On the wind mechanical forcing of the ocean general circulation

The wind mechanical forcing of the ocean general circulation and its seasonal variations are examined using available observational products, focusing on the role of the mean and fluctuating winds. It is found that including wind fluctuations in the stress calculation produces a qualitative change in the estimates of the mean and seasonal wind stress, particularly at mid and high latitudes where the synoptic wind variability is large. This effect of wind fluctuations on air-sea momentum exchange has immediate dynamical consequences for the large-scale ocean circulation. For example, power input to the ocean general circulation and subtropical gyre transport can be underestimated by more than 50% if the fluctuating winds are not taken into account. However, the impact of including wind fluctuations depends strongly on the presence of the mean winds. If the mean winds are ignored in the stress calculation, the net effect of the fluctuating winds is to take energy out of the ocean owing to the skewness of the near-surface wind field. Furthermore, covariances of wind fluctuations are found to explain most of the effect of the fluctuating winds, while the variable drag coefficient makes a non-negligible contribution in the Southern Ocean. These results imply that paleo and future climate studies need to take into account the changes of the large-scale low-frequency wind field as well as the synoptic weather systems.