Chap. 3 Regional climates in tropics

3.1 Regional climates

3.2 Ocean circulations

3.3 Structure of the InterTropical Convergence Zone (ITCZ)

3.4 Monsoon circulations and associated jets

sommaire

3.1 Regional climates

The tropical atmosphere is nearly barotropic = slack horizontal gradient of MSLP (mean sea level pressure) and

temperature

• Origin of subtropical highs in winter hemisphere : subsiding branch of winter Hadley cell

• But which origin for summer subtropical highs knowing that the summer Hadley cell is very weak ? : Hypothesis formulated by Chen and al. (2001) : the thermal forcing ( ) over asian monsoon initiate stationnary planetary Rossby wave. Their zonal group velocity (cgx) is eastwards which induce the formation of subtropical highs out of mid-oceans (Hawaii and Azores) : 0

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2

lk

kucg x

Cgx>0

MSLP : June /July/August Source : RéAnalyses du CEP

Surface circulations of the tropical Surface circulations of the tropical atmosphere in july :atmosphere in july :

3.1 Regional climates

• Southern subtropical highs : Easter Island, St Helen, Mascareign • Northen subtropical highs : Hawaii, Azores• Max. sea level pressure on eastern flank of highs : subsiding branch of Walker cell + local interaction ocean-air (coastal upwelling)

• 3 ascending branches of Walker cell located at ‘heat low’

Source : Atlas

Bordas, 1985, p. 5.

The tropical atmosphere in july : wind and streamline (a) 200 hpa (b) 850 hPa. The tropical atmosphere in july : wind and streamline (a) 200 hpa (b) 850 hPa. Source : Atkinson et Sadler, 1970

3.1 Regional climates

•Upper tropo ‣ High pressure over Tibetan Plateau originated by thermal forcing initiate easterlies called ‘Tropical Easterly Jet’ or TEJ.

‣ Subtropical Jet (STJ) is maximum in the winter hemisphere (100 kt at 30°S) because the winter Haldey cell is 10 times more developped than summer cell.

• Low tropo ‣ trades winds over Atlantic and Eastern Pacific; the confluence axis is located about 7 to 11°N ‣ summer monsoon flow over North Indian Ocean,

Southern China and Western Africa

H

TEJSTJ

200 hPa

850 hPa7 to 11°N

Surface temperature and SST in july.Surface temperature and SST in july. Source : RéAnalyse NCEP 68-90

- Max. located next to tropics over continents : at 25°N over Sahara, at 30°N over Tibetan Plateau

- Max. located closer to the equator over oceans, especially over their eastern flanks : ‣ 10°N over Atlantic and Eastern Pacific ‣ 20°N over Indian Ocean and 25°N over Western Pacific

3.1 Regional climates

Monthly rainfall in july (source : ERA40)Monthly rainfall in july (source : ERA40)

Hatched over 300 mm/month

- Max. collocated with ITCZ, at 10°N over Atlantic and Eastern Pacific

- Max. located over monsoon area : Western Africa (7°N/15°N) and over Asian/Indian Monsoon (10°N/30°N)

3.1 Regional climates

Surface circulations of the tropical Surface circulations of the tropical atmosphere in january :atmosphere in january :

3.1 Regional climates

• Max. sea level pressure on eastern flank of highs : subsiding branch of Walker cell + local interaction ocean-air (coastal upwelling)

• 3 ascending branches of Walker cell located at ‘heat low’

Source : Atlas

Bordas, 1985, p. 5.

The tropical atmosphere in january : wind (kt) and streamline (a) 200 hpa (b) 850 hPaThe tropical atmosphere in january : wind (kt) and streamline (a) 200 hpa (b) 850 hPaSource : Atkinson and Sadler, 1970Source : Atkinson and Sadler, 1970

3.1 Regional climates

Chap 3.4.1

• Upper tropo ‣ High pressure (H) over W. Pacific originated by

thermal forcing initiate easterlies (Tropical Easterly Jet : TEJ ) btw 0-10°S extending from Central Pacific (dateline) across the Indian Ocean into Eastern Africa (strong geostrophic wind having regarding to pressure gradient because f is nearly 0)

‣ Subtropical Jet (STJ) is maximum in the winter hemisphere (130 kt at 30°N)

200 hPa

850 hPa

HTEJ

STJ

STJ in the winter hemsiphereWind >50kt at 200 hPa – 05/03/2005. ARP 1.5

Source : Météo-France

3.1 Regional climates

The tropical atmosphere in january : wind (kt) and streamline (a) 200 hpa (b) 850 hPaThe tropical atmosphere in january : wind (kt) and streamline (a) 200 hpa (b) 850 hPaSource : Atkinson and Sadler, 1970Source : Atkinson and Sadler, 1970

3.1 Regional climates

• Upper tropo ‣ High pressure (H) over W. Pacific and TEJ

‣ Subtropical Jet (STJ) maximum at 30°N ‣ Westerlies (40kt) crossing Equatorial Central Pacific

200 hPa

850 hPa

HTEJ

STJ

Westerly canal duxt in upper troposphere over Central Equatorial Pacific during The northern winter.

This WNW canal duct favours the equatorwardpropagation of the troughs of the STJ, which modulatethe activity of the ITCZ over Central Pacific.

equator

3.1 Regional climates

Vent >30kt at 200 hPa –09/03/2005. Source : Météo-France

200 hPa

850 hPa

HTEJ

STJ

The tropical atmosphere in january : wind (kt) and streamline (a) 200 hpa The tropical atmosphere in january : wind (kt) and streamline (a) 200 hpa (b) 850 hPa. (b) 850 hPa. Source : Atkinson and Sadler, 1970Source : Atkinson and Sadler, 1970

3.1 Regional climates

200 hPa

• Low tropo ‣ Trades winds over Atlantic and Eastern Pacific; the confluence axis is located btw equator and 5°N

‣ Winter monsoon flow over North Indian Ocean, Southern China, Northen Africa (Harmattan)

‣ Summer monsoon flow over Eastern Africa and Madagasikara, Northern Australia and Indonesia

5°N

Surface temperature and SST in january Surface temperature and SST in january

(source : ERA40)(source : ERA40)

Hatched over 26°C

- Max. located next to tropics over continent : at 25°N over Southern Africa and America, Australia

- Max. located nearer equator, especially over eastern oceans : at 2/3°N over Atlantic and Eastern Pacific but at 10°S over Indian Ocean and btw 10°N/15°S over W. Pacific

3.1 Regional climates

Monthly rainfall in january (source : ERA40)Monthly rainfall in january (source : ERA40)

Hatched over 300 mm/month

- Max. collocated with ITCZ, at 2/3°N over Atlantic, E. Pacific , at 10°S over Indian Ocean

- Max. located over monsoon Indonesia (5°S/12°S), over the South Pacific Convergence Zone (SPCZ : 5°S/18°S), Amazonia (eq./25°S), Southern Africa (eq./18°S)

3.1 Regional climates

Chap 3.2

References

- Atlas Bordas historique et géographique, 1985. Editeur Hözel à Vienne

- Atkinson , G.D., Sadler, J.C., 1970 : Mean Cloudiness and gradient level wind charts over the tropic. USAF Air Weather Service, Technical Report N°215, vol.1 text, vol.2 charts.

- Atkinson , G.D., 1971 :’Forecaster’s guide to tropical meteorology. USAF, Air Weather Service, Technical Report N°240, 341pp.

- Chen, P., M. P. Hoerling, and R. M. Dole, 2001 : The origin of the subtropical anticyclones. J. Atmos. Sci., vol.37, pp. 912-913