Characteristics of the Zonal Sea Surface Temperature Perturbation and Its Role in Representing the Evolution of the Kuroshio Extension

Zonal sea surface temperature (SST) perturbation (hereafter ZSSTP) is proposed in order to correctly describe the strength of the Kuroshio Extension (KE) and highlight its heat transport effect on SST adjustment. Analysis of the NOAA (National Oceanic and Atmospheric Administration) high resolution SST data and GODAS (Global Ocean Data Assimilation System) re-analysis data indicates that the climatological ZSSTP agrees well the distribution of surface heat transport by currents. Analysis of interannual evolution of ZSSTP indicates that it can represent the interannual oscillation of the KE strength and better identify the ranges influenced by the KE and Oyashio compared to traditional SST anomalies. Empirical orthogonal function (EOF) analysis of ZSSTP in the western North Pacific indicates that the first two EOF modes reflect the contraction and elongation states of the KE system, while the third and fourth EOF modes reflect the impacts by the weakened and enhanced Oyashio, respectively. As an important dynamic parameter to identify the KE region, the eddy kinetic energy (EKE) is closely linked with ZSSTP through the strength of SST front and the stability of currents during the process of the KE evolution. Warm ZSSTP indicates strong surface heat transport, and the SST gradient increases and SST front enhances correspondingly, while currents become more stable and mesoscale eddy activities decrease, and vice versa. Finally, a thermal index T_p is introduced to describe the variation of the thermal state of the KE. It is demonstrated that this index can successfully describe the variation of the zonal stretch and meridional migration of the main axis as well as the evolution of the intensity of the KE.