Abstract
Polar lows are intense mesoscale cyclones developing in marine polar air masses. This study presents a new global climatology of polar lows based on the ERA-5 reanalysis for the years 1979–2020. Criteria for the detection of polar lows are derived based on a comparison of six polar-low archives with cyclones derived by a mesoscale tracking algorithm. The characteristics associated with polar lows are considered by the criteria: (i) intense cyclone: large relative vorticity, (ii) mesoscale: small vortex diameter, and (iii) development in the marine polar air masses: combination of low dry-static stability and low potential temperature at the tropopause. Polar lows develop in all marine areas adjacent to sea ice or cold landmasses, mainly in the winter half-year. The length and intensity of the season are regionally dependent. The highest density appears in the Nordic Seas. For all ocean sub-basins, forward-shear polar lows are the most common, whereas weak shear and those propagating towards warmer environments are second and third most frequent, depending on the area. Reverse-shear polar lows and those propagating towards colder environments are rather seldom, especially in the Southern Ocean. Generally, PLs share many characteristics across ocean basins and wind-shear categories. The most remarkable difference is that forward-shear polar lows are often occurring in stronger vertical wind shear, whereas reverse-shear polar lows feature lower static stability. Hence, the contribution to a fast baroclinic growth rate is slightly different for the shear categories.
Highlights
Polar lows (PLs) are intense mesoscale cyclones with a typical diameter of 200 - 500 km that develop when polar air masses advect over open water during the winter season (Rasmussen and Turner, 2003; Renfrew, 2015; Terpstra and Watanabe, 2020)
This study presents a new global climatology of polar lows based on the ERA-5 reanalysis for the years 1979 - 2020
This study presents a new climatology of PLs based on the ERA-5 reanalysis for the years 1979 - 2020
Summary
Polar lows (PLs) are intense mesoscale cyclones with a typical diameter of 200 - 500 km that develop when polar air masses advect over open water during the winter season (Rasmussen and Turner, 2003; Renfrew, 2015; Terpstra and Watanabe, 2020). To be alert of these destructive weather events, it is important to identify where and when PLs form In this context, Stoll et al (2018) has developed the first global climatology of PLs from the ERA-I reanalysis (Dee et al, 2011). The horizontal scale of the polar low is approximately between 200 and 1000 km and has surface winds near or above gale force (15 m s−1).” This definition and recent encyclopedia entries on PLs (Renfrew, 2015; Terpstra and Watanabe, 2020) agree generally on the following PL characteristics: (i) being intense cyclone, (ii) the development in marine polar air masses poleward of the main baroclinic zone, and (iii) the mesoscale size.
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