Winter dense water formation and marine cold spells in the Northwestern Mediterranean: Multi-scale dynamics and implications for dense shelf water cascading

Dense Shelf Water (DSW) cascading in the northwestern Mediterranean Sea is an important winter phenomenon for the ventilation of deep-water ecosystems and modulation of the physical and chemical properties of deep waters. This study combines reanalysis data and recent observations to explore the drivers of DSW formation and cascading, across multiple spatial and temporal scales.

At the subsynoptic scale, in the Gulf of Lion area, the action of cold and dry northern winds, named Tramuntana, and freshwater inputs from rainfall, rivers, and continental runoff regulate the shelf water density. At the synoptic scale, persistent winds and the intrusion of cold air masses from continental Europe can induce Marine Cold Spells (MCS), which are extreme events marked by sustained periods of below-average sea surface temperatures. Notably, MCS occurring around mid-February, when shelf water reaches peak density, are closely linked to DSW formation.

On a larger scale, variability in the East Atlantic (EA) climate mode influences the frequency, persistence, and intensity of cold Tramuntana winds, connecting regional ocean-atmosphere interactions to broader climatic oscillations. Additionally, the combined negative phases of EA and North Atlantic Oscillation climate modes contribute to the formation of denser shelf water.

Amid recent trends of reduced Tramuntana wind intensity, rising shelf water temperatures, and a decline in MCS frequency, an analysis of all the factors contributing to DSW formation and cascading is key to understanding its future. These insights, in turn, will help to anticipate its impact on deep-water circulation and ventilation, biodiversity and functioning of the deep ecosystems.