Beyond their strong seasonal influence on monsoon circulation, rainfall, and air–sea fluxes of heat and carbon, SST variations also drive interannual rainfall changes in Africa and South America and are linked to Pacific variability, making their understanding a priority. PIRATA observations have been essential in identifying the processes behind SST variability, particularly the role of vertical turbulent mixing in generating the Atlantic Cold Tongue. Data from the 0°, 23°W mooring revealed a semiannual cycle of mixed-layer cooling linked to vertical shear (Foltz et al., 2013), consistent with modeling studies (Jouanno et al., 2011). PIRATA, combined with shipborne and microstructure measurements, confirmed the dominant role of mixing in boreal spring–summer and highlighted the contribution of zonal advection in the western Cold Tongue (Hummels et al., 2013, 2014). Observations also showed that tropical instability waves, Rossby–gravity waves, and diurnal processes modulate vertical shear and mixing, directly influencing the seasonal SST cycle (Gasparin et al., 2025; Wenegrat & McPhaden, 2015). Off the equator, PIRATA revealed that surface heat fluxes generally dominate SST variability, though vertical processes can be locally important, and that salinity modulates turbulent cooling with seasonal and regional contrasts (Foltz et al., 2018; Foltz & McPhaden, 2008).
