# Climatic Indices over the Mediterranean Sea: A Review
## 3 Large-Scale Atmospheric Forcing: Climatic Indices
### The North Atlantic Oscillation (NAO)  
The NAO index is a fundamental mode of the climatic variability in the northern hemisphere ([44; 46; 47]; see [48] for a review). It represents the dipolar pattern of the SLP characteristic of the North Atlantic-European region, with one center in the Azores high and the other in the Iceland low. Stronger/weaker phases of the NAO are related to variations in the position and intensity of the North Atlantic jet stream (NAJS) and the storm track, along with large-scale changes in the zonal and meridional heat and moisture transport, which are reflected in changes in the temperature and precipitation patterns of wide regions, including the Mediterranean Sea. To clearly show this, we display the composites of sea level pressure anomalies (mbar) for the positive (higher quartile) and negative (lower quartile) phases of NAO (Figure 2 and the same for all indices in Figures 2-5). Monthly mean sea level pressure at \(2.5^{\circ}\times 2.5^{\circ}\) for the period 1948-2009 from the National Centre for Environmental Prediction (NCEP) has been used. Figure 3: Composites of sea level pressure anomalies (mbar) in the period 1948–2008. Panel (**a**) reflects the positive phase (higher quartile) and panel (**b**) the negative phase (lower quartile) of the EA. Figure 2: Composites of sea level pressure anomalies (mbar) in the period 1948–2008. Panel (**a**) reflects the positive phase (higher quartile) and panel (**b**) the negative phase (lower quartile) of the NAO. Positive NAO phases (Figure 2a) are associated with an increase in the SLP over most parts of continental Europe and the Mediterranean Sea. Both poles, the Azores high and the Iceland low, are intensified, modifying the direction of the westerlies and associated storm tracks, thus leading to a decrease in the precipitation over the Mediterranean and Southern Europe (south 45\({}^{\circ}\) N) and an increase over Northern Europe. On the other hand, negative NAO (Figure 2b) generates negative SLP anomalies in Southern Europe and the Mediterranean basin, increasing the precipitation in these regions [16; 34; 35; 36; 47; 49; 50; 51; 52; 53; 54]. Although the NAO index shows high interannual and multidecadal variability, long periods of positive and negative phases are common (see Figure 6). Switches from  
Figure 4: Composites of sea level pressure anomalies (mbar) in the period 1948–2008. Panel **(a)** reflects the positive phase (higher quartile) and panel **(b)** the negative phase (lower quartile) of the EA–WR. Figure 5: Composites of sea level pressure anomalies (mbar) in the period 1948–2008. Panel **(a)** reflects the positive phase (higher quartile) and panel **(b)** the negative phase (lower quartile) of the MO. negative to positive NAO phases are followed by noticeable changes in the average precipitation of the Mediterranean basin, such as the decrease observed between the mid-1960s and the 1990s. Positive NAO phases favor negative evaporation anomalies that can reach \(-160\) mm/y in the Gulf of Lions and the Levantine basin [53; 54], areas of formation of the WMDW and LIW, respectively. As a consequence, the reduction of latent heat losses associated with evaporation may result in a decrease in the amount of LIW and WMDW produced (see [32; 55] for a detailed discussion of convective processes). Anti-correlation is expected since, under a negative state, anomalously low pressure over the whole basin is observed (see Figure 2b) and more severe weather conditions over the Eastern and Northern Mediterranean are generated by the colder and drier air masses that flow from continental regions. Under this scenario, the enhancement of evaporative losses to the atmosphere is expected. In contrast, positive values are followed by higher than average pressure over the Mediterranean and North Africa (Figure 2a) that induce a change in the wind trajectories toward lower latitudes. Moister and warmer air masses are then advected to the Mediterranean, producing milder winters and, consequently, a decrease in the evaporative losses [34].