An objective global climatology of polar lows based on reanalysis data

Abstract

Here we present an objective global climatology of polar lows. In order to obtain objective detection criteria, the efficacy of several parameters for separating polar lows from other cyclones has been compared. The comparison and the climatology are based on the ERA‐Interim reanalysis from 1979 ‐ 2016 and the high‐resolution Arctic System Reanalysis from 2000 ‐ 2012. The most effective parameters in separating polar lows from other extra‐tropical cyclones were found to be the difference between the sea‐level pressure at the centre of the low and its surroundings, the difference in the potential temperature between the sea surface and the 500 hPa level, and the tropopause wind speed poleward of the system. Other parameters often used to identify polar lows, such as the 10 m wind speed and the temperature difference between the sea surface and the 700 hPa level, were found to be less effective. The climatologies reveal that polar lows occur in all marine basins at high latitudes, but with high occurrence density in the vicinity of the sea‐ice edge and coastal zones. The regions showing the highest degree of polar‐low activity are the Denmark Strait and the Nordic Seas, especially for the most intense polar lows. In the North Atlantic and Pacific, the main polar‐low season ranges from November to March. In the Southern Hemisphere, polar lows are mainly detected between 50 ‐ 65 S from April to October, indicating that this hemisphere compared to its northern counterpart has a two months longer, but less intense, polar‐low season. No significant hemispheric long‐term trends are observed, although some regions, such as the Denmark Strait and the Nordic Seas, experience significant downward and upward trends in polar lows, respectively, over the last decades. For intense polar lows, a significant decaying trend has been observed for the Northern Hemisphere.