Air-Sea Coupling in the Vicinity of Ocean Fronts

Air-Sea Interaction near Ocean Fronts

Observations in the vicinity of ocean fronts indicate that sea surface temperature (SST) is positively correlated with near-surface wind speed such that stronger winds are found over warmer SST. This indicates that the ocean is forcing the atmosphere, and is opposite to what is commonly observed on large scales, where a negative correlation is indicative of the atmosphere driving the ocean. I have been exploring several aspects of the coupling near ocean fronts, including the behavior in strong cross-front, mid-latitude regimes and the feedback of such coupling on the ocean circulation. Traditional mechanisms for the acceleration of near-surface winds over warm water are an adjustment to local pressure gradients and the downward mixing of high momentum from aloft due to increased instability of the planetary boundary layer over warm water. A third mechanism appears to be important for mid-latitude fronts when the cross front wind is suffienctly strong, namely the conversion of along front momentum to across front momentum through the Coriolis term (J. Climate paper below). It is also shown that winds from the warm side of the front to the cold side of the front can enhance the baroclinic instability of the ocean front, while winds from the cold side to the warm side tend to stabilize the front. A nondimensional parameter is derived that characterizes the strength of this coupling, which is strongest at low latitudes, for weak ocean flows, and in shallow water.