Polar Lows (PL)
Polar Lows
At high latitudes, favorable conditions for the developments of severe meso-cyclones or Polar Lows (PL), are expected to occur less often (Zahn and von Storch, 2013, Mallet et al., 2017, Landgren et al., 2019). Stringent geostrophic constraints and larger static stability may impose very large temperature differences between the ocean and the mid troposphere, i.e. O(40°) (Rasmussen & Turner, 2003). PLs are then often associated with cold air outbreaks, developing over the relatively warmer ocean, and form poleward of the main baroclinic zone. Sometimes referred to as Arctic hurricanes, PLs are short-lived (less than 48 h) and small-scale (less than 1000 km) cyclones. Some studies thus predict the decrease in both number of PLs in the Arctic and their intensity in the future due to the faster air temperature increase relative to water temperature and therefore higher stability of the atmosphere. Yet, so far, reported climatological studies did not prove out these predictions: small positive trends are found both for the PLs over the Nordic and Barents Seas (Smirnova et al., 2015; Rojo et al., 2015) and over the North Pacific seas (Chen and Storch, 2013).
In Smirnova et al., 2015, a high correlation is reported between the sea ice extent and the number of identified polar lows. Some other results show that because of the decrease in sea-ice extent over the Pacific sector of the Arctic during intermediate seasons, the Chukchi Sea might also see more PL developments especially in the fall season (e.g., Inoue et al., 2010). While the stability of the troposphere is expected to diminish the number and intensity of PLs (Zahn and von Storch, 2013), the rapid evolution of the Arctic sea ice extent further opens new regions for PL formation. These counteracting effects are confirmed by recent findings of new areas of PL formation over the Nordic Seas (Rojo et al., 2015) and the appearance of new open water regions potentially suitable for polar-low development (Comiso et al., 2008; Ivanov et al., 2013). Seas, such as East Siberian Sea, Chukchi Sea, and Laptev Sea, previously covered by ice throughout the whole year lose their ice cover by autumn and are thus subject to polar cyclone development (Zabolotskikh et al., 2016).
In the Nordic and Barents Seas, the number of polar lows is however increasing, following a relatively quiet period of 2001-2007 (Stoll et al., 2018). Although in the late XXIst century the polar low activity is projected to decrease due to the faster Arctic warming (Zahn and Von Storch, 2010), they are still becoming an important threat to the rapidly expanding economic activity in the Arctic.