Wind Stress Simulations and the Equatorial Momentum Budget in an AGCM

Abstract

A detailed examination of the atmospheric momentum budget over the equatorial Pacific and its relation to oceanic wind stresses is undertaken using the results of a 20-yr (1979?99), forced-SST, AGCM experiment. The results show that free-tropospheric pressure gradients play a significant role in forcing boundary layer flow in the model. In particular, the time-mean and interannual variability of wind stress at the surface is found to be dominated by forcing from the free troposphere. The NCEP reanalyses from 1979?99 are also examined and a similar result is found, although the relative importance of this free-tropospheric forcing is somewhat higher in the model. The seasonal cycle of free-tropospheric forcing in the model is found to be substantially stronger in the model than in the reanalysis, and that has a clearly negative impact on the simulated seasonal cycle of surface wind stresses. In the model, these free-tropospheric pressure gradients are not balanced by turbulent stresses or other dissipative forces. Rather, the momentum budget analysis shows that they are balanced by advective momentum tendencies, with vertical advection of momentum in the descending branch of the Walker circulation playing an important role.