Can Pacific Ocean Thermocline Depth Anomalies Be Simulated by a Simple Linear Vorticity Model?

Abstract

This study attempts to reproduce the salient features of the variability in the depth of the thermocline in the marginally eddy-resolving Parallel Ocean Climate Model (POCM) of Semtner and Chervin, using a simple linear vorticity model that only permits local Ekman pumping and the propagation of long Rossby waves. The dynamic upper-ocean variability in the POCM is examined in response to changes in daily European Centre for Medium-Range Weather Forecasts wind stresses across the tropical and subtropical Pacific Ocean (31°S?31°N) between 1983 and 1989. The POCM provides a complete and physically consistent representation of the state of the Pacific Ocean, with the phase of the thermocline depth anomalies being consistent with the observed El Niño/La Niña variations in the near-equatorial zone and southwest Pacific during the decade. A series of vorticity model sensitivity experiments, incorporating scaled Rossby wave speeds based on recent observations from the TOPEX/Poseidon satellite altimeter, is used to examine and compare the phase and amplitude variations in the depth of the internal surface against changes in the depth of the 14°C isotherm (D14, used as a proxy for the depth of the thermocline, or pycnocline) as simulated in the POCM. This study demonstrates that the simple linear vorticity model can reproduce the Pacific Ocean thermocline depth anomalies in the interior of the subtropical gyres as simulated by the POCM. These variations are both qualitatively and quantitatively consistent with an ocean forced by only Ekman pumping and Rossby waves that traverse the basin, with isolated topographic and background influences. Further, a number of experiments demonstrate that the phase similarities, from correlation analyses, between results from the POCM and those from the simple dynamical model are statistically significant (at the 95% level) across the majority of the 11°S, 11°N, and 21°N transects in the western, central, or eastern Pacific basin. At 11° and 21° latitude, the amplitude of the variability is similarly comparable across much of the basin. The model is generally less successful at 31° latitude where higher baroclinic modes of the mean flow become important.