Marine cold spells in the Baltic sea

Abstract Marine cold spells, characterized by anomalously low sea surface temperatures (SSTs), play a crucial role in shaping marine ecosystems and have significant socioeconomic implications. Unlike the extensively studied marine heatwaves, research on marine cold spells (MCSs) remains limited, particularly in the Baltic Sea, one of the fastest-warming seas globally. Its semi-enclosed nature, strong seasonal variations, and sensitivity to atmospheric–oceanic changes make it an ideal case study region for examining the occurrence and impacts of cold spells under ongoing climate change. This study analyzes MCS characteristics in the Baltic Sea from 1982 to 2023 during the extended summer–autumn season (June–November). The Cold spell occurrences declined, with coastal events being more intense (≈4 °C) than offshore (≈3 °C). The MCS event duration has decreased, especially in the western Baltic Sea. The decline is mainly driven by a sustained SST warming trend of ≈0.05 °C per year, with interannual variability being secondary. Cold spell parameters derived from detrended SST data remove long-term warming and were used to assess cold spell variability. In recent years, coastal regions, especially upwelling zones, have experienced fewer and shorter cold spells, consistent with other regions, but their intensity has significantly increased (0.047 °C per year since 2006). These intensifications may be linked to large-scale atmospheric circulation patterns such as the North Atlantic oscillation, Scandinavian pattern, and East Atlantic pattern.