Abstract
Summer upwelling occurs frequently off the southeast Vietnam coast in the western South China Sea (SCS), where summer phytoplankton blooms generally appear during June-August. In this study, we investigate inter-annual variation of Ekman pumping and offshore transport, and its modulation on summer blooms southeast of Vietnam. The results indicate that there are low intensities of summer blooms in El Niño years, under higher sea surface temperatures (SST) and weaker winds. However, a different pattern of monthly chlorophyll a (Chl-a) blooms occurred in summer of 2007, a transitional stage from El Niño to La Niña, with weak (strong) wind and high (low) SST before (after) early July. There is a weak phytoplankton bloom before July 2007 and a strong phytoplankton bloom after July 2007. The abrupt change in the wind intensity may enhance the upwelling associated with Ekman pumping and offshore Ekman transport, bringing more high-nutrient water into the upper layer from the subsurface, and thus leading to an evident Chl-a bloom in the region.
Highlights
Upwelling refers to the upward motion of the water body that brings subsurface water to the sea surface, with the surface water taken away from upwelling regions through horizontal advection [1]
Summer upwelling appears generally in the northern shelf and the western South China Sea (SCS) through offshore Ekman transport induced by the wind stress component parallel to the coastlines and through Ekman pumping caused by wind stress curls [3,4]
We investigate the inter-annual changes of phytoplankton blooms and responses of Chla patterns to the summer upwelling In addition, we examine other ocean conditions in this area, to discuss the different roles that the summer upwelling plays in increasing the concentration of phytoplankton and chlorophyll a (Chl-a)
Summary
Upwelling refers to the upward motion of the water body that brings subsurface water to the sea surface, with the surface water taken away from upwelling regions through horizontal advection [1]. The wind pattern in the South China Sea (SCS) is mainly controlled by the East Asian monsoon, with southwesterly and northeasterly monsoonal winds in summer and winter, respectively. Summer upwelling appears generally in the northern shelf and the western SCS through offshore Ekman transport induced by the wind stress component parallel to the coastlines and through Ekman pumping caused by wind stress curls [3,4]. A statistical correlation exists between the wind stress and the sea surface temperature (SST) on various spatial and temporal scales [3,5]. The magnitude of the wind has a positive effect on upwelling [6]. In the Pearl River plume, wind direction exerted a more substantial effect on upwelling than wind speed [7].
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