Preprints
https://doi.org/10.5194/wcd-2021-60
https://doi.org/10.5194/wcd-2021-60

  01 Oct 2021

01 Oct 2021

Review status: this preprint is currently under review for the journal WCD.

A global climatology of polar lows investigated for local differences and wind-shear environments

Patrick Johannes Stoll Patrick Johannes Stoll
  • Department of Physics and Technology, Arctic University of Norway, Tromsø, Norway

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.

Patrick Johannes Stoll

Status: open (until 19 Nov 2021)

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Patrick Johannes Stoll

Patrick Johannes Stoll

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Short summary
Polar lows are small, but intense cyclones and constitute one of the major natural hazards in the polar regions. To be alert when and where polar lows occur, this study maps polar lows globally by utilizing new atmospheric datasets. Polar lows develop in all marine areas adjacent to sea ice or cold landmasses, mainly in the winter half-year. The highest frequency appears in the Nordic Seas. Further, it is found that polar lows are rather similar in the different ocean sub-basins.