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

3.2 Ocean circulations Surface 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

In regions next to the equator (< 150 km), t rades 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. 100 m 40 cm WestEast z 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) 200 m 3.2 Ocean circulations Subsurface current Which is the origin of equatorial undercurrent btw 100/200 m ? SEC Equatorial undercurrent

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. 100 m 40 cm WestEast z 200 m 3.2 Ocean circulations Subsurface current Which is the origin of equatorial undercurrent btw 100/200 m ? SEC Equatorial undercurrent 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)

Which is the origin of the Equatorial upwelling ? 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. 3.2 Ocean circulation Ekman divergence and equatorial upwelling E E Source : Météo-France (F.Beucher)

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. 3.2 Ocean circulations Upwellings and SST Monthly mean of Sea surface température Source : RéAnalyse NCEP

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 3.2 Ocean circulations Ocean-atmosphere coupling : upwelling areas Annual Précipitations (m.). Sources : Dorman et Bourke (79,81), Dorman (82), Baumgartnet et Reichel (75)

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 ? 3.2 Ocean circulations Surface current Courants océaniques de surface (moyenne annuelle). Source : d’après White et al., 1992

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 z 15°N 2°N 3.2 Ocean circulations Surface current Location of the North ECC : Source : Pond et Pickard (83)

sommaire chap.3 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) ECC 3.2 Ocean circulations Surface current : ECC Sea Surface Température in August. Source : RéAnalyse NCEP

ECC sommaire chap.3 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. 3.2 Ocean circulations Surface current : ECC Sea Surface Température in August. Source : RéAnalyse NCEP

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 3.2 Ocean circulations Ekman convergence and downwelling sommaire chap.3 Source : Météo-France (F.Beucher)

chap 3.3 : ZCIT sommaire chap Circulations océaniques Ocean-atmosphere coupling : zone de TSM maxi 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 10°N Annual Précipitations (m.). Sources : Dorman et Bourke (79,81), Dorman (82), Baumgartnet et Reichel (75)

References - 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, Dorman, C., E., Bourke, R.,R., H., 1981 :’Precipitation over the Atlantic Ocean’, 30°N to 30°S. Mon. Wea. Rev., 109, 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