Atmosphere Response to an Oceanic Submesoscale SST Front: A Coherent Structure Analysis

The atmosphere response to oceanic submesoscale sea surface temperature (SST) fronts is neither resolved by operational nor climate models. Above the ocean, the atmospheric boundary layer (ABL) is in a convective regime and its turbulence levels and structure are altered by the SST variations. Using large eddy simulations, we investigate an ABL flowing above a 1D oceanic submesoscale SST front. The organization of atmospheric turbulence evolves from rolls to a cell-roll transition state but with a 3.5 km delay due to the strong advection. The SST gradient imprints on the atmosphere, creating a pressure gradient that accelerates the flow at all altitudes. On warm water, horizontal wind in the lower half of the ABL increases mainly due to turbulent mixing. Most of the flow is resolved, and we relate statistical quantities to coherent structures using a conditional sampling. The SST increase strengthens updrafts, whereas a downward motion develops in response, transporting high wind from the upper to the lower layers of the ABL. We show that this descending flow is not a downdraft but rather a less turbulent object similar to an enhanced compensating subsidence, which could be parametrized by a mass-flux term. The intense updrafts overshoot the boundary layer height and could trigger convection with moister environmental conditions.