The Effective Drag Coefficient for Evaluating Wind Stress over the Oceans

Abstract

Computations of the surface wind stress and pseudo-stress over the global oceans have been made using surface winds from the European Centre for Medium Range Weather Forecasts for 7 years. The drag coefficient is a function of wind speed and atmospheric stability, and the air density is computed for each observation. Assuming a constant density, the effective drag coefficient required to convert the pseudo-stress into a stress has been computed for each month of the year using several different methods. Because the drag coefficient varies from day-to-day and with the seasons, the effective drag coefficient cannot be uniquely defined and is a useful concept if only the very gross characteristics of he field are of interest and errors of the order of 10% are tolerable. Even then, the spatial and seasonal variations in Cd must be taken into account, and occasionally the wind stress may be greatly in error.