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.

Spatio-Temporal Variations of Winter Phytoplankton Blooms Northwest of the Luzon Island in the South China Sea

“…After removing EPV, we find that the multiple correlation coefficient has only a slight drop, also indicating a weak impact of EPV on Chl-a. This finding is obviously different from previous studies (Chen et al, 2006;Shang et al, 2012;Zhao et al, 2012) that Ekman pumping was thought to play an important role in the winter phytoplankton bloom, which however, focus on individual cases of winter blooms only. After EPV, SLA, and WS are removed from the multiple correlation analysis, the winter Chl-a bloom still shows a significant correlation (multiple correlation coefficient, R = 0.7714, p < 0.01) with ET and SST along with partial correlation coefficients of 0.6942 and −0.6650, respectively.…”

“…Wang et al (2010) emphasized that phytoplankton blooms are mainly regulated by the entrainment of the upper mixed layer by analyzing the seasonal variation of the phytoplankton southwest of Luzon Strait from 2000 to 2006. Using satellite remote sensing data from 1997 to 2007, Zhao et al (2012) analyzed the spatial and inter-annual variations of phytoplankton blooms northwest of Luzon Island, and indicated that the Ekman pumping induced by the wind stress is the main factor causing high Chl-a concentration in winter. Shang et al (2012) studied the phytoplankton blooms in January 2010, and their results suggested that phytoplankton blooms in the region near Luzon Strait were related to the mesoscale processes caused by the Kuroshio intrusion.…”

Summer Phytoplankton Blooms Induced by Upwelling in the Western South China Sea

“…The regions of low SST in Figure 3A, low temperature from 50-75 m in Figure 5D, and high nitrate concentration from 30-50 m in Figure 4, are all similar, and coincide with that of ET-induced upwelling (Figure 3). This might indicate that ET-induced upwelling provides a continuous source of cold and nutrient-rich water to surface layers, leading to the phytoplankton blooms (Chereskin and Price, 2001;Zhao et al, 2012). SLA (Figure 3C) reveals the occurrence of both cold and warm eddies in the WSCS in summer.…”

Progress on upwelling studies in the China seas

“…J . Wang et al ., 2010; Zhao et al ., ]. In the Luzon Strait, the winter bloom occurs during the relaxation period of intensified northeasterly winds when the deepened mixed layer starts to shoal, and the advection of relative vorticity primarily contributes to the subsurface upwelling that supplies nutrients to the region below the mixed layer [ Lu et al ., ].…”

“…Specifically, in the region northwest of Luzon Island, the winter phytoplankton bloom is primarily induced by Ekman pumping-driven upwelling (observed at about 50 m below the sea surface) and upper mixed layer entrainment [e.g., J. J. Zhao et al, 2012]. In the Luzon Strait, the winter bloom occurs during the relaxation period of intensified northeasterly winds when the deepened mixed layer starts to shoal, and the advection of relative vorticity primarily contributes to the subsurface upwelling that supplies nutrients to the region below the mixed layer [Lu et al, Reviews of Geophysics Upwelling in the ECS, the Yellow Sea, and the Bohai Sea can also be verified by the distribution of marine chemical and biological elements [Chen et al, 2004;Wong et al, 2004].…”