Climatic Indices over the Mediterranean Sea: A Review

The Mediterranean Sea, strategically situated across a dynamic frontier line that separates two regions with different climates (Europe and North Africa), has been the focus of attention of many studies dealing with its thermohaline circulation, deep water formation processes or heat and freshwater budgets. Large-scale atmospheric forcing has been found to play an important role in these topics and attention has been renewed in climatic indices that can be used as a proxy for atmospheric variability. Among them, the North Atlantic oscillation, the East Atlantic or the East Atlantic–West Russia patterns have been widely addressed but much less attention has been devoted to a Mediterranean mode, the Mediterranean oscillation. This overview summarizes the recent advances that have been achieved in the understanding of these climatic indices and their influence on the functioning of the Mediterranean from a physical point of view. The important role of the Mediterranean oscillation is emphasized and the most relevant aspects of the other indices are revisited and discussed.

Decadal acidification in Atlantic and Mediterranean water masses exchanging at the Strait of Gibraltar

Seawater pH is undergoing a decreasing trend due to the absorption of atmospheric CO₂, a phenomenon known as ocean acidification (OA). Biogeochemical processes occurring naturally in the ocean also change pH and hence, for an accurate assessment of OA, the contribution of the natural component to the total pH variation must be quantified. In this work, we used 11 years (2005-2015) of biogeochemical measurements collected at the Strait of Gibraltar to estimate decadal trends of pH in two major Mediterranean water masses, the Western Mediterranean Deep Water (WMDW) and the Levantine Intermediate Water (LIW) and assess the magnitude of natural and anthropogenic components on the total pH change. The assessment was also performed in the North Atlantic Central Water (NACW) feeding the Mediterranean Sea. Our analysis revealed a significant human impact on all water masses in terms of accumulation of anthropogenic CO₂. However, the decadal pH decline found in the WMDW and the NACW was markedly affected by natural processes, which accounted for by nearly 60% and 40% of the total pH decrease, respectively. The LIW did not exhibit a significant pH temporal trend although data indicated natural and anthropogenic perturbations on its biogeochemical signatures.

Trends of pH decrease in the Mediterranean Sea through high frequency observational data: indication of ocean acidification in the basin

A significant fraction of anthropogenic carbon dioxide (CO₂) released to the atmosphere is absorbed by the oceans, leading to a range of chemical changes and causing ocean acidification (OA). Assessing the impact of OA on marine ecosystems requires the accurate detection of the rate of seawater pH change. This work reports the results of nearly 3 years of continuous pH measurements in the Mediterranean Sea at the Strait of Gibraltar GIFT time series station. We document a remarkable decreasing annual trend of −0.0044 ± 0.00006 in the Mediterranean pH, which can be interpreted as an indicator of acidification in the basin based on high frequency records. Modeling pH data of the Mediterranean outflow allowed to discriminate between the pH values of its two main constituent water masses, the Levantine Intermediate Water (LIW) and the Western Mediterranean Deep Water (WMDW). Both water masses also exhibited a decline in pH with time, particularly the WMDW, which can be related to their different biogeochemical nature and processes occurring during transit time from formation sites to the Strait of Gibraltar.