Chap. 3 Regional climates in tropics

1. Chap. 3Regional 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

2. Cgx>0 3.1 Regional climates MSLP : June /July/August Source : RéAnalyses du CEP 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) :

3. 3.1 Regional climates Source : Atlas Bordas, 1985, p. 5. Surface circulations of the tropical atmosphere in july : • 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’

4. 3.1 Regional climates 200 hPa H The tropical atmosphere in july : wind and streamline (a) 200 hpa (b) 850 hPa. Source : Atkinson et Sadler, 1970 TEJ STJ 850 hPa 7 to 11°N • 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

5. 3.1Regional climates 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

6. 3.1 Regional climates 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)

7. 3.1 Regional climates Source : Atlas Bordas, 1985, p. 5. Surface circulations of the tropical atmosphere in january : • 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’

8. 3.1 Regional climates 200 hPa STJ The tropical atmosphere in january : wind (kt) and streamline (a) 200 hpa (b) 850 hPa Source : Atkinson and Sadler, 1970 H TEJ 850 hPa • 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) Chap 3.4.1

9. 3.1 Regional climates STJ in the winter hemsiphere Wind >50kt at 200 hPa – 05/03/2005. ARP 1.5 Source : Météo-France

10. 3.1 Regional climates 200 hPa STJ The tropical atmosphere in january : wind (kt) and streamline (a) 200 hpa (b) 850 hPa Source : Atkinson and Sadler, 1970 H TEJ 850 hPa • Upper tropo • ‣ High pressure (H) over W. Pacific and TEJ • ‣ Subtropical Jet (STJ) maximum at 30°N • ‣ Westerlies (40kt) crossing Equatorial Central Pacific

11. This WNW canal duct favours the equatorward propagation of the troughs of the STJ, which modulate the activity of the ITCZ over Central Pacific. 3.1 Regional climates Westerly canal duxt in upper troposphere over Central Equatorial Pacific during The northern winter. Vent >30kt at 200 hPa –09/03/2005. Source : Météo-France equator

12. 200 hPa STJ H TEJ 850 hPa 3.1 Regional climates 200 hPa The tropical atmosphere in january : wind (kt) and streamline (a) 200 hpa (b) 850 hPa. Source : Atkinson and Sadler, 1970 5°N • 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

13. 3.1 Regional climates Surface temperature and SST in january (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

14. 3.1 Regional climates 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) Chap 3.2

15. 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 References