A Wind-Driven Model Experiment of the Seasonal Cycle of the Main Thermocline in the Interior Midlatitude North Pacific

Abstract

A wind-driven model experiment is designed to test the effectiveness of the classical wind-driven theory of Veronis and Stommel (1956) in explaining the phase and amplitude of the seasonal cycle of the main thermocline in the interior portion of the midlatitude North Pacific. The model is a two-layer quasi-geostrophic model on the ?-plane. driven by the observed seasonal cycle of the wind-stress curl. It allows the first-mode baroclinic response to Ekman pumping by the wind stress curl, modified by baroclinic planetary (Rossby) wave effects. The horizontal phase and amplitude distributions of the seasonal cycle of temperature at 200 m over the interior midlatitude North Pacific from 25?45°N, 130°W?150°E, are determined based on XBT data collected by ship-of-opportunity from 1968?74. Quantitative comparison between these observed distributions of phase and amplitude with those of the model shows that the phase distributions have a pattern correlation of 0.72, while the amplitude distributions have a correlation of only ?0.15. Even worse is the fact that the rms magnitude for the model is 5?10 times smaller than that observed. There is some evidence that a portion of this amplitude mismatch may be due to an underestimation of the magnitude of the wind-stress curl by a factor of 2, but this does not even begin to explain the order of magnitude difference. Therefore, it must be concluded that the simple dynamical concepts put forth by Veronis and Stommel (1956) cannot account for the seasonal cycle of the main thermocline in the midlatitude North Pacific.