The influence of the ambient flow on the spreading of convected water masses

We investigate the influence of a cyclonic vortical flow on the lateral spreading of newly mixed fluid generated through localized deep convection. Localized open ocean deep convection often occurs within such a cyclonic gyre circulation, since the associated upwardly domed isopycnals and weaker stratification locally precondition the ocean for deeper convection. In the absence of ambient flow, localized convection has been shown to result in strong lateral fluxes of buoyancy generated by baroclinic instability, sufficient to offset the local surface buoyancy loss and limit the density anomaly of the convectively generated water mass. Here we examine the consequences of a cyclonic ambient flow on this baroclinic instability and lateral mixing. To isolate the influence of the circulation on this later stage of localized convection, we parameterize the convective mixing by the introduction of baroclinic point vortices ("hetons") in a two-layer quasi-geostrophic model, and prescribe the initial flow by a patch of constant potential vorticity. Linear stability analysis of the combined system of pre-existing cyclonic vortex and convectively generated baroclinic vortex indicates scenarios in which the pre-existing cyclonic circulation can modify the baroclinic instability. Numerical experiments with the two-layer QG model show that the effectiveness of the lateral heat fluxes can be strongly diminished by the action of the pre-existing circulation, thereby increasing the density anomaly of the convected water mass.