Wind Stress Measurements from the Open Ocean

An automatic inertial dissipation system was used during three cruises of the RRS Discovery in the Southern Ocean to obtain a large dataset of open ocean wind stress estimates. The wind speed varied from near calm to 26 m s−1, and the sea-air temperature differences ranged from −15° to +7°C. The data showed that the assumption of a balance between local production and dissipation of turbulent kinetic energy is false and that the sign and magnitude of the imbalance depends critically on both stability and wind speed. The wide range of stability conditions allowed a new formulation for the nondimensional dissipation function under diabatic conditions. A minimum in the 10-m neutral value of the drag coefficient occurred at 6 m s−1. At lower wind speeds the data were fitted by the relationship where U10n. is the 10-m neutral wind speed (m s−1). At higher wind speeds which gives drag coefficients that are about 10% higher than those from previous open ocean studies (which assumed a balance between production and dissipation). Wave measurement suggested that the sea state was not, on average, fully developed at wind speeds above 15 m s−1. However, contrary to findings from other studies, no large anomalies in the drag coefficient were detected despite the range of conditions and sea states encountered. It is believed that the ideal conditions (such as the absence of swell) needed to defect the effects of sea state on the wind stress may occur rather infrequently over the open ocean.