Abstract
The typhoon Wind-Pump induced upwelling and cold eddy often promote the significant growth of phytoplankton after the typhoon. However, the importance of eddy-pumping and wind-driven upwelling on the sea surface chlorophyll a concentration (Chl-a) during the typhoon are still not clearly distinguished. In addition, the air–sea heat flux exchange is closely related to the upper ocean processes, but few studies have discussed its role in the sea surface Chl-a variations under typhoon conditions. Based on the cruise data, remote sensing data, and model data, this paper analyzes the contribution of the vertical motion caused by the eddy-pumping upwelling and Ekman pumping upwelling on the surface Chl-a, and quantitatively analyzes the influence of air–sea heat exchange on the surface Chl-a after the typhoon Linfa over the northeastern South China Sea (NSCS) in 2009. The results reveal the Wind Pump impacts on upper ocean processes: (1) The euphotic layer-integrated Chl-a increased after the typhoon, and the increasing of the surface Chl-a was not only the uplift of the deeper waters with high Chl-a but also the growth of the phytoplankton; (2) The Net Heat Flux (air–sea heat exchange) played a major role in controlling the upper ocean physical processes through cooling the SST and indirectly increased the surface Chl-a until two weeks after the typhoon; (3) the typhoon-induced cyclonic eddy was the most important physical process in increasing the surface Chl-a rather than the Ekman pumping and wind-stirring mixing after typhoon; (4) the spatial shift between the surface Chl-a blooms and the typhoon-induced cyclonic eddy could be due to the Ekman transport; (5) nutrients uplifting and adequate light were two major biochemical elements supplying for the growth of surface phytoplankton.
Highlights
Chlorophyll a concentration (Chl-a) is an important index of phytoplankton biomass and the main marine green photosynthetic pigment, and plays an important role in the process of relevance to marine atmospheric carbon cycle, of material cycle and energy conversion, environment monitoring, ocean currents, fishery management, and so on [1,2]
The northeastern South China Sea (NSCS) is generally stratified, tropical, and oligotrophic with low phytoplankton biomass and frequent mesoscale processes, and it is frequently influenced by the tropical cyclones during the summer time [4]
The changes of the surface chlorophyll a concentration (Chl-a) in the study area can be analyzed from the remote sensing data in Figure 2 regardless of some data missing due to the thick cloud cover
Summary
Chlorophyll a concentration (Chl-a) is an important index of phytoplankton biomass and the main marine green photosynthetic pigment, and plays an important role in the process of relevance to marine atmospheric carbon cycle, of material cycle and energy conversion, environment monitoring, ocean currents, fishery management, and so on [1,2]. Tropical cyclones can induce cyclone eddies (or reduce anti-cyclone eddies), and these eddy-pumping upwelling can further increase the surface and subsurface Chl-a [4,12]. These typhoon wind-driven physical processes and air–sea exchanges that subsequently affect the ocean’s ecological status is defined as the “Wind Pump” [2,4,7,8,10,11]
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