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Basics of atmospheric circulation (as a premise to understand oceanic circulation)
The Earth can be considered as a physical system with an energy budget that includes all gains of incoming energy and all losses of outgoing energy. The planet is approximately in equilibrium, so the sum of the gains is approximately equal to the sum of the losses. Of the incoming solar energy = 100%, 6% is reflected from the atmosphere + 20% is reflected from clouds + 4% is reflected from the ground (including land, water and ice) + 51% is absorbed by land and water, then transferred into the atmosphere (different ways) and then reradiated into space + 19% is absorbed by the atmosphere and clouds, then reradiated into space = 100% outcoming energy. (Note: this is the net budget excluding the greenhouse effect)
Earth’s zonal climate. The energy that reaches the Earth’s surface is
distributed and absorbed/reflected over the globe as function of the angle
L at which sunlight strikes the Earth, which varies by location, time of day,
and season.
a
c
c = a / sinL setting a = 1 km
for L = 90°, c = 1 km
for L = 45°, c = 1.4 km
for L = 30°, c = 2 km
This implies that at high latitudes and/or during winter times, the same
amount of solar energy is spread over larger areas than at low latitudes
and/or during summer times.
This dependance of the absorbed energy distribution on the angle at which sunlight strikes the Earth is responsible for higher amounts of annually absorbed solar energy per unit area at low latitudes (30°S–30°N) than at high latitudes (blue line). The outgoing terrestrial radiation per unit area varies less with latitude (it actually decreases slightly at the equator for the reduced albedo effect of pluvial forests). This creates a positive energy budget at low latitudes (more solar energy is adsorbed than radiated back to space) and a negative budget at high latitudes.
The Greenhouse Effect Of the total amount of energy available at the top of the atmosphere (TOA), about 50% is absorbed at the Earth's surface and re-radiated upward as infrared (IR) thermal radiation. A greenhouse gas (H2O, CO2) is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. Without CO2, Earth's greenhouse effect would collapse as water vapor would quickly precipitate from the atmosphere, plunging the Earth into an icebound state
The 51% of energy that reaches the Earth’s surface and that is
transferred into the atmosphere as IR before being lost to space controls the
thermal state of the lower atmosphere as described by the ideal gas law.
The ideal gas law links pressure, density, and temperature of a gas (e.g., air):
P = r RspecT
where r is density g/cm3, Rspec = R / M where R is the gas constant =
8.314472 J/mol K and M = molar mass (g/mol) (the mole is defined as the
amount of substance that contains as many elementary entities (e.g., atoms,
molecules) as there are atoms in 12 g of the isotope carbon-12 (12C). Thus,
by definition, one mole of pure 12C has a mass of exactly 12 g.)
For what we’ve seen before, we would have, on an hypothetical non-
rotating planet, low pressures on the equator and high pressures at high
latitudes causing winds to blow from the poles to the equator on the
surface and from the equator to the poles at higher altitudes
High pression
Low pression
High pression
…but the Earth rotates around its axis… The Coriolis effect arises when Newton's laws are transformed to a rotating frame of reference. Coriolis force is a consequence of inertia, the resistance of any physical object to a change in its state of motion or rest, or the tendency of an object to resist any change in its motion. The Coriolis force disappears in a non-rotating, inertial frame of reference. The Coriolis force causes moving objects on the surface of the Earth to appear to veer to the right in the northern hemisphere, and to the left in the southern. Rather than flowing directly from areas of high pressure to low pressure, as they would on a non-rotating planet, winds and currents tend to flow to the right of this direction north of the equator, and to the left of this direction south of it. This effect is responsible for the rotation of large cyclones
Intuitive explanation The Earth's rotation causes the surface to move fastest at the equator, and not at all at the poles. A bird flying from the equator to the north maintains its faster eastward motion as objects sitting on the equator; as it moves to the north, it goes east faster than the earth beneath it, and the bird's flight curves eastward slightly. In general: objects moving away from the equator curve eastward; objects moving towards the equator curve westward. Flying objects deflect to the right in the northern hemisphere and to the left in the southern hemisphere.
