# Climatic Indices over the Mediterranean Sea: A Review
## 3 Large-Scale Atmospheric Forcing: Climatic Indices
### The Mediterranean Oscillation (MO)
On the other hand, the PC analysis captures the variability of the whole region, filtering out the small-scale events and hence providing a better representation of the climatological evolution of the atmospheric processes over the basin independent of the season. MO\({}_{\rm AC}\) shows better correlation for most winter-averaged variables and reveals more clearly the well-known dipole response of the eastern and western basins. However, all MO indices show fairly similar results, especially at decadal timescales [43]. It is worth pointing out that, while NAO, EA and EA-WR can be considered independent modes of the same EOF analysis, this is not the case for the MO. The NAO and MO patterns have important similarities, both with positive (negative) phases characterized by higher (lower) SLP anomalies over the Mediterranean (Figure 5). The two indices are strongly influenced by the Northeast Atlantic low systems that force the Mediterranean cyclogenesis [62], and thus their annual time series are highly correlated (\(\sim\)0.6 [54]; see also Figure 6). The MO can be understood as an oscillation of sea level pressure anomalies in the Central and Western Mediterranean, an important source of cyclogenesis. Since the appearance of these cyclones is partially connected with the activity of North Atlantic fronts governed by NAO, a high correlation is expectable. During winter, the southern center of the NAO is placed closer to the Mediterranean and, hence, the best correlation for all variables is always achieved by the winter NAO index. However, in summer and spring, the southern center of the NAO moves westward [34] and lower correlation is observed. On the contrary, since its center remains rather stable, the influence of MO in the Mediterranean is noticeable in all seasons. The MO index is also able to capture the effects of other low-frequency atmospheric modes in the Mediterranean SLP field. In particular, the influence of the EA (annual correlation of 0.43) but also the winter EA-WR and Scandinavian (SCAN) modes [29] are noticeable. Therefore, this provides a more complete picture of the large-scale atmospheric impact over the Mediterranean, emerging as a rather accurate index to describe the basin climatological freshwater and (especially) heat budgets. Particularly, it is important to point out some achievements of the MO index with respect to the other indices [54]: (i) the annual correlations between the MO index and the climatic variables (E, P, E-P and heat flux) are higher than for the other indices; (ii) specifically, the influence of the MO negative phase is stronger than the NAO and is linked to an increase in precipitation and, in particular, intensification of the evaporation in the Levantine basin. In both MO phases, the SLP pattern induces wind trajectories that are closely related to the evaporation and net heat flux variability (Figure 5): warmer and moister air masses are transported to the Central and Western Mediterranean in the MO positive phase, as a consequence of the positive SLP anomaly dipole structure between North Africa and Central Europe (Figure 5a), resulting in milder winters and the subsequent decrease in evaporation and heat loss. Conversely, negative MO phases are characterized by a dipole of low SLP anomalies between Turkey and Central Europe (Figure 5b), enhancing the flow of cold and dry air masses from continental regions to the Mediterranean. This results in severe winters in the Aegean and Levantine basins that increase evaporation and heat losses, favoring the convective processes that generate the LIW.