Role of SST in Triggering the Summer WAM

The Tropical Atlantic (TA) climate has great socio-economical impacts on the African countries bordering the Atlantic. The TA basin, however, is one of the less accurately represented areas in state-of-the-art climate models, so their predictions show large uncertainties and biases respect to the observations, especially in the simulation of the Seasonal Cycle (SC). A better representation of the role of ocean-land-atmosphere interactions in TA at seasonal timescales will likely lead to an enhancement of the climate models.

Along the equator, Sea Surface Temperature (SST), surface winds and deep convection exhibit a strong annual cycle, in stark contrast with the solar insolation showing a semiannual cycle in the top of the atmosphere. The SC of equatorial Atlantic SST is characterized by a rapid cooling from April to July. It is clear that the continental West African summer Monsoon (WAM) impact on ocean surface winds is the major cause of this cooling, but it remains in debate how the SST influence the monsoonal winds, and subsequently, the precipitation. Some studies point that annual cycle in equatorial Atlantic SST has little effect on equatorial winds, while others suggest that the interaction between the latter and the WAM, can play a major role on determining the SC.

In order to study the influence of equatorial SST SC on the atmosphere, we have run three different sensitivity experiments for the historical period 1982-2013 using the atmospheric model CAM4 in its finest resolution configuration (0.9x1.25) forced with different prescribed observed SSTs: a) climatological SST and b) time-independent SST.

Our results suggest that without the SC in SST, the monsoon starts earlier, the onset of the monsoon is missed and the ITCZ is constrained close to the equator, not showing the actual observed seasonal migration of the rainband moving northward during boreal summer. We have calculated a sort of statistical skill scores for further understanding of the difference between the experiments. Differences in precipitation are directly proportional to the difference in the prescribed SST patterns; seasons with cooler (warmer) SST exhibit less (more) precipitation in time-fixed SST run respect to climatological SST run, because the ocean-land temperature difference is reduced (enhanced).