The temperature, salinity, and chlorophyll‐a (Chl‐a) profiles of the upper ocean (0–200 m) were measured during the China‐Pakistan Joint Expedition, 2017. The cruise track passed through an identified cyclonic eddy, and the multi‐disciplinary records thus verify the eddy contributions to winter blooms in the northern Arabian Sea (AS). The records show that the strength of upper blooms within the mixed layer is mainly dominated by surface phytoplankton biomass (the Chl‐a concentration), which increases simultaneously with the shallow mixed layer and uplifted thermocline. A composite analysis of eddy‐tracked sea level anomalies (SLA) and Chl‐a reveals the pumping and stirring effects that cyclonic eddies make to the ocean surface nutrient supply. Based on the high‐resolution Ocean General Circulation Model for the Earth Simulator (OFES) outputs, a mixed layer nitrogen budget calculation was subsequently conducted to quantify the contributions of mesoscale eddies to 10‐year climatological winter blooms. The results indicate that eddy flows are primary contributors to the increased nutrients in the coastal north AS during the mature period (usually in January and February) of the winter blooms, with a concentration histogram double that of convective mixing. The concentration decreases from the north to south, agreeing well with the seasonal northward nutrition gradient. This confirms that (i) the growth of nutrients over the northern AS, which is known to be triggered by convective mixing, would be further maintained by eddy‐induced advection; and (ii) higher concentration of nutrients would facilitate an easier response of Chl‐a to eddy dynamics.

Chlorophyll-a (Chla) concentration derived from the Sea viewing Wide field of View sensor (SeaWiFS) data (January 1998 to December 2010) shows phytoplankton blooms in the western south equatorial Indian Ocean (WSEIO) during the summer monsoon. The mechanism that sustains the blooms is investigated with the high-resolution Ocean General Circulation Model for the Earth Simulator (OFES) products. The summer blooms in the WSEIO are separated from the coast; they occur in June, reach their maximum in August, and decay in October. With summer monsoon onset, cross-equatorial wind induces open-ocean upwelling in the WSEIO, uplifting the nutricline. The mixed layer heat budget analysis reveals that both thermal forcing and ocean processes are important for the seasonal variations of SST, especially wind-driven entrainment plays a significant role in cooling the WSEIO. These processes cause nutrient enrichment in the surface layer and trigger the phytoplankton blooms. As the summer monsoon develops, the strong wind deepens the mixed layer; the entrainment thus increases the nutrient supply and enhances the bloom. Horizontal advection associated with the Southern Gyre might also be an important process that sustains the bloom. This large clockwise gyre could advect nutrient-rich water along its route, allowing Chla to bloom in a larger area.

River plumes play an important role in the cross-margin transport of phytoplankton and nutrients, which have profound impacts on coastal ecosystems. Using recently available Soil Moisture Active Passive (SMAP) sea surface salinity (SSS) data and high-resolution ocean color products, this study investigated summertime high-frequency variations in the Pearl River plume of China and its biological response. The SMAP SSS captures the intraseasonal oscillations in the offshore transport of the Pearl River plume well, which has distinct 30–60 day variations from mid-May to late September. The offshore transport of freshwater varies concurrently with southwesterly wind anomalies and is roughly in phase with the Madden–Julian Oscillation (MJO) index in phases 1–5, thus implying that the MJO exerts a significant influence. During MJO phases 1–2, the southwest wind anomalies in the northeastern South China Sea (SCS) enhanced cross-shore Ekman transport, while the northeast wind anomalies during MJO phases 3–5 favored the subsequent southwestward transport of the plume. The high chlorophyll-a concentration coincided well with the low-salinity water variations, emphasizing the important role of the offshore transport of the Pearl River plume in sustaining biological production over the oligotrophic northern SCS. The strong offshore transport of the plume in June 2015 clearly revealed that the proximity of a cyclonic eddy plays a role in the plume’s dispersal pathway. In addition, heavy rainfall related to the landfall of tropical cyclones in the Pearl River Estuary region contributed to the episodic offshore transport of the plume.