Abstract
To develop a 14-winter (October–April; 1999–2013) climatic description of polar low (PL) occurrence for the Nordic Seas, systems have been tracked using images acquired from the Very High Resolution Radiometer (AVHRR). Also, the dominant PL characteristics – their temporal and spatial distributions, size, lifespan, distance travelled, speed of propagation and directions – have been determined. On average, 14 PL events occur per winter but there is strong inter-annual and intra-seasonal variability. Although systems may form and travel over the whole Nordic Seas, their genesis is enhanced in areas characterised by warm oceanic currents. At the start of the season (October–November), systems mainly form over the Greenland and Norwegian Seas, but further into winter they form increasingly over the Barents Sea. In connection with recent low-ice winters, new areas of PL formation are evident, particularly to the west of Spitsbergen and in the Barents Sea. PL speeds of propagation range between 5 and 13 m/s but are observed to be highly variable among cases and even during the lifespan of individual PLs. To a considerable extent, the direction of movement is controlled by the large-scale flow in the lowest atmospheric layers, but we also observed cyclonic co-rotation of some pairs of PLs due to their influence on the ambient flow. Although these generally move southward or southeastward, a substantial number of PLs have westward and even northward tracks. PLs in the western part of the region average larger than their eastern counterparts. This study also highlights that PLs characteristics and tracks differ according to weather regimes.
Highlights
IntroductionPolar mesoscale cyclones in cold air streams occur frequently over the Nordic Seas
During the winter season, polar mesoscale cyclones in cold air streams occur frequently over the Nordic Seas
polar low (PL) formation over the Nordic Seas is characterised by strong inter-annual variability (Fig. 3)
Summary
Polar mesoscale cyclones in cold air streams occur frequently over the Nordic Seas. The proximity of relatively warm open water to the cold snowcovered land masses and sea-ice, offer favourable conditions for the formation of these polar lows (PLs), which are the most intense of such mesoscale systems (Rasmussen and Turner, 2003). There are several definitions of PLs, the following one is adopted in this study: ‘A PL is a small but fairly intense low in cold air outbreaks (CAO) well north of the polar front, with a cyclonic cloud structure and a PLs over Nordic Seas generally develop in zones of lowaltitude baroclinic instability (e.g. near the ice margin of the eastern coast of Greenland and along the Barents ice edge), especially where there are large differences between air and sea-surface temperatures during CAO
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