东海黑潮上下游不同的涡动能季节变化特征及其产生机制

DISTINCTLY DIFFERENT SEASONAL EDDY KINETIC ENERGY VARIATIONS AND THEIR MECHANISMS IN THE UPSTREAM AND DOWNSTREAM KUROSHIO WITHIN THE EAST CHINA SEA SECTOR

用一种新的泛函工具——多尺度子空间变换(multiscale window transform,MWT),得到如实的东海涡动能分布,发现东海黑潮上下游区域的涡动能有着完全不同的季节变化特征。根据功率谱分析,东海黑潮流系可正交地重构到背景流尺度子空间(大于64 d)与涡旋尺度子空间(小于64 d),并用MWT得出相应子空间的能量,后者即为涡动能。结果表明,上游涡动能在8月达到峰值,冬季最弱;而下游涡动能在4月和9月存在两个峰值,冬季最弱。利用基于MWT的正则传输理论以及局地多尺度能量分析方法,发现正压不稳定和斜压不稳定是东海黑潮涡动能的主要来源,且这两个来源决定了其上下游不同的季节变化。其中上游涡动能的季节变化主要受正压不稳定路径(即背景流动能向涡动能的正则传输)控制,而下游涡动能在4月的峰值主要由斜压不稳定路径(即背景流有效位能向涡旋有效位能作正则传输并进一步转化为涡动能)决定,其在9月的峰值受到正压不稳定和斜压不稳定的共同影响。

Application of a new functional analysis tool,i.e.,multiscale window transform(MWT),reveals to us distinctly different seasonal variation patterns of the eddy kinetic energy(EKE)in the upstream and downstream of the Kuroshio in the East China Sea.Based on a power spectrum analysis,the fields are orthogonally reconstructed with MWT onto two scale windows,namely,the background flow window(>64 d)and the eddy window(<64 d).The kinetic energy on each window is obtained accordingly with the MWT transform coefficients.In the upstream,the EKE peaks in August,and reaches its minimum in winter;in contrast,the EKE downstream has two peaks,one in April and another in September,and attains its minimum in winter.Using the MWT-based canonical transfer theory and localized multiscale energetics analysis,we find that barotropic instability and baroclinic instability are the main mechanisms for the EKE variations in this region.They govern the different EKE seasonal cycles in upstream and downstream Kuroshio,respectively.In the upstream,the EKE seasonality is mainly generated through a barotropic instability pathway(i.e.the canonical transfer of kinetic energy from the background flow to the eddies).In contrast,in the downstream,the April peak is determined by a baroclinic instability(i.e.the canonical transfer of available potential energy from the background flow to the eddies,which is further converted to EKE),while the September peak is jointly generated by a baroclinic instability and a barotropic instability.