Simple Model Prediction of Horizontal Temperature Fields in the Subtropical–Subpolar System Caused by Sudden Change in Wind Stress Curl

Abstract

A four-level numerical model, which is driven by wind stress and surface heat flux, is used to study responses of horizontal temperature fields in the subtropical?subpolar system to a sudden change in the magnitude of the wind stress curl. Weakly nonlinear responses to O(1) change in the wind stress curl are examined, according to the effects on the Ekman pumping, convection, westward baroclinic wave, and advection. For this purpose, a quasi-analytical method, that is, characteristics associated with effects of both wave propagation and advection, is constructed based on a planetary geostrophic model with four-level geometry. Characteristics obtained for the first and second baroclinic modes are used to diagnose steady-state and time-dependent solutions. One feature of time-dependent motions predicted by the model is a westward propagation of the first baroclinic mode with a significantly higher speed than the combined speed of the nondispersive first-mode baroclinic Rossby wave and barotropic circulation. The speedy westward propagation of the first baroclinic mode is ascribed to the wave effect caused by the ambient potential vorticity gradient. The primary feature of time-dependent motions of the second baroclinic mode is a temperature change with different tendencies between the western and eastern portions in both the subtropical and subpolar gyres. The mechanism generating such a temperature change is not related to the vertical shift of the thermocline but is related to the horizontal shift of the thermocline caused by the temperature anomaly formation and its distribution along characteristics. The horizontal shift of the thermocline near the midlatitude jet induces strong temperature anomalies into the western closed region of the second baroclinic mode so that, in contrast to the purely wind-driven gyre, an intensive temperature change occurs in the western closed region of the subtropical gyre.