Estimates of Power Consumed by Mixing in the Ocean Interior

Abstract

Much attention has focused on the power required for driving mixing processes in the ocean interior, the thermohaline circulation, and the related meridional overturning circulation (MOC). Recent estimates range from roughly 0.5 to 2 TW (1 TW = 1 ? 1012 W), based on differing arguments for the closure of the MOC mass budget. While these values are both O(1) TW, the thermodynamic implications of the estimates are significantly different. In addition, these numbers represent an integral constraint on the global circulation, and the apparent discrepancy merits careful examination. Through basic thermodynamic considerations on water mass mixing, a mechanical power consumption of 3 ± 1 TW is found to be consistent with a basic knowledge of the distribution and magnitude of oceanic turbulence diffusivities. This estimate is somewhat independent of any specific model for mass closure of the MOC. In addition, this estimate is based on a thermocline diffusivity of only 0.1 cm2 s?1, with enhanced diffusivities acting only in the deep and bottom waters. Adding enhanced diffusivities in the upper ocean, or lowering the mixing efficiency below 20%, will increase the power estimate. Moreover, 3 TW is a reasonable estimate for the power availability to processes acting beneath the oceanic mixed layer.