Seasonal and interannual variations of surface chlorophyll-a in the Karimata Strait

This study aimed to evaluate the dynamics driving seasonal and interannual surface chlorophyll-a (chl-a) variability in the Karimata Strait (KS). The analysis shows that high chl-a concentrations were observed in the KS during December - February (northwest monsoon season) extending northwestward along the eastern coast of Peninsular Malaysia and northeastward along the northern coast of Borneo Island. This high chl-a concentration contrasts with the low chl-a concentration observed along the southern coast of Sumatra and Java during the same season. A substantial upwelling signal was identified in the central-eastern section of the South China Sea and along the northern coast of Java and the eastern coast of Sumatra from December to February. The positive wind stress curl forced downwelling in the southern section of the KS and the northern half of the Java Sea off the coast of Kalimantan. The elevated chl-a concentration in the KS during the northwest monsoon season could not be attributed to wind dynamics alone. The increase in surface chl-a concentration during the northwest monsoon season was associated with an increase in allochthonous nutrients from river discharges caused by increased precipitation over land. On an interannual timescale, a high chl-a concentration was observed during the southeast monsoon season when the La Nina event occurred in the tropical Pacific. The warm SST, associated with favourable downwelling winds, was observed over the entire the KS. It is suggested that anomalous chl-a that bloomed during the La Nina event was associated with anomalously high precipitation over land that transported nutrients to KS through river discharge.