On the drivers of Mediterranean Marine Cold Spells under Climate Change

Abstract. Marine Cold Spells (MCSs) are prolonged, extreme cold temperature events in the ocean that can disturb marine ecosystems. Here, we analyze for the first time historical (1980–2014) and future (2015–2100) MCSs in the Mediterranean Sea, using the output from a high-resolution regional climate system model, under global warming levels 1–4°C and the 25 high-emission climate change scenario SSP5-8.5. Historically, atmosphere forcing dominates open-ocean MCSs in all seasons, except winter, where Advection plays a more dominant role across the basin, reflecting the influence of alongshore currents and mesoscale variability in dynamically active regions. Mixing drives coastal and localized events mostly in the summer, likely due to regional wind forcing. As global temperatures rise, projections relative to a fixed historical baseline indicate up to 92 % decline in future MCS occurrence, up to a 50 % reduction in their duration and up to 19 % decrease in 30 their intensity already by 2036–2055. These changes are more pronounced during winter, with the events becoming increasingly confined to the northern Mediterranean and progressively disappearing. When defined relative to a shifted climatology, future MCSs represent extreme cold events of a warmer climate and are projected to become less frequent, slightly longer and more intense compared to the past by the end of the century. The driving mechanisms of both types of future MCSs shift from a dominant atmospheric control in the historical period towards an increasing influence of oceanic 35 advection and mixing, particularly in the summer, autumn and spring and by the end of the century.