Polar lows over the Nordic and Labrador Seas: Synoptic circulation patterns and associations with North Atlantic‐Europe wintertime weather regimes

Abstract

[1] Polar Lows (PLs) are intense meso-cyclones forming in winter at high latitudes over open water. Using several datasets of PLs over the North Atlantic, the synoptic environment conducive to their development is determined. The 500 hPa geopotential height, the difference between the sea surface temperature and the 500 hPa air temperature, the near-surface wind and air temperature, and the 300 hPa potential vorticity present significant anomaly patterns over large areas centered over PL genesis zones, suggesting cold air outbreaks and stratospheric intrusions. PLs develop within a northerly flow in the Norwegian Sea, a northeasterly flow in the Barents Sea and a westerly flow in the Labrador Sea. PLs form after a certain build-up, the outbreak day being marked by strong winds and PV intensification. The relationship between PLs and daily weather regimes over North Atlantic-Europe is then investigated. Regimes have a typical lifetime of 8-10 days, similar to the large-scale anomalies associated with PLs. Over the Norwegian and Barents seas from 1999 to 2011, 37% of PLs are observed during the Atlantic Ridge regime (AR) and 28% in the negative phase regime of the North Atlantic Oscillation (NAO), whereas PL probability of occurrence is reduced by half for the positive phase of NAO and the Scandinavian blocking (SB) regimes. Over the Labrador Sea, most PLs occur during NAO+ while they are almost absent during NAOÀ. Demonstrating the temporal variation of key factors based on an updated dataset and relating PLs to weather regimes will introduce novel and important elements in PL forecasting methodology.

Polar low tracks over the Nordic Seas: a 14-winter climatic analysis

“…Figure 8 displays the diagnosed 14-winter PL tracks over the Nordic Seas according to WRs. Mallet et al (2013) showed a relationship between WRs and PL occurrence, but Norwegian and Russian coasts, and possibly make landfall more often than during other WR configurations. A more detailed analysis of links between WRs and PL directions will be provided in Section 3.2.…”

“…Both the negative and positive phases of NAO appear more favourable for development of multiple PL events than the AR and SB WRs. This difference is particularly evident for the AR and NAO (; the most favourable regimes for PL formation in general (Mallet et al, 2013). Only 8% of multiple events occurred in the AR configuration while 40% of multiple systems occurred during NAO (.…”

“…This product yields SST at 0.058 latitude )0.058 longitude horizontal resolution, using in-situ data and satellite retrievals from both TIR and microwave radiometers. Mallet et al (2013) documented the associations of observed PLs in the Noer and Lien (2010) shorter list (1999Á 2009) with North AtlanticÁEurope WRs over the Nordic Seas. The four most common large-scale flow patterns pertinent to the cold season (NovemberÁMarch) are as follows: Scandinavian Blocking (SB); the positive and negative phases of the North Atlantic Oscillation (NAO); and a pronounced ridge over the Atlantic (Atlantic Ridge; AR) (e.g.…”

“…Certain synoptic situations favour PL development, particularly those promoting instability due to condensational heating and sea surfaceÁair heat fluxes, and within cold upper-lows that trigger positive vorticity advection (e.g. Businger, 1985;Claud et al, 2007;Blechschmidt et al, 2009;Mallet et al, 2013 for the Nordic Seas). *Corresponding author.…”

“…Accordingly, this paper presents a 14-winter climatic analysis of PL tracks for the Nordic Seas, with a statistical description of PL dominant characteristics that includes their associations with 'weather regimes' (WRs) (Mallet et al, 2013) and also considers their inter-annual variability. Attributes of dual and multiple PLs are analysed and presented.…”