The Mediterranean Sea is very sensitive to climatic changes due to its semi-enclosed nature and is therefore defined as one of the hotspots in future climate change projections. In this study, we use Argo float data to assess climatologies and trends in temperature and Ocean Heat Content (OHC) throughout the Mediterranean Sea and for specific 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 from 2001 to 2020, increases from west to east in the Mediterranean Sea. Time series of temperature and OHC from 2005 to 2020, estimated in the surface 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 temperature trend of 0.041 ± 0.012°C·yr^-1, corresponding to an annual increase in OHC of 3.59 ± 1.02 W·m^-2. The Western Mediterranean Sea (5-700 m) is warming fastest with an increase in temperature at a rate of 0.070 ± 0.015°C·yr^-1, corresponding to a yearly increase in OHC of 5.72 ± 1.28 W·m^-2. Mixing and convection events within convection sites and along boundary currents 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°C·yr^-1, corresponding to a yearly increase in OHC of 9.43 ± 0.85 W·m^-2. The warming of the different water masses will show its feedback on ocean dynamics and air-sea fluxes in the next years, decades, and even centuries as these warming waters spread or re-emerge. This will provide more energy to the atmosphere, resulting in more extreme weather events and will also 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 act as another wake-up call for policy makers and society.

Levantine intermediate water (LIW) is formed in the Levantine Sea (Eastern Mediterranean) and spreads throughout the Mediterranean at intermediate depths, following the general circulation. The LIW, characterized by high salinity and relatively high temperatures, is one of the main contributors of the Mediterranean Overturning Circulation and influences the mechanisms of deep water formation in the Western and Eastern Mediterranean sub-basins. In this study, the LIW and Levantine deep water (LDW) formation processes are investigated using Argo float data from 2001 to 2017 in the Northwestern Levantine Sea (NWLS), the larger area around Rhodes Gyre (RG). To find pronounced events of LIW and LDW formation, more than 800 Argo profiles were analyzed visually. Events of LIW and LDW formation captured by the Argo float data are compared to buoyancy, heat and freshwater fluxes, sea surface height (SSH), and sea surface temperature (SST). All pronounced events (with a mixed layer depth (MLD) deeper than 250 m) of dense water formation were characterized by low surface temperatures and strongly negative SSH. The formation of intermediate water with typical LIW characteristics (potential temperature > 15 °C, salinity > 39 psu) occurred mainly along the Northern coastline, while LDW formation (13.7 °C < potential temperature < 14.5 °C, 38.8 psu < salinity < 38.9 psu) occurred during strong convection events within temporary and strongly depressed mesoscale eddies in the center of RG. This study reveals and confirms the important contribution of boundary currents in ventilating the interior ocean and therefore underlines the need to rethink the drivers and contributors of the thermohaline circulation of the Mediterranean Sea.

Addressing the need for clear communication in Mediterranean oceanography, this study presents a revised and comprehensive catalogue of water mass acronyms. Although there is no universal set of standard rules for assigning acronyms to water masses, as these can vary among different research groups, regions, or scientific communities, scientists generally aim for clarity and consistency when naming and abbreviating water masses to facilitate communication within the scientific community. The team of experts, guided by the CIESM (the Mediterranean Science Commission) C2 Committee on the Physics and Climate of the Ocean, reviewed existing literature and established new acronyms and naming guidelines. This standardized system, emphasizing clarity and consistency, replaces the current variable practices employed by different research groups.

The Mediterranean marine heat wave (MHW) during the warm season (May–September) and the fall period (October–December) of 2022 is analyzed using Argo float in situ observations in the upper 2000 m of depth. The five study regions (North Western Mediterranean, South Western Mediterranean, central Ionian Sea, Pelops Gyre, and South Adriatic Pit) most affected by warming in different layers were selected and investigated. The primary goal is to provide insights into how the water column responds to the onset and progression of the MHW during the warming period, characterized by peak stratification and reduced vertical mixing. Additionally, this study aims to examine how the heat accumulated in the upper layers is redistributed to deep layers within regions with different dynamic characteristics through advection and/or mixing during the subsequent fall period. Temperature anomaly profiles, T_a (z), computed for each area and for both periods were divided into three categories based on vertical heat penetration: Category 1 (shallow, 0–150 m), Category 2 (intermediate, 150–700 m), and Category 3 (deep, >700 m). During the warm season, Category 1 profiles had a temperature anomaly near zero or slightly negative in a thin layer between 50 and 150 m depth, while warming was observed in the 0–50 m layer and below the middle layer. Profiles characterized by greater vertical heat penetration (categories 2 and 3) were mainly in mesoscale or subbasin structures and showed the largest positive temperature anomaly in the surface and intermediate layers. All profile categories showed a warming between 200 and 800 m depth. This increase is roughly split, with half attributed to the impact of the 2022 MHW and the other half linked to the ongoing long-term trend in ocean temperatures. During the fall period and in the layer below 200 m depth, the shapes of the T_a profiles are similar for all sectors with the exception of the South Adriatic Pit, which depicts a +0.5 °C warming at 800 m depth. The present work highlights the warming characteristics throughout the entire water column across different regions of the Mediterranean Sea, and it seeks to connect the impacts of the warm season on the cold period with oceanic dynamic processes, such as dense water formation, upwelling, or water column stratification. These regions are characterized by dynamic activities (e.g., dense water formation, upwelling); therefore, any variation in these ocean processes can influence the thermohaline circulation and, consequently, the climate system.

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.

The Mediterranean marine heatwave during the warm season (May–September) and the fall period (October–December) of 2022 is analysed using Argo floats in-situ observations in the upper 2000 m of depth. Based on the 2022 ocean heat content anomaly, five study regions (North Western Mediterranean, South Western Mediterranean, central Ionian Sea, Pelops Gyre, southern Adriatic Sea) most affected by warming in different layers were selected and investigated. Temperature anomaly profiles T’(z) computed for each area and for both periods, were divided into three categories based on vertical heat penetration: Category 1 (shallow, 0–150 m), Category 2 (intermediate, 150–700 m) and Category 3 (deep, > 700 m). Category 1 profiles had a temperature anomaly near zero or slightly negative in a thin layer between 50 m and 150 m depth, while warming was observed below the middle layer. Profiles characterized by greater vertical heat penetration (categories 2 and 3) were mainly in mesoscale or sub basin structures and showed the largest positive temperature anomaly in the surface layer and thermocline. All profile categories showed warming between 200 and 800 m depth. During the fall period all sectors show similar warming in the layer below 200 m depth, except for the SAP, which records an anomalous warming in the intermediate and deep layer. The present work highlights the warming characteristics along the entire water column in different regions of the Mediterranean Sea, some of which are characterised by dynamic activities. This sheds light on possible scenarios changes with implications on the variation of the ocean processes that regulate the thermohaline circulation and thus, the climate system.