DETERMINING TORQUES OVER THE OCEAN AND THEIR ROLE IN THE PLANETARY MOMENTUM BUDGET

Knowledge of wind torques at the atmosphere-ocean interface is important in understanding the global angular momentum balance. A surface wind data set that combines special sensor microwave/imager (SSM/I) and European Centre for Medium Range Weather Forecasts (ECMWF) wind products and covers the period July 1987 to June 1989 is used to estimate zonal torques over the global ocean. Comparison of these fields and fields computed from ECMWF wind analyses alone shows substantial differences in the statistics of both locally and globally integrated torques; the impact of SSM/I data on the torque analysis is largest over the Indian Ocean and eastern Pacific regions. The characteristics of the torques from the combined SSM/I-ECMWF data set are evaluated. In general, local torques show the largest means and variances over mid-latitudes (especially in the southern ocean), with fluctuations at periods shorter than 1 month dominating the records. In contrast, variability in the globally integrated ocean torque is largest at seasonal time scales (3 months to 1 year); midsouthern and tropical northern latitudes contribute substantially to this variability. Comparison between the global ocean torque and atmospheric angular momentum M indicates that indirect exchanges of momentum between the atmosphere and solid Earth via oceanic stresses cannot be neglected at seasonal time scales. Intraseasonal fluctuations (1-3 months) in both tropical and global ocean torques are coherent with M but too weak to account alone for the variability in M. At submonthly periods the ocean torque is apparently not important in the atmospheric momentum balance.