# Climatic Indices over the Mediterranean Sea: A Review
## 3 Large-Scale Atmospheric Forcing: Climatic Indices
### The Mediterranean Oscillation (MO)  
The first definition of the Mediterranean oscillation (MO) [30] described it as a dipolar behavior of the atmosphere between the Western and Eastern Mediterranean. Since then, some authors have attributed the differences in key atmospheric and oceanographic parameters between both basins to this mode [30; 57; 58; 59]. The index measuring the intensity of the dipole was primarily defined as the normalized 500 hPa height difference anomalies between Algiers (36.4\({}^{\circ}\) N, 3.1\({}^{\circ}\) E) and Cairo (30.1\({}^{\circ}\) N, 31.4\({}^{\circ}\) E) [30]. An alternative definition [31] estimated the MO index as the difference in the normalized SLP between the northern frontier of the Strait of Gibraltar (36.1\({}^{\circ}\) N, 5.3\({}^{\circ}\) W) and the Lod Airport in Israel (32.0\({}^{\circ}\) N, 34.5\({}^{\circ}\) E) [60]. A third definition [61] is offered for a better representation of the Central Mediterranean behavior, choosing the normalized SLP difference between Marseille and Jerusalem. Using this index, the authors found good (anti-)correlation with the precipitation and the number of wet days in Italy (around \(-\)0.4 on a yearly basis and up to \(-\)0.7 for wintertime depending on the location). More recently [29; 32], the Mediterranean index has been introduced as the sea level pressure difference between South France (45\({}^{\circ}\) N, 5\({}^{\circ}\) E) and the Levantine Sea (35\({}^{\circ}\) N, 30\({}^{\circ}\) E). These two points are orientated in a NW-SE direction and are likely to reflect more accurately the realistic dipole pressure pattern. An approach based on the analysis of the EOF of the SLP anomaly fields over an extended Mediterranean region has been proposed [38]. Elsewhere [39], this paradigm is also used to analyze the influence of the MO index on the variability of the flow exchange through the Strait of Gibraltar. More recently [43], it was analyzed in detail these different paradigms of the Mediterranean oscillation teleconnection index: station-based definitions (Algiers-Cairo, MO\({}_{\text{AC}}\), Gibraltar-Israel, MO\({}_{\text{GI}}\), Marseille-Jerusalem, MO\({}_{\text{MJ}}\) or France-Levantine, MO\({}_{\text{FL}}\)) and the principal component (PC) approach in which the MO\({}_{\text{PC}}\) index was obtained as the time series of the first mode of normalized SLP anomalies over the extended Mediterranean region included in the limits [30\({}^{\circ}\) W-40\({}^{\circ}\) E, 30\({}^{\circ}\) N-60\({}^{\circ}\) N], which exhibits a single center located over the Central and Western Mediterranean that remains fairly steady in all seasons. They correlated interannual to interdecadal precipitation (P), evaporation (E), E-P and net heat flux with the different MO indices to compare their relative importance in the long-term variability of heat and freshwater budgets over the Mediterranean Sea. The accuracy of the index based on the different definitions to describe the interannual and interdecadal variability of the basin freshwater and heat budget components has also been analyzed [43]. They concluded that the most effective representation of the basin large-scale atmospheric forcing is achieved by the MO index based on PC analysis since it provides optimal representation of the full spatial pattern. Station-based indices show very poor correlation with the climatic variablesanalyzed (P, E and E-P) and only affect a reduced region of the basin. The noise introduced by the local small-scale and transient meteorological events in the SLP measurements causes the poorer results of the station-based indices [41; 42]. During winter, when the dynamic activity of the atmosphere increases and the impact of the large-scale atmospheric variability is higher, the large-scale patterns over the basin are more marked and stable, improving the capacity of the station-based indices to capture the atmospheric variability. Unfortunately, this capacity is strongly reduced for the rest of the year.