Seasonality of the ventilation of the tropical Atlantic thermocline in an ocean general circulation model

This paper presents an analysis of subduction and advection within the shallow subtropical-tropical connections (STC) of the Atlantic Ocean. We carried out a kinematic study of the seasonal cycle of subduction, entrainment, and subsurface circulation in an upper Atlantic Ocean general circulation model (GCM), with a particular emphasis on the southern STC that is less studied but more important for the water supply of the equatorial upwelling. It is found that the southern STC, like the northern, has a typical water mass formation rate of 25-50 m/year, depending on the wind product used. This formation lasts 1 to 2 months in the south (longer in the northern tropics) and starts from early winter to late spring, in contrast to the common view of the end of March start. Such a large span of starting time is explained by a key control mechanism of the wind through the spatial distribution and seasonal variations of its speed. Each of these results stresses the importance of the variability as well as the uncertainties of the wind forcing for understanding the sustenance and variability of the characteristics of the tropical and equatorial thermocline. The slow currents within the thermocline branches of the STC may play a role in setting an interannual to decadal timescale for tropical or equatorial SST variability; we estimate an Atlantic STC timescale associated with the main advective pathways that at maximum reaches 5 years.