General Circulation of the Atmosphere. René Garreaud

Transcription

1 General Circulation of the Atmosphere René Garreaud

2 General circulation of the Atmosphere 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 ~ T 4 ), 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.

3 General circulation of the Atmosphere Simple solution: Direct thermal cell t F C t+!t F C

4 General circulation in an aqua-planet Perpetual Equinox Surface winds " = " 0 /100 " = " 0 = 2#/24 hrs -1

5 General circulation in an aqua-planet Perpetual Equinox Tropical Tropopause (15 km) Belt of lower pressure NE trades SE trades ITCZ: Intertropical Convergence Zone 0 Belt of higher pressure Surface westerlies 45 Surface wind (arrows) Precipitation (green shadow)

6 General circulation in an aqua-planet Perpetual Equinox ITCZ 0 Midlatitude precipitation maximum and westerly belt 60 Jet stream (westerly flow) aloft (10-12 km): long term mean. Boundary between subtropical and extratropical air masses

7 General circulation in an aqua-planet Perpetual Equinox Daily view of the jet stream: Highly unstable Z300: Meanders of the jet associated with development of troughs and ridges aloft

8 General circulation in an aqua-planet Perpetual March or September Daily Z300 (colors) SLP (contours) Precipitation area (blue lines) H L H Trough aloft! surface low Ridge aloft! surface high { Acomplish poleward transport of heat and westerly momentum Produce midlatitude weather& precipitation

9 General circulation in an aqua-planet Perpetual March or September Schematics of poleward heat transport by transient eddies in midlatitudes

10 Real world general circulation Continents and seasonality Nice we still have ITCZ, jet streams, midlatitude precipitation maximum, but with considerable zonal asimetry

11 Real world general circulation Continents and seasonality

12 Real world general circulation Continents and seasonality Ocean energy transport ~0.3 total transport

13 Real world general circulation Continents and seasonality

14 Real world general circulation Continents and seasonality Jan J WB 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.

15 Real world general circulation Continents and seasonality Jul J SP JST WB SAPF In 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.

16 Real world general circulation Continents and seasonality Long term mean rainfall (colors) & surface winds (arrows) Jan July * 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.

17 Real world general circulation Surface wind (contours) & 300 hpa wind (colors) Jan Jul Westerlies J ST J ST+SP (Surface) Westerly belt is more continuous and stronger in austral summer than in winter. Stronger westerlies at surface not always under strong westerlies aloft...

18 Real world general circulation Surface wind (colors) & Sea Surface Temperature (contours) Jan Jul Note that maximum westerlies tend to coincide with SST maximum gradient (APFZ). Implication for ice ages

19 Real world general circulation Surface wind (contours) & 300 hpa wind (colors) Jan Jul Storm Track Storm Track J ST J ST+SP Storm track: band of preferred displacement of transient disturbances. Usually quantified as $ 2 (Z250). In principle, eddy growth rate % &[T]/&y ~ U 300, but not always the storm track is under stronger westerlies

20 Real world general circulation Surface wind (contours) & precipitation (colors) Jan Jul Midlatitude precipitation tends to coincide with max. U sfc and storm track, but also depends of other factors, including tropical connection.

21 Upper tropospheric jet stream and surface depresions 30 day animation of 300 hpa zonal wind speed (shaded, 25 and 50 m/s) and 925 hpa relative vorticity (contours, -3 and s -1 ). Time resolution is 6 hours

22 Corredores de tormentas (Varianza de vorticidad, viento zonal, precipitación) Anual Junio Enero