Heat content and temperature trends in the Mediterranean Sea as derived by Argo float data (2005 – 2020)

The Mediterranean Sea is very sensitive to climatic changes due to its semi-enclosed nature and is defined as one of the hotspots in future climate change projections. In this study we use Argo float data to describe spatial variabilities and trends in Ocean Heat Content (OHC) within the entire Mediterranean Sea and for specified sub-basins (e.g. Western and Eastern Mediterranean, Gulf of Lion, South Adriatic). The amount of the OHC, spatially averaged in bins of 1 °x1 ° over the period 2001-2020, increases from west to east in the Mediterranean Sea.
Time series of temperature and OHC from 2005 to 2020, estimated in the upper and intermediate layers (5-700 m) and deeper layer (700-2000 m), reveal significant warming trends and an increase of OHC: the upper 700 m of the Mediterranean Sea show a warming trend of 0.041±0.012 °Cyr^-1, corresponding to a yearly increase in OHC of 3.59±1.02 Wm^-2. The upper 700 m of the Western Mediterranean Sea are warming fastest with an increase in temperature at a rate of 0.070±0.015 °Cyr^-1, corresponding to a yearly increase in OHC of 5.72±1.28 Wm^-2.

Mixing and convection events transport and disperse the temperature and OHC changes: significant warming trends are evident in the deeper layers (700-2000 m) of the two deep convection sites in the Mediterranean Sea (Gulf of Lion, South Adriatic), with an exceptionally strong warming trend in the South Adriatic from 2013 to 2020 of  0.058±0.005 °Cyr^-1, corresponding to a yearly increase in OHC of 9.43±0.85 Wm^-2. 

The warming of different water masses will show its feedback on ocean dynamics and the atmosphere (air-sea fluxes) in the next years, decades or even centuries when these warming waters spread or re-emerge. This will stress ecosystems and accelerate the extinction of several marine species. This study contributes to a better understanding of climate change in the Mediterranean region and should be another wake-up call for policy makers and society.