General Circulationof the Atmosphere

René Garreaudwww.dgf.uchile.cl/rene

Low latitude areas receive more solar energy than high latitudes (because of earth sphericity). Low latitudes are also warmer, so they emit more infrared energy back to space (OLR ~ T4), but this effect doesn’t compensate excess of insolation. There is an radiative imbalance between low and high latitudes, that would produce an increase (decrease) of temperature at low (high) latitudes.

Actually, thermal structure of the planet is nearly in steady state. There must be a net transport of energy to compensate the radiative imbalance.

General circulation of the Atmosphere

Simple solution: Direct thermal cell

F

F

C

C

t

t+t

General circulation of the Atmosphere

= 0 = 2/24 hrs-1 = 0 /100

General circulation in an aqua-planetPerpetual Equinox

Surface winds

General circulation in an aqua-planetPerpetual Equinox

0°

45°

NE trades

SE trades

ITCZ: IntertropicalConvergence Zone

Belt of higher pressure

Tro

pica

l Tro

popa

use

(15

km)

Belt of lower pressure

Surface westerlies

Surface wind (arrows)Precipitation (green shadow)

0°

60°

General circulation in an aqua-planetPerpetual Equinox

Jet stream (westerly flow)aloft (10-12 km): long term mean.Boundary between subtropicaland extratropical air masses

ITCZ

Midlatitude precipitation maximum and westerly belt

General circulation in an aqua-planetPerpetual Equinox

Daily view of the jet stream:Highly unstable

Z300: Meanders of the jet associated with developmentof troughs and ridges aloft

General circulation in an aqua-planetPerpetual March or September

H LH

Trough aloft → surface low Ridge aloft → surface high

Daily Z300 (colors)SLP (contours)

Precipitation area (blue lines)

Acomplish poleward transportof heat and westerly momentum

Produce midlatitude weather& precipitation

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Schematics of poleward heat transport by transient eddies in midlatitudes

General circulation in an aqua-planetPerpetual March or September

Real world general circulationContinents and seasonality

Nice…we still have ITCZ, jet streams, midlatitude precipitation maximum, but with considerable zonal asimetry

Real world general circulationContinents and seasonality

Ocean energy transport~0.3 total transport

Real world general circulationContinents and seasonality

Real world general circulationContinents and seasonality

Jan

WB

J

SAPF

In austral summer, weak convection takes place in the SH low latitudes. There is moderate subsidence over the SH subtropics driving a weak jet stream at about 45°S. The upper-level jet stream is, however, over the SAPF producing a rapid growth of the eddies that force strong westerlies near the surface.

Real world general circulationContinents and seasonality

JulJSP

JST

WB

SAPFIn austral winter, strong convection takes place in the NH low latitudes, fostering strong subsidence over the SH subtropics and driving an intense jet stream at about 30°S. The subpolar jet stream and westerly belt at about 45°S tends to weaken but they are still there because of the baroclinicity around the SAPF.

Real world general circulationContinents and seasonality

* NH bias in east Pacific ITCZ position* Higher tropical precipitation in July compared with January (SST)* South American monsoon (austral summer precipitation)* Subtropical anticyclone stronger in winter.

Long term mean rainfall (colors) & surface winds (arrows)

Real world general circulationContinents and seasonality

Jan July

(Surface) Westerly belt is more continuous and stronger in austral summer than in winter. Stronger westerlies at surface not always under strong westerlies aloft....

Jan Jul

Real world general circulationSurface wind (contours) & 300 hPa wind (colors)

JST

JST+SP

Wes

terli

es

Jan Jul

Real world general circulationSurface wind (colors) & Sea Surface Temperature (contours)

Note that maximum westerlies tend to coincide withSST maximum gradient (APFZ). Implication for ice ages…

JST

JST+SP

StormTrack

StormTrack

Real world general circulationSurface wind (contours) & 300 hPa wind (colors)

Jan Jul

Storm track: band of preferred displacement of transientdisturbances. Usually quantified as 2(Z250).In principle, eddy growth rate [T]/y ~ U300, but not always the storm track is under stronger westerlies

Real world general circulationSurface wind (contours) & precipitation (colors)

Jan Jul

Midlatitude precipitation tends to coincide with max. Usfc and storm track, but also depends of other factors, including tropical connection.

30 day animation of 300 hPa zonal wind speed (shaded, 25 and 50 m/s) and 925 hPa relative vorticity (contours, -3 and -6 10-5 s-1). Time resolution is 6 hours

Upper tropospheric jet stream and surface depresions

Corredores de tormentas(Varianza de vorticidad, viento zonal, precipitación)

Anual

Junio

Enero