# Climatic Indices over the Mediterranean Sea: A Review
## 1 Introduction  
The Mediterranean Sea acts as a natural laboratory since, despite its reduced dimensions, most of the processes typical of global ocean circulation, such as deep water formation or thermohaline circulation, take place in the basin. Moreover, semi-enclosed basins such as the Mediterranean are appropriate for characterizing heat and water flux since it makes possible a basin budget closure. There is a lot of research devoted to the Mediterranean heat [1; 2; 3; 4; 5; 6; 7; 8; 9] and water [1; 5; 6; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18] budgets but, despite these efforts, estimation of the surface freshwater flux has been found to depend on the methodology and datasets used and, hence, is subject to significant uncertainty, particularly in its seasonal and interannual variability. For instance, estimates of the water deficit (evaporation minus precipitation) are found to range from 421 [5] to 1230 mm-y\({}^{-1}\)[14], confirming the difficulty of obtaining a reliable estimate.  
More than 60 years of climatological datasets were analyzed in recent research [9], with longer time series available. The seasonal cycle of the net heat is positive (that is, toward the ocean) between March and September, with a maximum in June, whereas negative values (that is, toward the atmosphere) are found during the rest of the year, with a minimum in December. On a yearly basis, a nearly neutral budget of 0.7 Wm\({}^{-2}\) is reported for the whole Mediterranean but there are remarkable differences between the eastern and western sub-basins: \(\sim\)12 Wm\({}^{-2}\) (toward the ocean) is found for the western Mediterranean and around \(-\)6.4 Wm\({}^{-2}\) (toward the atmosphere) for the eastern Mediterranean [9], mainly related to the high latent heat losses that take place in this sub-basin (up to 100 Wm\({}^{-2}\)).  
For the freshwater deficit, a seasonal cycle with an amplitude of around 600 mm\(\cdot\)y\({}^{-1}\), with maximum in August-September and minimum in May, is described [9]. The long-term mean of the basin-averaged deficit is 680 \(\pm\) 70 mm\(\cdot\)y\({}^{-1}\) but it is almost 70% higher in the Eastern Mediterranean due to higher evaporation and lower precipitation in this basin. A higher deficit (positive trend) of 1.6 \(\pm\) 0.9 mm\(\cdot\)y\({}^{-2}\) is reported for the entire 60-year period in which the decrease in precipitation seems to be the dominant factor, although one study [17] also underlines the role of evaporation changes.  
With respect to interannual variability, only a few of the most recent works have focused on this topic and the associated forcing mechanisms, since longer datasets are necessary. For example, in one study [9], several different periods of evaporation and precipitation anomalies are described: from the early 1950s to the late 1960s, a positive trend was reported; then, it changed to negative, until the late 1980s, when a new change was observed. This variability is also found in the net heat flux exchanged between the atmosphere and the ocean and indicates a \(\sim\)40-year period multidecadal oscillation that is likely to be related to large-scale atmospheric forcing. To explore this linkage, several climatic indices representing this large-scale atmospheric forcing can be tested as a proxy to monitor the long-term variability of climatic variables that influence the Mediterranean. In this review, we revisit the relevance of the most important modes of atmospheric variability in the Mediterranean region from a physical point of view. The work is organized as follows: Section 2 describes the geographical frame; in Section 3, the most outstanding climatic indices are analyzed and the main results presented by different authors are summarized and discussed. Finally, Section 4 provides some concluding remarks and anticipates some future work.