Upwelling Mechanism in Banggai Sea Waters and Its Influence on Chlorophyll-a and Net Primary Productivity in Tomini Bay, Indonesia

Tomini Bay, located near the equator within the Indonesian archipelago, represents a unique marine system where mesoscale and regional-scale oceanic processes regulate regional productivity. This study examines the upwelling mechanism in the Banggai Sea and its impact on the hydrobiological conditions of Tomini Bay, utilizing secondary satellite and reanalysis datasets from January to December 2022. The analyzed variables include sea surface temperature (SST), salinity, surface currents, wind fields, Chlorophyll-a concentration (Chl-a), and Net Primary Productivity (NPP). Results indicate that upwelling in the Banggai Sea intensifies during the southeast monsoon (July), characterized by stronger southeasterly winds (~4.05 m s⁻¹), enhanced wind-driven Ekman pumping, surface cooling of approximately 2–3 °C, and a shoaling of the thermocline depth. This physical forcing coincides with increased surface Chl-a (up to ~1.0 mgm⁻³) and elevated NPP (>700 mg C m⁻² day⁻¹), particularly along the southeastern sector of Tomini Bay. In contrast, during the northwest monsoon (January), weaker wind forcing reduces Ekman-driven upwelling intensity, while higher surface Chl-a in the inner and central bay is likelyinfluenced by terrestrial (riverine) nutrient inputs and reduced stratification, rather than verticalnutrient transport. The findings demonstrate that seasonal monsoon variability governs upwellingdynamics in the Banggai Sea, which in turn modulates the spatial distribution of biologicalproductivity in Tomini Bay. This study emphasizes the significance of monsoon-driven physical–biological coupling in maintaining marine productivity in equatorial Indonesian seas.