Radiating Instability of Nonzonal Oceanic Shear Flows with Application to the Kuroshio Extension Current

Abstract

This paper proposes a mechanism to explain the fairly energetic internal temperature fluctuations observed in the regions far away from the axis of the Kuroshio Extension Current (KEC). Based upon the typical observed time and length scales, flow speeds and orientations, phase propagation, and length scale of decay in the cross-flow direction, the radiating instabilities of nonzonal shear flows are explored for the cases of different flow orientations, vertical shears, and Rossby radii of deformation. The results show that, as the orientation of an eastward flow becomes more northward, the number of radiating unstable modes within the typical range of the observed wavelengths becomes larger, the length scale of decay becomes larger, and the surface layer of the far field in the two-layer system tends to become more intensified than the bottom layer. A similar effect is found in the cases of a stronger vertical shear and a larger Rossby radius of deformation. Therefore, it is concluded that the radiating instabilities of the KEC in the western North Pacific can be a source of internal temperature fluctuations observed in the regions away from the KEC, especially in the region near the western boundary, where the flow orientation may be nonzonal, the vertical shear is strong, and the Rossby radius of deformation is large.