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Coupling between the Atlantic Cold Tongue and the West African Monsoon in boreal spring and summer

Coupling between the Atlantic Cold Tongue and the West African Monsoon in boreal spring and summer. G. Caniaux 1 , H. Giordani 1 , J.L. Redelsperger 1 , F. Guichard 1 , E. Key 2 and M. Wade 3 1. CNRM/GAME (Météo-France/CNRS), Toulouse, France

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Coupling between the Atlantic Cold Tongue and the West African Monsoon in boreal spring and summer

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  1. Coupling between the Atlantic Cold Tongue and the West African Monsoon in boreal spring and summer G. Caniaux1, H. Giordani1, J.L. Redelsperger1, F. Guichard1, E. Key2 and M. Wade3 1.CNRM/GAME (Météo-France/CNRS), Toulouse, France 2. LDEO (Colombia University), Palissade, N.Y., U.S.A. 3. LPAOSF/UCAD, Dakar, Sénégal

  2. Mean Surf. 1982-2008 5.9°C 26.0°C 4 months The Atlantic Cold Tongue (ACT) Reynolds SSTs at 1.5°S 0.5°W 1982-2008 • Looks like a northward extension of the S.H. cold waters, in the eastern equatorial Atlantic, south of the equator • The cold anomaly occurs between June and November • Large cooling observed between April and August

  3. 1992 1982 2005 2000 0.61±0.12°C 1.37±0.31°C 1987 1988 1984 ACT Properties Temperature index Reynolds SSTs 1982-2008 Domain sampled: 30°W-15°E et 5°S-5°N • Presence every year; begining in May, max. end July, end in December • Cooling faster than warming (May-July / August-November) • Strong interannual variability (1982 three times colder than 1984 and 1988)

  4. 1995 1988 1996 1998 2.4±5x106km² 11 June ± 12 d 1992 1997 1983 2005 ACT Properties Surface Date of formation: SCT >0.4x106 km² • Max. surface : nearly one quarter the surface of Sahara • Begining between May, 19th and July, 4th (= 46 days between the earliest year, 2005 and the latest year, 1995) • No long term trend in dates of formation

  5. 1 2 3 4 Zonal wind stress Meridional wind stress Wind stress divergence Wind stress curl ACT Formation Equations in an homogeneous frictional surface layer, on a beta-plan centered on the equator (Zebiak and Cane, 1987) Ekman pumping

  6. Total pumping f(Tx) i(Curl) h(Div) g(Ty) ACT Formation Hovmøller 10°W-4°E ECMWF wind stress 1998-2007 • Pumping over 3°S-3°N • >>0 pumping confined 3°S-0°N explaining the form of the ACT • S the equator, all terms contribute to the pumping • Leading term: meridional wind stress • Strengthening in April (curl + meridian wind stress) • Pumping lasts till September even if f(Tx)<0

  7. Reynolds SSTs 1998-2007 A B C ECMWF Winds 1998-2007 ~2 months 1/2 Influence of the ACT on the Atmosphere • In B, Ssts cool as soon as winds strengthen at 3°S • In B, cooling increases in May-June • Sharp SST gradients between A and B • SST gradients relax in August-September • S.H. winds increase and reach the N.H., never the contrary • As soon as a SST gradient threshold is reached, winds: (1) weaken S of the equator; (2) strengthen N of the equator up to the continent in July-August

  8. SST Gradients 0.5°N Heat Flux Gradients 0.5°N Winds 2°N – 2°S Influence of the ACT on the Atmosphere • Differential cooling generates SST and net heat flux gradients from May to September in the band 2°S-2°N • Max. SST gradients and min. net heat flux gradients do not concide due to differential solar heat fluxes • N of the equator, winds increase as soon as N.H.F. gradients increase SST Meridional Gradients SST Gradients 1998-2007 Net Heat Flux Meridional Gradients

  9. Perturbations Generated by a Sea Surface Heat Flux Discontinuity • Formation of a density front • Upstream wind weakening and upwelling; downstream wind strengthening and subsidence • Northward migration of convection • Upwelling thickens mixed layer heigths • Accelaration in the mixed layer N of the equator and convection inhibited -80 W/m² 0 W/m² Density and winds Surface heat fluxes W<0 W>0 Vertical velocities Mixed layer heights

  10. June 27±9 June 11±12 Influence of the ACT on the Atmosphere ACT index: Date at which the CT surface exceeds 0.4x106 km² WAM onset index: Filtered rain (CMAP, GPCP); excess of rain North of 7.5°N / South (Fontaine and Louvet, 2006) Spearman rank correlation coef.

  11. Conclusions • ACT formation • The ACT develops with the strengthening of the south hemispheric winds • It forms south of the equator, because of southeasterlies in the Gulf of Guinea (and weak mld) • Differential cooling across the equator generate SST gradients and surface heat flux gradients 2. the ACT modifies the atmospheric circulation • When sea surface heat flux gradients are strong enough, winds weaken S of the equator and • Winds strengthen N of the equator • Wind strengthening contributes to push atmospheric convection northward

  12. Conclusions • Importance of the Santa Helena Anticylone and southeasterlies of the Southern Hemisphere • Coupled mechanism in two distinct stages, limited in time (May-June) • Strong ACT/WAM interaction: • strong and early southeasterlies, shallow MLD • early ACT set up • strong atmospheric convection (cloud cover) over 0°N-4°N • early and intense cross-equatorial heat flux gradients • wind strengthening untill waters N of the equator cool and weakens the heat flux gradients See Caniaux et al., JGR Oceans, 2011, 116, C04003, doi:10.1029/2010JC006570

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