Uncertainties in Seasonal Wind Torques over the Ocean

Abstract

Changes in axial atmospheric angular momentum M are related to zonal torques on the atmosphere, but studies reveal large imbalances between the estimated torques and M variations on seasonal timescales. The observed imbalances are commonly attributed to uncertainties in the torque estimates. One particularly important torque component at the seasonal period is that due to zonal wind stresses over the ocean TO. The uncertainties in TO are explored by calculating different multiyear time series based on surface wind products derived from passive and active microwave satellite data. The satellite-based TO are compared to available reanalysis products. Results indicate that there are indeed substantial uncertainties in the seasonal TO, and that these uncertainties are related mostly to the wind fields rather than to the particular parameterizations of the surface stress in the boundary layer. Regional analyses point to the need to improve knowledge of the wind fields over extensive areas of the ocean, particularly in many tropical and southern latitude regions. Resolving subweekly variability in surface winds is also found to be important when determining the seasonal cycle in TO. The current satellite-based TO estimates can lead to a better seasonal momentum budget, but results are tempered by the uncertain effects of gravity wave torque in that budget.