Importance of Latent Heating in Mesocyclones for the Decay of Cold Air Outbreaks: A Numerical Process Study from the Pacific Sector of the Southern Ocean

Abstract

Abstract In this study the dynamical mechanisms shaping the evolution of a marine cold air outbreak (CAO) that occurred over the Ross, Amundsen, and Bellingshausen Seas in June 2010 are investigated in an isentropic framework. The drainage of cold air from West Antarctica into the interior Ross Sea, its subsequent export, and the formation of a dome of cold air off the sea ice edge are shown to be intimately linked to a lower-tropospheric cyclone, as well as the cyclonic breaking of an upper-level potential vorticity trough. The dome formation is accompanied by an extreme deepening of the boundary layer, whose top reaches to the height of the low-lying tropopause within the trough, potentially allowing for deep stratosphere–troposphere exchange. A crucial finding of this study is that the decay of the CAO is essentially driven by the circulation associated with a train of mesocyclones and the release of latent heat in their warm sectors. Sensitivity experiments with switched off fluxes of sensible and latent heat reveal that the erosion of the CAO air mass depends critically on the moistening by latent heat fluxes, whereby the synergistic effects of sensible heat fluxes and moist processes amplify the erosion. Within the CAO air mass, the erosion is inhibited by cloud-top radiative cooling and the dissolution of clouds by the entrainment of dryer air. These findings potentially have implications for the representation of CAOs in coarse-resolution climate models.