Distribution and interannual variation of winter phytoplankton blooms northwest of Luzon Islands from satellite observations

Abstract

Phytoplankton blooms often occur in the offshore region northwest of Luzon Islands in winter. Using remote sensing data, including sea-surface temperature, monthly-mean mixed layer depth (MLD), wind speed (SP), and SeaWiFS-derived chlorophyll-a (Chl-a) data from September 1997 to February 2007, we investigate the spatial and interannual variation of the winter phytoplankton in the region along with in situ GTS (the Global Telecommunications System) XBT (Expendable Bathythermograph) data and climatological nitrate and temperature observations from World Ocean Atlas 2005. The results clearly show high winter phytoplankton biomass (i.e. Chl-a) and its interannual variation in the region. The lower Chl-a concentrations (∼0.2 mg m−3) appeared often in winters (e.g. 1997 and 1998) when there was weaker wind speed, Ekman pumping velocity (EPV) and entrainment velocity (EV), and shallower MLD; conversely higher Chl-a (>0.5 mg m−3) appeared in winters (e.g. 2001 and 2003) when there was higher subsurface temperature, stronger wind speed, EPV and EV, and deeper MLD. It is thought that wind-induced upwelling (Ekman pumping) and entrainment mixing may be important factors leading to the high winter Chl-a, and the wind speed and wind stress curl were good indicators of higher Chl-a; the positive correlation between subsurface temperature (and MLD) and Chl-a represented more warm water intrusion from the Northwest Pacific Ocean into the region in the higher Chl-a years.