Physical processes associated with the tropical Atlantic SST meridional gradient

The major modes of seasonal sea surface temperature (SST) meridional gradient and their connection with some regional mean SST indices in the Atlantic Ocean are examined using reanalysis data. The focus of the work is on the evolution of the dominant mode of the meridional SST gradient in boreal spring and the associated physical processes. The spatial distribution of the dominant mode in boreal spring is a seesaw pattern, reflecting the opposite variation of the meridional SST gradient between the subtropical and tropical North Atlantic, which resulted from a coherent warming or cooling with maxima along 10 degrees-15 degrees N. It is confirmed that this mode is dominated by the wind-evaporation-SST feedback. The feedback persists a longer time in the western Atlantic than in the eastern. The contribution to the SST variation is mainly from latent heat flux. The surface longwave and shortwave cloud radiative forcings are mainly determined by low cloud cover variations. The authors also found that the thermodynamic mode that peaked in boreal spring becomes weak in the following boreal summer. A similar thermodynamic mode appears in a northward position in boreal autumn, and its life cycle is shorter than the one in boreal spring. In contrast to the leading mode in boreal spring, it is shown that the leading mode in boreal summer is a dynamical air-sea feedback mode, reflecting a coherent warming or cooling pattern extending from the Angolan coast toward the equator in the Gulf of Guinea. The thermodynamic processes act as a negative feedback. The net surface latent heat flux anomalies are the leading damping factor, while the sensible heat flux plays the same role on a smaller scale.