Propagation, diffusion, and decay of SST anomalies beneath an advective atmosphere

A simple conceptual model is used to illustrate aspects of how the midlatit ude atmosphere, in the absence of ocean dynamics, responds to and feeds bac k on sea surface temperature (SST) anomalies. In the model, a dynamically p assive ocean mixed layer of fixed depth exchanges heat with a single-level, energy-balance atmosphere with a constant mean wind, U. The temperatures o f the two subsystems, T-O and T-A, respectively, strive to equilibrate thro ugh surface heat exchange, which is parameterized as lambda(T-0 - T-A). Atmospheric advection of heat has two important effects on the evolution of SST anomalies. First, the SST anomalies propagate downwind at the speed (c (A)/c(O))U, where c(A) and c(O) are the heat capacities of the atmosphere a nd the oceanic mixed layer, respectively. Second, the damping rate of SST a nomalies is scale dependent: the distance an atmospheric column travels bef ore it equilibrates with the ocean through surface hear exchange (Uc(A)/lam bda) introduces a length scale that discriminates between small-scale and l arge-scale SST anomalies. Local bulk formulas of surface heat exchange dete rmine the damping of small-scale anomalies, which decay exponentially over the timescale c(O)/lambda. Large-scale anomalies, on the other hand, decay essentially diffusively and propagate downwind, until longwave radiation fi nally extinguishes them. The apparent diffusive decay results from the join t 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 diffusi on coefficient associated with the phenomena is c(A)(2)c(O)(-1) U(2)lambda( -1).