Importance of rotation shear stress for entrainment in the ocean mixed layer.

Abstract

The interaction of the northward component of planetary rotation and the east-west Reynolds stress affects the isotrophy of the integral scale turbulence in the upper ocean by redistributing turbulent kinetic energy (TKE) among the components. This "rotation stress" mechanism is incorporated into a vertically integrated model of the ocean mixed layer. Simulations of Ocean Weather Stations P (50˚N, 145˚W) and N (30˚N, 140˚W) are used to compare this model with the Garwood (1977) model and with observations. The significant effect is the augmentation (for easterly winds) or reduction (for westerly winds) of the ratio of vertical to horizontal TKE. The rate of entrainment is affected by the change in the vertical convergence of TKE at the interface between the mixed layer and the pycnocline. Rotation stress significantly alters the mixing on diurnal and synoptic time scales during late winter and early spring. With rotation stress, retreat events occur more frequently, and the mixed layer depth change during retreat is 10-30% greater than without rotation stress. Typically, the ratio of vertical to total TKE is three times larger when rotation stress is included and the dissipation enhancement of Garwood (1977) is neglected. The resulting TKE distribution is more isotropic and in better agreement with laboratory results for neutrally stratified shear flows. This study demonstrates the need for measurements of the TKE budget in the upper ocean to confirm these findings and to further test the hypotheses of TKE models in oceanic applications.