Simulations of Topographically Forced Mesocyclones in the Weddell Sea and the Ross Sea Region of Antarctica

Abstract

Mesocyclones (MCs) are a frequently observed phenomenon in the coastal regions of Antarctica. Numerical simulations of topographically forced MCs in the Weddell Sea and the Ross Sea region are presented using a three-dimensional mesoscale weather forecast model. Simulations are performed for an idealized case without synoptic forcing, three realistic cases of smaller MCs with diameters of 200?300 km, and two larger systems with diameters of up to 1000 km. The simulation results show that the orography of the coastal regions can play an important role in mesocyclogenesis. One key factor is the katabatic wind system, which is able to initiate low-level MCs in areas of suitable orography structure. The second key factor is the support of the synoptic environment, leading to vorticity production by vertical stretching of the synoptically supported katabatic winds. Besides this stretching mechanism, katabatic winds can have a second impact on the generation of MCs by transporting cold air into the coastal areas and thereby enhancing the low-level baroclinicity. A large fraction of short-lived coastal MCs seem to be generated by these mechanisms. For larger-scale and long-lived MCs, the amplification of a near-surface perturbation is found to occur in association with the approach of an upper-level potential vorticity anomaly. The initial low-level perturbations, however, seem to be strongly connected to specific topographical features. Since satellite-based climatologies only include MCs associated with cloud formation, a considerable fraction of topographically forced MCs will remain undetected in areas without a mesoscale observational network.