Feedback Of Mesoscale Wind Stress Perturbations In The Kuroshio Extension

Mesoscale perturbations(with a size of 100-1000 km)of wind stress magnitude,divergence and curl in the Kuroshio Extension(KE)region are observed to tightly link to those of sea surface temperature(SST),downwind and crosswind SST gradients,respectively.Based on long term satellite observation,empirical relationships between mesoscale wind stress responses and SST forcing are established.These relationships are further used to represent mesoscale wind stress-SST coupling in an ocean model which is based on the Regional Oceanic Modelling Systems(ROMS).This study examines the covariability characteristics of mesoscale wind stress and SST perturbations in the KE,and illustrates the effect of the wind stress perturbations on the ocean.The mesoscale wind stress and SST perturbations are firstly isolated by using a locally weighted regression method,and then analyzed using the linear regression and Singular Value Decomposition(SVD)methods.It is observed that mesoscale wind stress magnitude,divergence and curl perturbations are positively correlated to SST,downwind and crosswind SST gradients perturbations.The strength of mesoscale wind stress and SST perturbations displays a consistent seasonal variability,with the maximum appeared in winter while the minimum appeared in summer.Coupling coefficient between mesoscale wind stress and SST perturbations also displays distinct seasonal variability,with large value appeared in winter while small value appeared in summer.The leading three SVD modes for mesoscale wind stress and SST perturbations show highly consistent variability in both spatial eigenvectors and temporal expansion coefficients.These temporal expansion coefficients show seasonal and interannual variability.Based on the observed coupling relationship between mesoscale perturbed wind stress magnitude and SST,and those between wind stress divergence/curl and downwind/crosswind SST gradients,two methods are devised to get the mesoscale wind stress field perturbations from the SST data.The two methods provide convenient ways in which the coupling relationship between mesoscale perturbed wind stress and SST can be hold adequately,and are feasible in eddy-resolving ocean models.The effect of mesoscale wind stress-SST coupling in the KE is studied by using a high resolution ocean model.This ocean model can simulate the seasonal variability of mesoscale SST perturbations.The mesoscale wind stress field is diagnostically determined from modelled SST by using the empirical relationship between them.Through comparing two experiments with and without the mesoscale wind stress-SST coupling,it is found that the mesoscale perturbed wind stress has a negative feedback on SST perturbations.Analyses of sensitivity experiments suggest that the mesoscale wind stress perturbations inhibit SST perturbations mainly by means of surface heat flux.The mesoscale wind stress perturbations also influence the ocean mean state and seasonal variability of SST in the KE,with large effect in autumn and winter when the mesoscale perturbations are most active.Analyses of sensitivity experiments demonstrate that the mesoscale wind stress perturbations can affect long term mean SST through either the way of surface heat flux or momentum flux.The mesoscale wind stress perturbations also modulate the kinetic energy along the Kuroshio pathway,with little effect on the position of the Kuroshio extension jet.The results derived in this study help to understand the mesoscale air-sea interaction in the Kuroshio Extension.