Interannual variability of the West African Monsoon has been statistically and physically linked for a long time to the occurrence and intensity of particular SST anomaly (SSTA) patterns such as the cross-equatorial SST anomaly gradient in the Atlantic or ENSO variability. Diagnostic analyses show however that this teleconnexion is unstable in time over the last 50-years.

So two questions arise:

1- What could explain the unstability of the teleconnections?

2- How do the aforementioned SST variability modes modulate their respective impacts over West Africa?

These topics are documented mainly in the divergent circulation frame since it allows to unify a global view of the modification of the E-W and N-S over-turnings during an ENSO event and a more regionally anchored view of the West African Monsoon as a meridional overturning driven by low level boundary (thermal and moist) conditions. A series of 16 numerical experiments have been performed with the AGCM Arpege-Climat (T42, 11 levels in the troposphere) in August, the peak of the rainy season in the Sahel. The model has been forced with specific SSTA based on EOF decomposition in order to better document the respective Atlantic, Pacific and Indian SST forcings.

The results confirm previous analyses but show that:

1- The reversal of the Atlantic SSTA mainly impacts in the Atlantic the strength of the southern Hadley cell, hence ITCZ location.

2- The ENSO-related SST variability mainly affects the central part of the Atlantic region through an atmospheric bridge.

3- The recent long term warming in the Indian Ocean modulates 1 & 2 by forcing east-west anomalous circulations and modifying the subsidence location in the Atlantic region shifting it towards the West African region.