Associations between large‐scale atmospheric circulation and polar low developments over the North Atlantic during winter

Abstract

Polar lows are intense mesoscale weather systems which develop in response to combinations of upper tropospheric vorticity advection, low‐level baroclinicity near the sea ice‐ocean margin and strong air‐sea thermodynamical fluxes. In this paper, an evaluation of the associations between dominant patterns of low‐frequency variability in the atmospheric circulation and polar low developments over the North Atlantic during winter is presented. Because polar lows are rather poorly represented in reanalyses, the temperature (T500) and geopotential height at 500 hPa, the wind at 925 hPa, the SST, and the difference (SST‐T500), which are critical for their formation, are considered. The North Atlantic Oscillation (NAO) has a strong impact on all these variables, but it is contrasted spatially and temporally. Polar low formation is stimulated in the Labrador Sea for a positive NAO phase, while over the Norwegian Sea, there is an increase of polar lows forming on the flank of synoptic storms. Still over the Norwegian Sea, higher SST from January on favor the formation of polar lows of predominantly convective nature, while to the east of Greenland, the reduction of sea ice plus the northerly low‐level flow lead to a larger number of polar lows. Other teleconnection patterns also exhibit associations with polar low developments. The Scandinavia pattern shows a strong negative association with the studied variables over the Norwegian and Barents seas, but only marginally affects the Labrador Sea. The formation of polar lows is encouraged over the Norwegian and Greenland seas when the Polar/Eurasia pattern is in its negative phase.