Numerical experiments to determine the influence of sea surface temperatures over tropical oceans on the seasonal distribution of precipitation in the equatorial Amazon

How the seasonal cycle of sea surface temperatures in the tropical Atlantic and Pacific influences the seasonality of the Amazon rainfall has been tested through a set of numerical experiments using the National Center for Atmospheric Research (NCAR) Community Climate Model Version (CCM3). The comparison between the results of the control run and observations suggests that the model qualitatively captures the seasonal patterns of rainfall in the equatorial Amazon, although it overestimates the amount and misses the time of onset and end of the wet season by one to two months. The experiments suggest that a) the seasonal pattern of precipitation in the eastern equatorial Amazon, where the influence of ENSO is observed, is very sensitive to the changes in SSTs, whereas the rainfall in the western Amazon is relatively insensitive to SSTs; b) When the seasonal cycle of Atlantic SSTs is removed, a wet season, rather than the observed dry season, appears in the eastern equatorial Amazon during austral spring. This is because the southward migration of warm SSTs and ITCZ in the tropical Atlantic Ocean lags that of the maximum insolation during austral spring. Subsidence over the eastern equatorial Amazon induced by the ITCZ centered near 10°N stabilizes and dries the air above the PBL and also reduces the low-level moisture convergence. This suppresses convection despite strong surface heating. The removal of the seasonal cycle in the tropical Pacific SSTs has a similar but weaker effect on the rainfall in the same region and season; c) A weakening of deep convection in the western Pacific and an associated change in the Walker circulation due to weaker SST gradients in the tropical Pacific can suppress the wet season in the eastern Amazon during austral fall and spring, despite the semi-annual maximum land surface solar heating. The changes in moisture transport from the Atlantic Ocean and Atlantic ITCZ associated with ENSO contribute more to the drought in the equatorial eastern Amazon (0°-5°S, 50°W-60°W) than those from the eastern Pacific, even thought Atlantic SSTs remain unchanged in the experiment. This mechanism may be useful in explaining the El Niño- related droughts in northeastern Brazil.