuring Atlantic or Pacific HLB
associated with the linked simultaneous HLB events randomly replaced.

(Lupo, 1997; Tyrlis and Hoskins, 2008; and references (a) 0.7
therein). Note, however, that the events studied here are
Ratio Pacific/Atlantic
high-latitude, rather than midlatitude, blocks, which may 0.6
explain the differing results.
0.5
The daily NAM does appear to at least partly represent
this physical link, but not all of the most negative NAM 0.4
days are simultaneous HLB days. Out of the most
negative NAM days, about half are simultaneous HLB 0.3
days (using any of –1, –1.5 or –2 standard deviations as
a NAM index threshold). Thus, we do not claim that the 0.2
100 101 102
negative phase of the daily NAM solely represents simul-
taneous blocking, but that the simultaneous HLB events (b) 6
play a role in determining the pattern of the daily NAM.
5
The focus of this paper has been on HLB, which resem-
Centre (hpa/std)

bles the negative phase of the NAM, but it is possible 4
that the two basins are linked by dynamical processes
which resemble the positive phase. This possibility has 3
been investigated using a modified version of the wave-
breaking index. This suggests there is no simultaneous 2
link, but that strong westerly wind events in the Pacific
1
tend to lead similar Atlantic events by around a week, 100 101 102
contrasting strongly with the HLB behaviour. This anal- Averaging period (days)
ysis is described more fully in the Appendix.
It is interesting that the NAM, as usually derived Figure 12. (a) Ratio of the Pacific and Atlantic centres of the first EOF
of MSLP, as a function of the number of days over which the data are
from monthly mean data, does not appear to represent averaged. (b) shows the separate values of the Atlantic (solid line) and
the physical link through HLB, but the version derived Pacific (dashed line) centres.
directly from daily data does. There is clearly low-
frequency variability in the occurrence of the linked
events (Table I), as evidenced by the trend, so the lack of NAM is not as robust as teleconnection patterns such
a signal in the monthly NAM is surprising. This may as the NAO and PNA, which are relatively insensitive to
be because the NAM pattern is quite sensitive to the the averaging period used (Feldstein, 2000).
length of the period over which data are averaged. The It is possible that the sensitivity seen in Figure 12 could
Pacific centre is weak when daily data are used, as shown be an effect of the seasonal cycle, especially given the
here, but also when seasonal data are used (e.g. Quadrelli very different nature of Pacific variability in different
and Wallace, 2004, Figure 13). To summarise this aspect, winter months (e.g. Eichelberger and Hartmann, 2007).
the first EOF was obtained using the daily winter MSLP In agreement with this, the first EOFs of monthly mean
anomalies averaged over fixed, non-overlapping windows MSLP for the separate months are quite different (not
of length 1, 3, 5, 9, 15, 30, 45 and 90 days, and the shown). For example, in the EOFs for December, January
Pacific and Atlantic centres are compared in Figure 12. and February the ratio of the Pacific to Atlantic centres is
This confirms that the NAM is sensitive to the averaging 0.74, 0.43 and 1.26 respectively. This is suggestive of a
period used, with the Pacific centre being of maximum strong seasonal cycle, but this variation does not in fact
importance with 30-day averaging. In this respect the appear to be significant. In datasets comprising 44 winter

ATLANTIC–PACIFIC BLOCKING AND THE NORTHERN ANNULAR MODE 1645

months chosen at random, the pattern of the first EOF is whether the technique used to identify HLB also detects
very variable, and can often resemble the EOFs of each such a link.
of the individual months. Sampling is therefore a problem The blocking index uses the difference between θ PV2
with datasets of this size. averaged over boxes to the north and south of each
It is also possible that the differing results for the point to indicate the reversal of the usual meridional
daily and monthly NAM could arise because different contrast (when the northern value minus the southern
dynamical processes link the Atlantic and Pacific on daily value is positive). Following Berrisford et al. (2007),
and monthly time-scales. For example, it is possible that this can be modified to identify episodes of enhanced
the Branstator–Kushnir travelling wave patterns have a negative contrast, which would be associated with strong
larger impact on the monthly time-scale, although the westerly winds. Over the Atlantic the difference between
intrinsic time-scale of these events is similar to that of the northern and southern boxes in the climatological
the simultaneous blocking events. mean is –15 K, so a threshold of –30 K was chosen
Regardless of their contribution to the NAM, the simul- to indicate strong westerly events. When the northern
taneous blocking events described here are of interest in value minus the southern value is less than –30 K an
that they have a profound effect on tropospheric flow, instantaneous event is defined, and then the same time-
show a clear link to stratospheric disturbances, and also and space-scales as for blocking are applied to identify
exhibit a dramatic frequency change over the ERA-40 strong westerly episodes.
period. Figure A.1 shows maps of the lagged correlation of
strong westerly episodes around the Northern Hemisphere
with strong westerly episodes in the Atlantic region
Acknowledgements
(as in Figure 1 for HLB). There are no significant
We are indebted to Steven Feldstein and to the editors simultaneous correlations, suggesting that strong westerly
and reviewers for constructive feedback which has been events do not occur simultaneously over both basins any
particularly helpful. We would also like to thank ECMWF more than would be expected by chance. There are,
and BADC for supplying the data. TW was supported however, significant lag correlations, but the influence
by a grant from the NERC RAPID Climate Change is in the other direction to that seen for HLB, with
programme. Pacific events leading Atlantic events. The time-scale
is of the order of a week, which is consistent with the
downstream propagation of synoptic-scale Rossby waves
Appendix: Strong westerly events from the Pacific to the Atlantic. A similar time-scale was
The focus of this paper has been on the occurrence of seen by Franzke et al. (2004), who applied wave-like
simultaneous Atlantic–Pacific HLB, which gives flow perturbations in the Pacific in a primitive-equation model
anomalies similar to the negative phase of the NAM. which led to NAO-like anomalies over the Atlantic a
There could also be an analogous dynamical link between week later.
the two basins which results in anomalies similar to the This index does not, therefore, identify a physical
positive phase of the NAM. This appendix investigates Atlantic–Pacific link which is directly analogous to the

Figure A.1. Maps of the correlation of strong westerly episodes throughout the Northern Hemisphere with their occurrence over the North
Atlantic, showing lags from −8 to +8 days with respect to the Atlantic series, with the maps ordered in the clockwise direction. Contours are
drawn at 0.05 intervals, with negative contours dashed and the zero contour omitted, and the 95% significance level is marked by the bold
contour.

1646                                            T. J. WOOLLINGS AND B. J. HOSKINS

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                                                          Lau N-C, Nath MJ. 1999. Observed and GCM-simulated westward-
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Copyright 
          c 2008 Royal Meteorological Society                                                    Q. J. R. Meteorol. Soc. 134: 1635–1646 (2008)
                                                                                                                                DOI: 10.1002/qj

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