The Effective Drag Coefficient for Evaluating Wind Stress over the Oceans

Computations of the surface wind stress and pseudostress 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 pseudostress into a stress has been computed for each month of the year using several 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 the 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 amount, and occasionally the wind stress may be greatly in error.