SETTING THE SCALES OF THE OCEAN RESPONSE TO ISOLATED CONVECTION

The ocean response to negative buoyancy flux, applied in an isolated r egion at the surface, is investigated to determine the scales of the e quilibrium state, that is, the time to reach equilibrium, the equilibr ium density anomaly within the convecting chimney, and, in the case of deep convection, the equilibrium depth of the chimney. Two types of i solated convection, with fundamentally different parameter dependencie s, are distinguished based on the importance of the forcing decay regi on: a region surrounding the isolated forcing region, across which the buoyancy flux decreases to zero. A narrow forcing decay region produc es ''internally constrained'' convection in which the baroclinic Rossb y radius is the dominant horizontal length scale, and the resulting eq uilibrium scales are those found by Visbeck et al. A wide forcing deca y region produces ''externally constrained'' convection in which the f orcing decay width is the dominant horizontal length scale, and the eq uilibrium scales are those found by Chapman and Gawarkiewicz. Some sim ple theoretical ideas are presented that provide an estimate of the tr ansition between the two types of convection, given by [GRAPHICS] wher e W is the width of the forcing decay region, B-0 is the surface buoya ncy flux. (0) is the radius of the forcing region, f is the Coriolis p arameter, and I-rot = (B-0/f(3))(1/2). If W is less (greater) than 3.2 (l(rot)/r(0))(2/2), then internally (externally) constrained convectio n results. This estimate is obtained for both shallow convection in wh ich the chimney reaches the bottom almost immediately and deep convect ion in which the chimney never reaches the bottom. Furthermore, the tr ansition is independent of the ambient stratification and the total wa ter depth. Calculations made with a primitive equation numerical model support the theoretical ideas and show that the transition between th e two types of convection is smooth and well behaved. The results sugg est that the forcing decay region may be important in ocean convection situations, especially for large forcing regions.