Seasonal variability of SeaWiFS chlorophyll a in the Malacca Straits in relation to Asian monsoon

The influences of Asian monsoon on chlorophyll a (chl a) in Malacca Straits (MS) are investigated using Sea-viewing Wide Field-of-view Sensor (SeaWiFS) data (November 1997–January 2003). Results show that chl a in MS experienced seasonal variability. At northern MS, SeaWiFS chl a increased as much as twice during northeast monsoon (NEM) compared to southwest monsoon (SWM). At the southern MS, SeaWiFS OC4 algorithm was found over-estimating chl a at the areas of high normalized water leaving radiance at 555 nm value (nLw 555). Strong relationship between the nLw 555 and total suspended sediment concentration suggested that high turbidity could contribute to the over-estimation of SeaWiFS chl a. Monsoon wind strongly influenced the spatial distribution and seasonal variation of the chl a at northern MS. During NEM, bloom of phytoplankton started from eastern MS, and it was advected towards Andaman Sea by northeasterlies. NEM wind created positive wind stress curl along 4.5–6.5°N in the Straits, which caused positive Ekman pumping resulting upwelling. Consequently, upwelling helped replenish the surface water nutrients, thus, maintaining the phytoplankton bloom over the continental shelf. Low chl a was observed during SWM, and this was attributed to negative wind stress curl and downwelling. The unique topology of MS, where the mountain ranges at Malay Peninsular and Sumatra Island acted as the monsoon windshield, enable both upwelling and downwelling at northern MS during different monsoons.