Simulations of Subtropical Cyclones in a Baroclinic Channel Model

Abstract

Abstract The present study considers a variety of cyclone developments that occur in an idealized, baroclinic channel model featuring full condensation heating effects over an ocean with prescribed sea surface temperature variation. The geostrophic basic-state jet is specified by the tropopause shape, and horizontal shear is included by specifying the meridional variation of zonal wind on the lower boundary. The horizontal shear induces anticyclonic wave breaking of baroclinic waves. Normal mode perturbations are computed using a “fake-dry” version of the model but integrated forward using full physics. Low-latitude moist convection is particularly strong in simulations with strong surface easterlies that destabilize the troposphere through water vapor fluxes from the ocean surface. Deep convection produces a locally elevated dynamic tropopause and an associated anticyclone. This modified zonal flow supports moist baroclinic instability. The resulting cyclones, identified as subtropical cyclones, occur in deep westerly vertical wind shear but are nearly devoid of lower-tropospheric baroclinicity initially. These systems are distinguished from baroclinically dominated secondary cyclones that also form at relatively low latitudes in the simulations. For weak jets and strong subtropical surface easterlies, subtropical cyclone development dominates formation on the midlatitude jet. For strong westerly jets or weak horizontal shear, the situation is reversed and the midlatitude baroclinic wave can help or hinder the ultimate intensification of the subtropical cyclone. The similarity of this cross-latitude influence to the extratropical transition of tropical cyclones is noted.