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. 3.2 Ocean circulationsSurface current Reminder : wind stress is generated by the effect of the wind on the surface ocean Which is the origin of the South Equatorial Current (SEC) and the North Equatorial Current (NEC) ? ⇨ these surface current are driven by the wind stress Courants océaniques de surface (moyenne annuelle). Source : d’après White et al., 1992

3. Equatorial undercurrent 3.2 Ocean circulationsSubsurface current Which is the origin of equatorial undercurrent btw 100/200 m ? 40 cm SEC 100 m z 200 m West East Circulations along the Equatorial Pacific Green arrows above sea-level ocean show the wind stress Arrows in the ocean show oceanic circulations Source : Météo-France (F.Beucher) • In regions next to the equator (< 150 km), trades winds pile up • water on the western Pacific, and thus give rise to eastwards • pressure force (next to equator, no influence of the Coriolis Force), • which in turns drives a subsurface current between 100 and 200 m • deep, called ‘Cromwell current’ or Equatorial undercurrent.

4. Equatorial undercurrent 3.2 Ocean circulationsSubsurface current Which is the origin of equatorial undercurrent btw 100/200 m ? 40 cm SEC 100 m z 200 m West East Circulations along the Equatorial Pacific Green arrows above sea-level ocean show the wind stress Arrows in the ocean show oceanic circulations Source : Météo-France (F.Beucher) • The surface and subsurface currents explain why downwelling • occur on the western flank of Pacific (high SST) and upwelling on • the eastern flank (low SST). • For Atlantic Ocean : same circulation.

5. E E 3.2 Ocean circulationEkman divergence andequatorial upwelling Which is the origin of the Equatorial upwelling ? Source : Météo-France (F.Beucher) • The oceanic Ekman mass transport, E, is directed at right angles to the right (left) of τ in the northern (southern) hemisphere. The magnitude of E is proportional to the strenght of τ. • Following this rule, at the equator, E is directed away • from the equator producing divergence and upwelling • along the equator.

6. 3.2 Ocean circulationsUpwellings and SST Monthly mean of Sea surface température Source : RéAnalyse NCEP 1981-2002 • The equatorial upwelling and • the coastal upwelling are pronounced • in the sectors of Eastern Pacific and Eastern Atlantic, • which explains that cold tongues of SST occur • in these area.

7. 3.2 Ocean circulationsOcean-atmosphere coupling : upwelling areas Annual Précipitations (m.). Sources : Dorman et Bourke (79,81), Dorman (82), Baumgartnet et Reichel (75) • As atmosphere-ocean coupling plays an important role in tropics (latent heat and sensible fluxes are linked with the SST) shallow convection (St/Sc or shallow Cu) and rare rain ( ) occur in upwelling areas : along the equator + E. Pacific + E. Atlantic

8. 3.2 Ocean circulationsSurface current Which is the origin of the north Equatorial Counter Current (ECC) located at the surface and between a latitud band of 4°N-10°N ? Courants océaniques de surface (moyenne annuelle). Source : d’après White et al., 1992

9. 15°N 2°N 3.2 Ocean circulationsSurface current Location of the North ECC : z Source : Pond et Pickard (83) • The minimum of trades winds occur between 2°N and • 15°N in phase with the location of the North ECC. • The piling-up of water on the western side of ocean is • backing through the North ECC in surface and through • the Equatorial undercurrent in subsurface

10. ECC 3.2 Ocean circulationsSurface current : ECC Sea Surface Température in August. Source : RéAnalyse NCEP 1981-2002 • Since the north Equatorial Counter Current (ECC) is directed eastwards, warm waters are also advected from the Western Pacific towards the Eastern Pacific • ⇨ we observe within the latitud band of ECC (2°N-15°N) a zone of TSM maximale (>= 28°C). • By coupling ocean-atmosphere, deep convection ( ) is enhanced within this latitud band = mean location of the InterTropical Convergence Zone (ITCZ) sommaire chap.3

11. ECC 3.2 Ocean circulationsSurface current : ECC Sea Surface Température in August. Source : RéAnalyse NCEP 1981-2002 • We can also explain the fast increasing os SST (+2 à 3 °C) just northwards the equator by the process of downwelling situated at 4-5°N. sommaire chap.3

12. 3.2 Ocean circulations Ekman convergence anddownwelling Source : Météo-France (F.Beucher) • We remind that the Ekman transport E is proportional to the intensity of the wind stress τ. • Since the southeasterlies decrease while they approach the ITCZ, • the Ekman transport decrease too : • ⇨ we observe a strong convergence of Ekman towards 4°N • ⇨ producing downwelling and fast increasing of SST sommaire chap.3

13. As the ocean-atmosphere coupling plays an important role under tropics (flux of latent heat and sensible heat are linked to SST), we observe heavy rains over areas of SST maximum (>28°C) • Under annual mean, the ITCZ ( ) is located between 5°N-10°N over Central Pacific – Eastern Pacific - Atlantic 3.2 Circulations océaniquesOcean-atmosphere coupling : zone de TSM maxi Annual Précipitations (m.). 10°N Sources : Dorman et Bourke (79,81), Dorman (82), Baumgartnet et Reichel (75) sommaire chap.3 chap 3.3 : ZCIT

14. - Baumgartner, A., Reichel, E., 1975 : The World water balance. Elsevier, Amsterdam, Oxford, New York, 179 pp. • Dorman, C. E. , 1982 :4Indian Ocean Rainfall’. Tropical Ocean-Atmosphere Newsletter,10,4. • Dorman, C., E., Bourke, R.,R., H., 1979 :’Precipitation over the Pacific Ocean’, 30°N to 30°S. Mon. Wea. Rev., 107, 896-910 • Dorman, C., E., Bourke, R.,R., H., 1981 :’Precipitation over the Atlantic Ocean’, 30°N to 30°S. Mon. Wea. Rev., 109, 554-563 • Pond, S., PIckard, G. L., 1983 : Introduction dynamic oceanography. Second Edition, Pergamon Press, Oxford, New York, Toronto, Sidney, Paris, Frankfurt, 329 pp. • White et Warren B., 1992 :’Reflection of interannual Rossby waves at the maritime western boundary of the tropical Pacific’; Journal of Geophysical Research, Washington, DC, vol.97, n°C9, pp. 14305-14322. References