Ocean current and wave effects on wind stress drag coefficient over the global ocean

The effects of ocean surface currents and dominant waves on the wind stress drag coefficient (CD) are examined over the global ocean. Major findings are as follows: (1) the combination of both ocean wave and current speeds can result in reductions in daily CD (>10%), but the notable impact of the latter is only evident in the tropical Pacific Ocean; (2) the presence of waves generally makes winds weaker and CD lower almost everywhere over the global ocean; (3) strong ocean currents near the western boundaries (Kuroshio and Gulf Stream) do not substantially influence CD since the winds and currents are not always aligned; and (4) the change in speed used in bulk flux parameterization also causes large changes in fluxes. Globally, the combined outcome of ocean currents and waves is to reduce CD by about (2%), but spatial variations (0% to 14%) do exist.