The occurrence of coreless winters in the South Shetland Islands region is related to increase in the intensity of cyclonic circulation and to the presence of massive and rapid advection of warm air northerly and westerly. Coreless winter developments depend on large-scale oceanic processes – the presence of positive anomalies in sea surface temperature (SST) in the Bellingshausen Sea over the range 080°–092°W and the retreat of sea ice extent southwards. When negative anomalies of SST in the same region are observed and the sea ice extent advances northwards, a winter with clearly marked cold core is experienced at the Arctowski Station on the South Shetlands.
The coreless winters ( i.e. not having a cold core) were distinguished in four stations within the European sector of the Arctic. Anomalies of the frequency of the Niedźwiedź’s (2011) circulation types were calculated separately for the mid−winter warm months and for cold months preceding and following the warm−spells. Furthermore, composite and anomaly maps of the sea level pressure as s well as anomaly maps of the air temperature at 850 gpm (geopotential meters) were constructed separately for the mid−winter warm events and for the cold months before and after warming. Different pressure patterns were recognized among the days of mid−winter warm spells, using the clustering method. The occurrence of coreless winters in the study area seems to be highly controlled by the position, extension and intensity of large scale atmospheric systems, mainly the Icelandic Low. When the Low spreads to the east and its centre locates over the Barents Sea the inflow of air masses from the northern quadrant is observed over the North Atlantic. This brings cold air of Arctic origin to the islands and causes an essential drop in the air temperature. Such situation takes place during the cold months preceding and following the warm mid−winter events. During the warm spells the Icelandic Low gets deeper−than−usual and it is pushed to the northeast, which contributes to the air inflow from the southern quadrant.