The effects of a surface stress-driven ambient circulation on open ocean convection

Preconditioned regions of the open ocean are usually characterised by local ised cyclonic gyres that make these areas susceptible to deep convection. I n this study the effect of the ambient circulation on the formation and sub sequent lateral spreading of deep water formed during open ocean convection was investigated in the laboratory. Oceanic convection was modelled using a circular tank mounted on a rotating table. For experimental convenience t he ambient circulation was produced by a differentially rotating false bott om and convection was initiated by heating from below, rather than cooling from above. The dominant mechanisms for lateral spreading of the warm conve ctive fluid varied depending on the speed of the bottom rotation. Lateral t ransport of heat was dominated by the propagation of baroclinic eddies from the convective region into the ambient region for low bottom rotation rate s and by Ekman transport for high bottom rotation rates. The cyclonic flow field hindered lateral transport by baroclinic instabilities in the laborat ory, thereby weakening the lateral transport of heat by this mechanism. At the same time, the efficiency of Ekman transport as a means for lateral hea t or buoyancy flux increased as bottom rotation rate increased. Together, t hese two processes resulted in a slight increase in the lateral density ano maly at steady state with increasing bottom rotation rate for anl Ohm < 0.4 and a rapid decrease in the lateral density anomaly for Delta Ohm/Ohm > 0. 4, where Delta Ohm is the rotation rate of the false bottom and Ohm is the planetary rotation rate.