In meteorology, a rotating frame (the Earth) with its Coriolis force proves a
more natural framework for explanation of air movements than a
hypothetical, non-rotating, inertial frame without Coriolis forces.
For winds moving N-S:
Where m is the mass of the
particle, v is the velocity of the
Fc = 2mvwsinf particle (m/sec), and w is the
angular velocity of Earth
(7.27*10E-5 rad/sec) and f is the
latitude.
Setting mass m = 1 kg and velocity v = 1m/s:
At the equator (f = 0), Fc = 0; at 45°, Fc = 1.2 *10-4 N; at the Pole, Fc =
1.3*10-4 N. (gravity = 9.8 N). In N hemisphere, Fc is to the right of v; in S
hemisphere, Fc is to the left of v (left hand rule).The balance between the Coriolis effect and the pressure gradient force produces geostrophic wind. The geostrophic wind is directed parallel to isobars (lines of constant pressure at a given height). This balance seldom holds exactly in nature. The true wind almost always differs from the geostrophic wind due to other forces such as friction from the ground. Thus, the actual wind would equal the geostrophic wind only if there were no friction and the isobars were perfectly straight. Despite this, much of the atmosphere outside the tropics is close to geostrophic flow much of the time and it is a valuable first approximation.
The geostrophic interaction between pressure gradient and Coriolis force generates zonally distributed trade winds and jet streams arranged in Hadley cells. Between 30°N and 30°S latitude, atm circulation is relatively simple, with rising motion near the equator, poleward motion near the tropopause, sinking motion in the subtropics, and an equatorward return flow near the surface. Near the tropopause, as the air moves polewards in the Hadley cell it is turned eastward by the Coriolis effect, which turns winds to the right in the Northern hemisphere and to the left in the Southern Hemisphere, creating the subtropical jet streams that flow from west to east. Analogously, near the surface, the equatorward return flow is turned to the west by the Coriolis effect. These resulting surface winds are referred to as the trade winds.
..as the air moves polewards in the Hadley cell it is turned to the right in the Northern hemisphere and to the left in the Southern Hemisphere, reaching geostrophic ~balance (little friction at that altitude) and creating the subtropical jet streams that flow from west to east. Analogously, near the surface, the equatorward return flow is turned to the west by the Coriolis effect (trade winds).
The Intertropical Convergence Zone (ITCZ): where the trade winds of the Hadley cells meet.
The zonal climate controls the distribution of ecosystems and vegetation
The dynamic theory of Monsoon During the northern Summer (May and June), the ITCZ moves northwards, along with the vertical sun, towards the Tropic of Cancer. With the ITCZ at the Tropic of Cancer, the South East Trade winds of the Southern Hemisphere have to cross the equator to reach the ITCZ. However, due to Coriolis effect, these South East winds are deflected to the right in the Northern Hemisphere transforming into South West trades. These pick up the moisture while traveling from sea to land and cause orographic rain once they hit the highlands of the Indian Peninsula. This results in the summer Monsoon.
Global atmospheric circulation
Circolazione Oceanica
Sommario Temperatura e salinità degli oceani Circolazione superficiale indotta dai venti Circolazione termoalina profonda Interazioni oceano-atmosfera
Gli oceani Oceano Pacifico: 52% dell’oceano totale, profondità media di 4028 metri; Oceano Indiano: 20% area, profondità media di 3897 m; Oceano Atlantico: 25% area, profondità media di 3332 m; Le parti meridionali dei tre oceani costituiscono l’Oceano Meridionale (Southern Ocean).
