Uncertainties in Seasonal Wind Torques over the Ocean

Changes in axial atmospheric angular momentumMare related to zonal torques on the atmosphere, but studies reveal large imbalances between the estimated torques andMvariations 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 oceanTO . The uncertainties inTO are explored by calculating different multiyear time series based on surface wind products derived from passive and active microwave satellite data. The satellite-basedTO are compared to available reanalysis products. Results indicate that there are indeed substantial uncertainties in the seasonalTO , 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 inTO . The current satellite-basedTO estimates can lead to a better seasonal momentum budget, but results are tempered by the uncertain effects of gravity wave torque in that budget.