THE INFLUENCE OF THE AMBIENT FLOW ON THE SPREADING OF CONVECTED WATERMASSES

We investigate the influence of a cyclonic vortical flow on the latera l spreading of newly mixed fluid generated through localized deep conv ection. Localized open ocean deep convection often occurs within such a cyclonic gyre circulation, since the associated upwardly domed isopy cnals and weaker stratification locally precondition the ocean for dee per convection. In the absence of ambient flow, localized convection h as been shown to result in strong lateral fluxes of buoyancy generated by baroclinic instability, sufficient to offset the local surface buo yancy loss and limit the density anomaly of the convectively generated water mass. Here we examine the consequences of a cyclonic ambient fl ow on this baroclinic instability and lateral mixing. To isolate the i nfluence of the circulation on this later stage of localized convectio n, we parameterize the convective mixing by the introduction of barocl inic point vortices (''hetons'') in a two-layer quasi-geostrophic mode l, and prescribe the initial flow by a patch of constant potential vor ticity. Linear stability analysis of the combined system of pre-existi ng cyclonic vortex and convectively generated baroclinic vortex indica tes scenarios in which the pre-existing cyclonic circulation can modif y the baroclinic instability. Numerical experiments with the two-layer QG model show that the effectiveness of the lateral heat fluxes can b e strongly diminished by the action of the pre-existing circulation, t hereby increasing the density anomaly of the convected water mass.