Mechanisms of spring intraseasonal cooling in the Northern Gulf of Guinea

Boreal springtime intraseasonal sea surface temperature cooling (SIC) events are a recurrent feature of the northern Gulf of Guinea (NGoG), yet the processes governing their intermittency and magnitude remain incompletely understood. Using satellite observations, ocean reanalysis, and objective composites, we investigate SIC mechanisms and quantify the mixed-layer heat budget. SIC events preferentially occur under a preconditioned upper ocean characterized by a shallow mixed layer, thin barrier layer, and enhanced upper-ocean stratification (0–30 m). This state reduces mixed-layer heat capacity and strengthens air–sea coupling, allowing modest atmospheric perturbations to produce large temperature tendencies. The heat budget shows that surface heat-flux anomalies emerge as the dominant resolved contribution to the intraseasonal temperature tendency throughout the SIC lifecycle. Cooling during the growth phase is primarily driven by reduced shortwave radiation and enhanced latent heat loss associated with strengthened winds and suppressed convection, while recovery reflects the rapid reversal of these flux anomalies. Oceanic processes contribute coherently but remain secondary, with zonal advection partially offset by meridional and vertical advections. Subsurface adjustments primarily reflect vertical redistribution rather than sustained entrainment-driven cooling. These results support a two-stage SIC mechanism involving background-state preconditioning followed by rapid thermodynamic adjustment.