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

The Atlantic Cold Tongue (ACT)

Reynolds SSTs at 1.5°S 0.5°W1982-2008

1. Looks like a northward extension of the S.H. cold waters, in the eastern equatorial Atlantic, south of the equator

2. The cold anomaly occurs between June and November 3. Large cooling observed between April and August

5.9°C

4 months

26.0°C

Mean Surf.1982-2008

ACT PropertiesTemperature index

)(

)(

)).(25(

)).(25()).(25(

xA

xACT

dAxSSTC

dAxSSTCxSSTC

T

Reynolds SSTs 1982-2008Domain sampled:

30°W-15°E et 5°S-5°N

1. Presence every year; begining in May, max. end July, end in December2. Cooling faster than warming (May-July / August-November) 3. Strong interannual variability (1982 three times colder than 1984 and 1988)

1982

1984

1988

0.61±0.12°C

19922005

1987

2000

1.37±0.31°C

Surface

1. Max. surface : nearly one quarter the surface of Sahara2. Begining between May, 19th and July, 4th (= 46 days between the earliest

year, 2005 and the latest year, 1995)3. No long term trend in dates of formation

ACT Properties

Date of formation: SCT >0.4x106 km²

)(

)).(25(xA

CT dAxSSTCS

2005

1992

19961988

11 June ± 12 d

1983 1997

1995

1998 2.4±5x106km²

Equations in an homogeneous frictional surface layer, on a beta-plan centered on the equator (Zebiak and Cane, 1987)

Hrvyu

Hyvru

yss

xss

0

0

ACT Formation

Ekman pumping

yr

yr

yr

yr

ry

yrHw yx .

²²²

²2

²²²

²)²²(

²)²²(

1)(

0

1 2 43

Zonal wind stress

Meridional wind stress

Wind stress divergence

Wind stress curl

Total pumping

1. Pumping over 3°S-3°N2. >>0 pumping

confined 3°S-0°N explaining the form of the ACT

3. S the equator, all terms contribute to the pumping

4. Leading term: meridional wind stress

5. Strengthening in April (curl + meridian wind stress)

6. Pumping lasts till September even if f(Tx)<0

ACT Formation

f(Tx) g(Ty)

h(Div) i(Curl)

Hovmøller 10°W-4°EECMWF wind stress

1998-2007

Influence of the ACT on the Atmosphere

1. In B, Ssts cool as soon as winds strengthen at 3°S

2. In B, cooling increases in May-June

3. Sharp SST gradients between A and B

4. SST gradients relax in August-September

1. S.H. winds increase and reach the N.H., never the contrary

2. 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

ECMWF Winds 1998-2007

Reynolds SSTs 1998-2007

A

B

C

~2 months 1/2

1. Differential cooling generates SST and net heat flux gradients from May to September in the band 2°S-2°N

2. Max. SST gradients and min. net heat flux gradients do not concide due to differential solar heat fluxes

3. N of the equator, winds increase as soon as N.H.F. gradients increase

Influence of the ACT on the Atmosphere

SST Gradients 1998-2007

Net Heat Flux Meridional GradientsWinds 2°N – 2°S

Heat Flux Gradients 0.5°N

SST Gradients 0.5°N

SST Meridional Gradients

Perturbations Generated by a Sea Surface Heat Flux Discontinuity

1. Formation of a density front

2. Upstream wind weakening and upwelling; downstream wind strengthening and subsidence

3. Northward migration of convection

4. Upwelling thickens mixed layer heigths

5. Accelaration in the mixed layer N of the equator and convection inhibited

Surface heat fluxes

-80 W/m²

0 W/m²Density and winds

Mixed layer heightsVertical velocities

W>0

W<0

ACT index: Date at which the CT surface exceeds 0.4x106 km²WAM onset index: Filtered rain (CMAP, GPCP); excess of rain North of 7.5°N / South

(Fontaine and Louvet, 2006)

Influence of the ACT on the Atmosphere

Fontaine and Louvet

(2006) CMAP

Fontaine and Louvet

(2006) GPCP

Fontaine and Louvet

(2006) GPCP

1988, 95, 96, 98

omitted

0.32 0.49 0.80

June 11±12

June 27±9

Spearman rank correlation coef.

Conclusions

1. 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

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