Radiazione entrante e uscente
Temperature superficiali
Evaporazione, Precipitazione, Salinità
Dry
Wet
Dry
La circolazione atmosferica definisce le zone E-PE-P
Salinità superficiale
L’acqua marina contiene circa
3.49% di sali in soluzione. I
costituenti principali sono il Cloro
[55%] e il Sodio [30.6%]
La densità dell’acqua
marina è funzione della
temperatura e della salinità.Densità superficiale
Stratificazione Acque più calde di 10°C dominano la superficie fino a ~500 m di profondità. Il decremento di temperatura in profondità è chiamato termoclino.
Circolazione oceanica Esistono due forze che determinano la circolazione oceanica: 1. Stress del vento che agisce sulla superficie dell’acqua, T = rair Cd W2 dove rai è la densità dell’aria,, W è la velocità del vento a 10 m, e Cd è il “drag coefficient” 2. Variazioni di temperatura e salinità = densità (termoalina)
La circolazione indotta dai venti è più vigorosa di quella termoalina, ma agisce essenzialmente nel primo km di profondità, mentre quella termoalina agisce a tutte le profondità e può causare “overturning” col quale le acque profonde possono venire a contatto con l’atmosfera e influenzare direttamente il clima. Iniziamo con la circolazione indotta dai venti e poi passiamo a quella termoalina…
Circolazione superficiale
indotta dai ventiVenti superficiali
Circolazione
superficialeCome il vento muove l’acqua: La spirale di Ekman
Lo stress del vento
(~velocità del vento2)
agisce sullo strato di
acqua superficiale e
50-200 m
produce trasporto
Ekman L’acqua è
deflessa verso
destra nell’emisfero
N e verso sinistra
nell’emisfero S
PER EFFETTO
DELLA FORZA DI
CORIOLISAnticiclone: ESEMPI EMISFERO NORD area di alta pressione atmosferica. Rotazione oraria dei venti in emisfero nord, antioraria in emisfero sud. Convergenza superficiale+ downwelling Ciclone: area di bassa pressione atmosferica. Rotazione antioraria dei venti in emisfero nord, oraria in emisfero sud. Divergenza superficiale+uwelling
Circolazione oceanica superficiale
Anticiclone,
downwelling
Anticiclone,
downwelling“valli” e “rilievi” sulla superficie degli oceani
Gli Alisei (Trade Winds) e la Zona di
Convergenza Intertropicale
(Intertropical Convergence Zone, ITCZ)
ITCZUpwelling equatoriale. I venti all’equatore provengono da oriente.
Per effetto dell’inversione della forza di Coriolis attraverso
l’equatore, il trasporto di Ekman è verso i poli in entrambi gli
emisferi. In altre parole, la circolazione oceanica superficiale è
divergente all’equatore. Per compensare la divergenza, acqua
profonda fredda viene richiamata in superficie (upwelling).
Trasporto di Ekman
upwelling
X EQ
ventoUpwelling equatoriale
Divergenza di Ekman e temperature superficiali
…Circolazione oceanica superficiale
Anticiclone,
downwelling
Divergenza, upwelling
Anticiclone,
downwelling
Divergenza, upwellingCircolazione oceanica profonda North Atlantic Deep Water (NADW) Antarctic Bottom Water (AABW)
Densità dell’acqua marina
(dovuta a salinità e temperatura)
Circolazione termoalina dovuta a variazioni di temperatura
e salinità = densità. Le acque più profonde e fredde derivano le
loro proprietà dall’esposizione in superficie alle alte latitudini.Corrente del Golfo e NADW
Perchè le acque subtropicali sono salate:
E-PAcque superficiali calde e salate si raffreddano muovendosi verso N e affondano, muovendosi quindi verso S a ~2-4 km di profondità
Sezione N-S del bacino atlantico L’acqua profonda si forma alle alte latitudini settentrionali e meridionali
Corrente Circumantartica e AABW
Anticiclone,
downwelling
Divergenza, upwelling
Anticiclone,
downwelling
Divergenza, upwellingFormazione di acque profonde antartiche
The Global Ocean Conveyor
The Great Ocean Conveyor Belt
Sommario
La circolazione profonda (>1 km) risulta da
cambiamenti di densità delle acque.
