黑潮延伸区气旋式涡旋浮游植物繁殖个例

A Case Study of Cyclonic Eddy Phytoplankton Bloom in the Kuroshio Extension

【目的】研究黑潮延伸体区域内的涡旋活动对浮游植物繁殖的影响。【方法】根据高分辨率的卫星遥感和再分析资料,对李亚诺夫指数(FSLE)、海表面温度(SST)、海表面高度异常(SLA)、叶绿素(Chl-a)浓度进行归一化合成分析,并计算气旋式涡旋内混合层厚度(MLD)以及引起的埃克曼抽吸速率。【结果与结论】黑潮延伸体区域内的气旋式涡旋引起的埃克曼抽吸作用和冬季混合是Chl-a浓度升高的主要驱动机制。周期为44周的涡旋在16～23周(春季)期间Chl-a浓度高于同期背景场,最大Chl-a浓度是周围的3倍。冬季强湍流混合和涡-风导致的埃克曼抽吸作用使得混合层深度加深至由埃克曼抽吸主导营养盐输送的跃层深度,结合涡致埃克曼抽吸,有利于携带跃层内更多的营养盐输送至混合层,配合春季充足的光照条件,则会更进一步促进浮游植物繁殖。长时间的闭合拉格朗日拟序结构,对于维持高浓度Chl-a有一定的作用。

【Objectives】The influence of the cyclonic eddy of the Kuroshio extension with more active vortex characteristics on phytoplankton blooms.【Method】Based on high-resolution remote sensing satellite and re-analysis data, composite averages of finite size lyapunov exponents(FSLE), sea surface temperature(SST), sea level anomaly(SLA) and Chlorophyll-a(Chl-a) concentrations, calculate the changes of mixed layer depth(MLD) and Ekman pumping velocities.【Result and Conclusion】The results show that the main mechanisms are eddy-Ekman pumping and winter low-temperature mixing.CHL-a concentrations of the cyclonic eddy with life cycle of 44 weeks was higher than the background field during the period of 16-23 weeks(spring), and it was three times that of the surrounding. The winter mixed layer deepening velocities was three times that of the Ekman pumping velocities. Strong turbulent mixing in winter and Ekman pumping caused by eddy-wind deepened the depth of the mixed layer to the depth of the eutrophic layer. Combined with eddy-induced Ekman pumping, which was beneficial to carry more nutrients to the mixed layer and to further promote phytoplankton bloom with sufficient light during the spring. Meanwhile, the long-term closure of the lagrangian coherent structures may maintain high Chl-a concentrations.