The Somali upwelling system in the northwestern tropical Indian Ocean exhibits a distinctive bimodal seasonal sea surface temperature (SST) cycle, with two warming and two cooling phases each year. This behavior contrasts with the predominantly unimodal SST evolution in the Java upwelling region, despite similarly pronounced monsoonal wind seasonality. Here, we combine satellite observations, atmospheric reanalysis, ocean reanalysis, and a mixed-layer heat budget to identify the phase-dependent processes governing the Somali SST cycle.

The four phases are characterized by distinct balances between atmospheric forcing and ocean dynamics. During spring warming, enhanced shortwave radiation and weakened winds increase net surface heat input and reduce latent heat loss, while horizontal advection and vertical entrainment remain weak. In contrast, summer cooling occurs under the southwest monsoon, when ocean dynamical processes overcome positive net surface heat flux. Vertical entrainment and horizontal advection each contribute strong cooling, consistent with monsoon-driven upwelling and the intrusion of colder subsurface water into the mixed layer. Autumn warming presents a marked asymmetry: net surface heat flux is nearly three times stronger than in spring, but persistent entrainment and horizontal advection substantially offset this atmospheric heating, resulting in a warming rate comparable to that in spring. Winter cooling is fundamentally different from summer cooling. It is controlled mainly by enhanced latent heat loss and reduced solar heating associated with the cold and dry northeast monsoon, whereas ocean dynamical contributions are relatively small.

These results show that the Somali bimodal SST cycle does not arise from a uniform response to the monsoon. Instead, it reflects a seasonal succession of distinct heat-budget regimes, in which the relative roles of surface forcing, horizontal advection, and vertical entrainment change sharply across the monsoon cycle. This phase-dependent monsoon–ocean interaction distinguishes the Somali upwelling system from the more thermally controlled seasonal evolution off Java and provides a regional perspective on upper-ocean climate variability in the tropical Indian Ocean.