Acque calde e salate tropicali si raffreddano
muovendosi verso N (Corrente del Golfo).
Il raffreddamento di acque salate causa
aumento di densità e affondamento, e
movimento in profondità verso S (NADW).
Nell’Oceano Meridionale si ha formazione di
AABW.Interazioni Oceano-Atmosfera ENSO El Nino/La Nina North Atlantic Oscillation
El Nino/La Nina Condizioni ‘normali’ In condizioni normali i venti tropicali (Alisei) soffiano in direzione ovest causando risalita di acque fredde profonde ad est (upwelling) e accumulo di acque calde superficiali ad ovest. In prossimità dell’Indonesia la superficie dell’oceano è circa 0.5 m più alta che non in Perù e la temperatura superficiale è di circa 8 °C più elevata ad ovest che ad est. La piovosità è connessa con la zona a temperatura più elevata ad ovest, mentre la zona est è relativamente più secca. Condizioni El Niño Nella fase di El Niño gli Alisei si attenuano. La massa di acqua calda superficiale precedentemente accumulata ad ovest migra verso est con conseguente abbassamento del termoclino, blocco della risalita di acque fredde profonde ad est (upwelling) e migrazione della piovosità con inondazioni in Perù e siccità in Indonesia e Australia.
Condizioni ‘normali’ • Prentice Hall Textbook animation link
Condizioni El Nino • Prentice Hall Textbook animation link
Condizioni La Nina • Prentice Hall Textbook animation link
https://www.youtube.com/watch?v=DbNzw1CCKHo
The positive NAO index phase
/The positive NAO index
phase shows a stronger than
usual subtropical high pressure
center and a deep than normal
Icelandic low.
/The increased pressure
difference results in more and
stronger winter storms
crossing the Atlantic Ocean on
a more northerly track.
/This results in warm and wet
winters in Europe and in cold
and dry winters in northern
Canada and Greenland.
/The eastern US experiences
mild and wet winter
conditions.
Martin Visbeck 9 May, 2018The negative NAO index phase
/The negative NAO index
phase shows a weak
subtropical high and weak
Icelandic low.
/The reduced pressure gradient
results in fewer and weaker
winter storms crossing on a
more west-east pathway.
/They bring moist air into the
Mediterranean and cold
weather to northern Europe.
/The US east cost experiences
more cold air outbreaks and
hence snowy winter
conditions.
/Greenland, however, will
have milder winter
temperatures. Martin Visbeck 9 May, 2018Impacts of the NAO in Europe
/Northern Europe experiences
mild and wet winter during the
positive NAO index phase.
/This has dramatic
consequences for hydro-
electric power generation and
heating oil consumption.
/South-Eastern Europe
receives less rain and hence
causes significant problems
with drinking water supply
and reduced stream flow
volume in the Middle East.
/Harvest yield of grapes and
olives have been shown to
depend significantly on the
NAO. Martin Visbeck 9 May, 2018NAO and Energy in Norway
/Norway experience cold
winters during a negative NAO
phase.
/Heating Oil consumption in
Norway varies by 30% in good
(anti) correlation with the
NAO.
/Correlation with precipitation
results in variability in
hydropower generation.
Martin Visbeck 9 May, 2018The North Atlantic Oscillation
Lugano T medie
Davos T medie
Milano T medie
Basel T medie
Temp medie*.KG
20
15
10
5
0
1860 1880 1900 1920 1940 1960 1980 2000The North Atlantic Oscillation
NAO Milano T medie
NaoTemp
2 15.5
1.5 15
1 14.5
0.5 14
0 13.5
-0.5 13
-1 12.5
-1.5 12
1900 1920 1940 1960 1980 2000You can also read
