南海海气CO2交换对两个热带气旋“风泵”的不同响应机理分析

Different mechanisms of air-sea CO2 exchange responding to“Wind Pump”effects of two tropical cyclones in South China Sea

利用实测和遥感数据,对比分析南海海气二氧化碳(CO2)交换对2011年4月热带气旋Tropical Depression One(TDO)和2013年9月热带气旋Wutip(WU)的响应及其机理。结果表明,两个热带气旋过境的“风泵”作用均对海气CO2交换产生显著影响:TDO和WU过境后,海水表层CO2分压(pCO2,sw)分别增加42.56μatm、30.88μatm,TDO过境导致区域海洋由强碳汇(-4.7±1.8)mmol·CO2·m^-2·d^-1变为弱碳源(2.0±3.1)mmol·CO2·m^-2·d^-1,而WU过境使区域海洋由弱碳源(1.9±1.0)mmol·CO2·m^-2·d^-1变为强碳源(4.0±1.6)mmol·CO2·m^-2·d^-1。pCO2,sw的增加均由于次表层富含溶解无机碳(DIC)海水的入侵,但是“风泵”的作用机制不同:TDO过境强风引起的艾克曼抽吸导致水体涌升作用显著,而WU过境导致的垂直混合作用强烈。“风泵”的作用机制差异可归因于热带气旋过境前海水的初始状态和过境时移动速度不同:TDO过境前该海域存在冷涡,强风引起的艾克曼抽吸使次表层海水的向上涌升作用增强;而WU过境前存在暖涡,强风引起的艾克曼抽吸造成次表层海水与下沉的表层水垂直混合。TDO的移动速度慢,对海水的作用时间更长,强风引起的艾克曼抽吸作用更强;而WU移动速度快,短时间强风过境造成的水体垂直混合效应作用更显著。

The mechanisms of air-sea carbon dioxide(CO2)exchange responding to Tropical Depression One(TDO)and Wutip(WU)in South China Sea,were studied in the research using both ship measured and satellite data.Results indicated that CO2 fluxes(F)were significantly enhanced by the effect of“wind pump”after the two tropical cyclones.The partial pressure of CO2 in sea surface(pCO2,sw)increased by 42.56μatm and 30.88μatm after the passage of TDO and WU,respectively.The regional ocean changed from a strong carbon sink(-4.7±1.8)mmol·CO2·m^-2·d^-1 to a weak carbon source(2.0±3.1)mmol·CO2·m^-2·d^-1 due to the passage of TDO,while a weak carbon source(1.9±1.0)mmol·CO2·m^-2·d^-1 was changed to a strong carbon source(4.0±1.6)mmol·CO2·m^-2·d^-1 due to the passage of WU.The increase of pCO2,sw due to both tropical cyclone events were consequently from the infusion of deeper subsurface waters,which were rich in dissolved inorganic carbon.However,the mechanisms of“Wind Pump”effects were apparently different,by which a water upwelling process was driven by an Ekman pumping caused by TDO,whereas a strong vertical mixing was driven by strong winds caused by WU,respectively.Such difference was attributed to the difference of the initial oceanographic condition states and the transition speeds of those two tropical cyclones.On the aspect of initial oceanographic conditions,a pre-existed cold eddy before the passage of TDO enhanced the strong Ekman pumping causing upwelling,whereas a pre-existed warm eddy before the passage of WU made a vertical mixing.For the aspect of translation speed,the low translation speed of TDO was slow so that the interaction time was long and leading to a strong Ekman pumping,whereas the speed of WU was fast so that vertical mixing was more significant due to a short-term strong wind transition.