An objective climatology of Polar Lows: structure and forcing mechanisms

Abstract

Polar lows are strong mesoscale cyclones occurring during outbreaks of cold arctic air over relatively warm seas. A long-term objective polar low climatology is constructed by identifying and tracking cyclones in a high-resolution dynamical downscaling of ERA-40 covering a part of the North Atlantic, the North Sea and the Nordic Seas. Regions of high polar low occurrence are found to the south of Iceland and in the eastern Norwegian Sea and Barents Sea. In these regions on average 2 polar lows per year can be found within a radius of 200 km. The polar low climatology (~2000 polar low tracks) is used to study polar low structure and forcing mechanisms. Polar low structure is analyzed by calculating for each polar low two different parameters based on the geopotential height fields surrounding the cyclone: thermal asymmetry (frontal / non-frontal) and thermal core (warm / cold). Plotting these parameters against each other shows a wide spectrum with the very symmetric hurricane-like polar lows on the one end of the spectrum and the asymmetric cold-core structures on the other side of the spectrum. It is shown that most polar lows (~70\%) are characterized by a shallow warm core and occur in a forward-shear environment with the thermal wind aligned with polar low motion. Warm core polar lows show a larger pressure-drop before their mature stage than systems with a colder core, which is consistent with existing theories. Also mean values of surface heat fluxes are found to be strongest for the warm-core systems. The large number of polar lows allows for investigating regional variations in the forcing parameters. We find that polar lows in the northern Norwegian Sea are characterized by stronger low-level baroclinicity and a larger air stability than polar lows more to the south. For future studies it would be interesting to include the roles of latent heat release and upper-level disturbances, by using potential vorticity diagnostics.