Abstract
Estimating long-term modifications of the sea surface temperature (SST) is crucial for evaluating the current state of the oceans and to correctly assess the impact of climate change at regional scales. In this work, we analyze SST variations within the Mediterranean Sea and the adjacent Northeastern Atlantic box (west of the Strait of Gibraltar) over the last 37 years, by using a satellite-based dataset from the Copernicus Marine Environment Monitoring Service (CMEMS). We found a mean warming trend of 0.041 ± 0.006 ∘ C/year over the whole Mediterranean Sea from 1982 to 2018. The trend has an uneven spatial pattern, with values increasing from 0.036 ± 0.006 ∘ C/year in the western basin to 0.048 ± 0.006 ∘ C/year in the Levantine–Aegean basin. The Northeastern Atlantic box and the Mediterranean show a similar trend until the late 1990s. Afterwards, the Mediterranean SST continues to increase, whereas the Northeastern Atlantic box shows no significant trend, until ~2015. The observed change in the Mediterranean Sea affects not only the mean trend but also the amplitude of the Mediterranean seasonal signal, with consistent relative increase and decrease of summer and winter mean values, respectively, over the period considered. The analysis of SST changes occurred during the “satellite era” is further complemented by reconstructions also based on direct in situ SST measurements, i.e., the Extended Reconstructed SST (ERSST) and the Hadley Centre Sea Ice and Sea Surface Temperature dataset (HadISST), which go back to the 19th century. The analysis of these longer time series, covering the last 165 years, indicates that the increasing Mediterranean trend, observed during the CMEMS operational period, is consistent with the Atlantic Multidecadal Oscillation (AMO), as it closely follows the last increasing period of AMO. This coincidence occurs at least until 2007, when the apparent onset of the decreasing phase of AMO is not seen in the Mediterranean SST evolution.
Highlights
Sea surface temperature (SST) is one of the essential climate variables (ECVs), as defined by the Global Climate Observing System (GCOS): it plays a key role in regulating climate and its variability
Examples include the interannual fluctuations in the eastern equatorial Pacific mean SST, identified with the El Niño Southern Oscillation (ENSO) [7], a tripole of SST anomalies in the North Atlantic [8] associated to the North Atlantic Oscillation (NAO) [9], and the multidecadal SST fluctuation in the North Atlantic domain associated to the Atlantic Multidecadal Oscillation (AMO) [10]
Comparing the Copernicus Marine Environment Monitoring Service (CMEMS) satellite-based data to the longer HadISST and Extended Reconstructed SST (ERSST) timeseries, we found that the Mediterranean warming trend from 1982 to 2018 is approximately 3.7 times higher than the global ocean warming trend from 1980 to 2005 (=0.011 ◦C/year, [12])
Summary
Sea surface temperature (SST) is one of the essential climate variables (ECVs), as defined by the Global Climate Observing System (GCOS): it plays a key role in regulating climate and its variability. From seasonal to longer timescales (i.e., interannual, decadal, and multidecadal), SST responds to both natural climate variability [1] and human-driven climate change [2]. SST annual cycle is the dominant oscillation induced by the solar forcing. On seasonal timescales, a change in the seasonal amplitude/phase can substantially contribute to the interannual variability in mean temperatures [6]. On interannual and (multi-)decadal timescales, the nonseasonal or slow variation in SST is modulated by atmospheric and oceanic circulation variability that leads to quasi-periodic oscillations and/or dominant patterns. SST is a key parameter to track climate change, monitor and characterize the state of the global climate system and contribute to the definition of the present state of the oceans, at both global and regional scales
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