The Deflection Angle of Surface Ocean Currents From the Wind Direction

Abstract

Wind stress drives the upper ocean circulation in nonequatorial regions by means of an interplay with the vertical turbulent friction and the Coriolis force, generating horizontal wind drift currents which spiral and decay with depth. Classical Ekman theory—applied almost universally in oceanography—predicts that the angle between the vectors of the surface current and surface wind is 45°, if the coefficient of vertical turbulent mixing is constant. However, observations show that the deflection angle is usually close to 30° in arctic regions and larger than 45° in some low-latitude areas, because the vertical turbulent mixing varies with depth. In contrast to Ekman's classical solution, the solutions that are available for depth-dependent eddy viscosity are quite involved and, as in data-driven studies, it is difficult to sort out spurious correlations that obscure the underlying structure. We propose a perturbative approach, providing a formula for the deviation of the deflection angle from the 45° reference value and discussing its implications.