Mediterranean Sea climatic indices: monitoring long-term variability and climate changes

Athanasia Iona,Charles Troupin,Sarantis Sofianos,Athanasios Theodorou,Sylvain Watelet,Jean-Marie Beckers

doi:10.5194/essd-10-1829-2018

Abstract

Abstract. We present a new product composed of a set of thermohaline climatic indices from 1950 to 2015 for the Mediterranean Sea such as decadal temperature and salinity anomalies, their mean values over selected depths, decadal ocean heat and salt content anomalies at selected depth layers as well as their long time series. It is produced from a new high-resolution climatology of temperature and salinity on a 1∕8∘ regular grid based on historical high-quality in situ observations. Ocean heat and salt content differences between 1980–2015 and 1950–1979 are compared for evaluation of the climate shift in the Mediterranean Sea. The two successive periods are chosen according to the standard WMO climate normals. The spatial patterns of heat and salt content shifts demonstrate that the climate changes differently in the several regions of the basin. Long time series of heat and salt content for the period 1950 to 2015 are also provided which indicate that in the Mediterranean Sea there is a net mean volume warming and salinification since 1950 that has accelerated during the last two decades. The time series also show that the ocean heat content seems to fluctuate on a cycle of about 40 years and seems to follow the Atlantic Multidecadal Oscillation climate cycle, indicating that the natural large-scale atmospheric variability could be superimposed onto the warming trend. This product is an observation-based estimation of the Mediterranean climatic indices. It relies solely on spatially interpolated data produced from in situ observations averaged over decades in order to smooth the decadal variability and reveal the long-term trends. It can provide a valuable contribution to the modellers' community, next to the satellite-based products, and serve as a baseline for the evaluation of climate-change model simulations, thus contributing to a better understanding of the complex response of the Mediterranean Sea to the ongoing global climate change. The product is available in netCDF at the following sources: annual and seasonal T∕S anomalies (https://doi.org/10.5281/zenodo.1408832), annual and seasonal T∕S vertical averaged anomalies (https://doi.org/10.5281/zenodo.1408929), annual and seasonal areal density of OHC/OSC anomalies (https://doi.org/10.5281/zenodo.1408877), annual and seasonal linear trends of T∕S, OHC/OSC anomalies (https://doi.org/10.5281/zenodo.1408917), annual and seasonal time series of T∕S, OHC/OSC anomalies (https://doi.org/10.5281/zenodo.1411398), and differences of two 30-year averages of annual and seasonal T∕S, OHC/OSC anomalies (https://doi.org/10.5281/zenodo.1408903).

Highlights

During the twentieth century the Mediterranean Sea has undergone profound and rapid changes
An analysis of these results shows that there are differences between them arising from (a) the input data, (b) their spatial and temporal variability, (c) the choice of the climatological reference, (d) the quality control procedures, (e) the instruments’ accuracy, and (f) the mapping techniques, e.g. the gridding and infilling methodologies such as optimal interpolation or variational inverse methods used to fill the data gaps and obtain a gridded 3-D continuous field and time series thereafter as well as which assumptions are made in areas of missing data (Jordà et al, 2017)
To get a more detailed insight into the long-term fluctuations we show in Fig. 4c, time series of the decadal ocean heat content (OHC) and ocean salt content (OSC) anomalies were integrated over the whole column depth and area of the Mediterranean Sea

Summary

Introduction

During the twentieth century the Mediterranean Sea has undergone profound and rapid changes. The most important contributions to global and regional mean sea level rise are a) increase in the ocean volume as a result of increase in the mass of the water (due to melting of ice sheets and shrinking of glaciers), and b) increase in the ocean volume as a result of decrease in ocean water density (the ocean expands as it warms). The projected future changes show that the GMSL will continue to rise during the twenty-first century with a rate that will very likely exceed that observed during 1971 to 2010 due to increased ocean warming and increased loss of mass from glaciers and ice sheets. In the Mediterranean region, climate model projections show an acceleration of warming, salinification as well as sea level rise during the twenty-first century (Somot et al, 2008; Mariotti et al, 2008; Giorgi, 2006; Giorgi and Lionello, 2008; Adloff et al, 2015; IPCC, 2014) with a potential strong impact on the marine environment, its effective management and human welfare (IPCC, 2014; Füssel et al, 2017)

Objectives

Results

Conclusion