Climatology and extreme cases of sea‐effect snowfall on the southern Baltic Sea coast

Abstract

Heavy snowfall events of possible sea‐effect origin, occurring on the southern Baltic Sea coast, are described in this study. The analysed region is not very snowy, with an average of 40 days with snow cover in the winter season and a mean maximum snow depth of 8–12 cm. Twenty‐five snowfall events were selected using the threshold of 20 cm/2 days (i.e., 20 cm of snow depth increase during 2 days), and the synoptic climatology, as well as triggering factors of their occurrence, were defined. The bipolar pattern of sea level pressure with a high‐pressure over Scandinavia and a low‐pressure system southward of it was identified as favourable for sea‐effect snowfall on the southern Baltic Sea coast. This pressure field, causing northeastern circulation in the lower troposphere, brings cold air masses over the relatively warm sea basin, and the negative anomalies of air temperature at 850 hPa geopotential level, amounting to −9°C, encompass Scandinavia, as well as central and northeastern Europe. This makes the temperature difference between the air at the 850 hPa geopotential height and the sea surface exceeding 15°C. This large temperature difference establishes favourable conditions for convection in cold air masses, which become unstable while crossing over the warm sea. The formed clouds give snowfall in the coastal areas. In the further part of the study, three sea‐effect snowfall cases were chosen for detailed analysis, including atmospheric sounding, radar and satellite images, air parcel backward trajectories, and synoptic maps. They reveal high‐pressure wedges associated with low air temperature over Scandinavia and northern airflow over the Baltic Sea. The wind direction and the temperature of air masses are key components in the occurrence of sea‐effect snowfall, and global warming conditions seem to reduce the frequency of snow events over the Polish Baltic Sea coast.

Climatology of sea‐effect snow in Finland

“…Recent study by Bednorz et al . (2022) produced climatology of extreme cases of sea‐effect snowfall on the southern Baltic Sea coast. They found out that temperature difference between SST and T850 was over 15°C when snowfall accumulated at least 20 cm/2 day of new snow.…”

“…Climatology of extreme snowfall cases (>20 cm/2 day) on the Polish Baltic Sea coast for 1981–2020 revealed that most of those cases resulted from sea‐effect snowfall (Bednorz et al ., 2022). In addition, occurrence of atmospheric parameters favouring snowband formation in the east coast of Sweden have been studied from 1970 to 2099 with the aid of a coupled regional atmosphere‐ice‐ocean model RCA4‐NEMO (Dieterich et al ., 2020).…”