Impact of QuikSCAT-Derived Wind Stress on High-Resolution Kuroshio Modeling

Spectral bandwidth of the wind stress over the ocean is broad in time and space. The scale of the oceanic responses, therefore, would vary from an upper-ocean nonlinear mixing to a basin-wide general circulation largely governed by linear Sverdrup dynamics. What is the impact of such broad-banded wind stress to a local mid-latitude mesoscale ocean dynamics? The Kuroshio south of Japan is known to have two preferred paths: a straight one and a meandering one. In its bimodal state, the choice of the path state is sensitive to perturbations such as mesoscale eddy. Because of its sensitivity to external forcing, the Kuroshio, in its bimodal state, provides a unique setting to test the impact of high-frequency wind forcing on the ocean currents. We, therefore, used a regional inflow-outflow Kuroshio/Oyashio GCM as a test-bed and forced the model by QuikSCAT-derived daily wind stress (2000-2001). The same model forced by an annual mean Hellerman-Rosenstein wind stress exhibited a bimodal Kuroshio path state at a fixed 35 Sv Kuroshio inflow. In addition, the variability of the Kuroshio path was large because of energetic eddy activities. The exact timing of the transition of the path was sensitive to changes in the initial condition as well as model parameters. It is, therefore, likely that the evolution of the modeled Kuroshio path would alter if a broad-spectrum wind stress was used. In a preliminary study using different initial conditions, we have seen a case where the meander growth was enhanced by the use of QuikSCAT wind. In another case, however, no statistically relevant change was observed. It is anticipated that there also can be a case where the reduction of the amplitude of the meander is caused by QuikSCAT wind. It is obvious, therefore, that the use of high-frequency wind stress will add perturbations to the flow, which is large enough to cause the evolution of the Kuroshio to become more chaotic. In an on going experiment, we will continue using our regional Kuroshio model as a test-bed to elucidate the dynamics of how the high-frequency wind forcing impacts the underlying oceanic flow, and also to understand what frequency band of the wind-stress influences most the local current system. As an outcome of this study, we may be able to identify the role of the high frequency wind forcing in the predictability of the ocean currents.