On the Offshore Decay Scale of Coastally Upwelled Cold SST

A simple analytic model is developed to represent the offshore decay of cold SST signals that originate from wind-driven upwelling at a coastal boundary. The model couples an oceanic mixed layer to an atmospheric boundary layer through wind stress and air-sea heat exchange. The primary mechanism that controls SST is a balance between Ekman advection and air-sea exchange. The offshore penetration of the cold SST signal scales with the product of the Ekman velocity and a time scale derived from the air-sea heat flux and radiative heating of the atmosphere. This cold SST signal imprints strongly on the atmosphere in terms of both temperature and surface wind. Weakly nonlinear theory shows that: the feedback between SST and atmospheric wind, baroclinic instability, and thermal wind in the atmospheric boundary layer all slightly modify this linear theory. The decay scale diagnosed from two-dimensional and three-dimensional eddy resolving numerical models are in close agreement with the theory, as is an analysis of climatological SST off the west coast of the US.