Marine Heat Waves in the Mediterranean: Heat spreads in deep waters

Marine heat waves in the Mediterranean Sea have become more frequent in the last decade, affecting the regional and European weather system. While extreme warming of the surface ocean is well-documented by remote sensing, the heat transport and modification of the thermohaline structure of the water columns remain elusive. This study examines historical ARGO data (years 2000-2025) and indicates that heat is no longer confined to the upper mixed layers but has penetrated deeper layers. Preliminary analysis shows that after a series of record-breaking surface temperatures, a distinct warming trend has emerged in the deep-water masses at 800–1200 meters in the western Mediterranean Sea. Historically more stable, these depths have warmed by 0.4 °C since 2019 relative to the climatological mean between 2000 and 2015. Data shows a general increase in salinity by 0.1 g kg^-1. The warming and more saline deeper layers are persistent throughout the seasons. In the upper layer (0-200 meters), extreme warming by > 4°C in the summer time led to a decrease in density by 0.5 kg m^-3 compared to the climatological mean. The implications of continued warming, including in the deeper layers, are substantial and include shifts in the Mediterranean Overturning Circulation and thermal persistence, with a long-term "thermal memory". It further affects local and regional weather extremes, with implications for ecosystems and the economy.