Propagation, Diffusion, and Decay of SST Anomalies beneath an Advective Atmosphere

Abstract

A simple conceptual model is used to illustrate aspects of how the midlatitude atmosphere, in the absence of ocean dynamics, responds to and feeds back on sea surface temperature (SST) anomalies. In the model, a dynamically passive ocean mixed layer of fixed depth exchanges heat with a single-level, energy-balance atmosphere with a constant mean wind, U. The temperatures of the two subsystems, TO and TA, respectively, strive to equilibrate through surface heat exchange, which is parameterized as ?(TO ? TA). Atmospheric advection of heat has two important effects on the evolution of SST anomalies. First, the SST anomalies propagate downwind at the speed (cA/cO)U, where cA and cO are the heat capacities of the atmosphere and the oceanic mixed layer, respectively. Second, the damping rate of SST anomalies is scale dependent: the distance an atmospheric column travels before it equilibrates with the ocean through surface heat exchange (UcA/?) introduces a length scale that discriminates between small-scale and large-scale SST anomalies. Local bulk formulas of surface heat exchange determine the damping of small-scale anomalies, which decay exponentially over the timescale cO/?. Large-scale anomalies, on the other hand, decay essentially diffusively and propagate downwind, until longwave radiation finally extinguishes them. The apparent diffusive decay results from the joint effect of atmospheric advection and surface heat exchange. And the effect becomes significant when the distance the atmosphere carries heat downwind is small compared to the scale of the SST anomaly. The kinematical diffusion coefficient associated with the phenomena is c2Ac?1O U2??1.