The Influence of Arctic Oscillation For Dry Conditions Over Eastern Mediterranean

The larger Mediterranean basin is an important region for the study of the climate, hydrological cycle, and future climate change and variability because of its links with the large-scale atmospheric and oceanic teleconnections and circulations. In this study, the relations between seasonal moisture budget (precipitation minus evaporation) and Arctic Oscillation (AO) associated with dry conditions in the Eastern Mediterranean Basin were investigated. According to this study, AO index is a better indicator of driest/wettest conditions variability over larger Mediterranean Basin. The seasonal moisture budget was calculated using the National Center for Environmental Prediction (NCEP)—National Center for Atmospheric Research (NCAR) reanalysis data for the period 1949–2014. The atmospheric moisture budget equation proposed by (Trenberth and Guillemot 1995) was used which was derived from the equation of state, the hydrostatic equation, and the continuity equation. Winter circulation patterns for explaining the changes in driest and wettest conditions were focused on, since winter is the season in which atmospheric circulation is more important to define the climate variability in the Mediterranean basin.

The moisture fluxes during wet conditions over the Mediterranean Basin are substantially different from long term moisture fluxes. However moisture fluxes shows similarity during dry and normal conditions over Mediterranean in terms of direction and magnitude. As a result, the large scale atmospheric conditions during the average normal and dry conditions are similar and both Western and Eastern Mediterranean prone to dry conditions. Therefore, it is hard to detect influence of atmospheric and oceanic teleconnections for the larger Mediterranean Basin for dry conditions. The driest Eastern Mediterranean winters were selected according to the Standardized Precipitation Index (SPI) (Figure a,b), and the differences in moisture budget between them and average conditions (i.e. climatology) were investigated (Figure c,d). Then, we calculated the correlation of SPI with Arctic Oscillation Index conditional on driest years. Thus, we were able to obtain the spatial configuration of AO patterns related to driest conditions. This approach gave the opportunity to compare teleconnection patterns which affect driest/wettest conditions over larger Mediterranean basin. According to the results; higher pressure at mid-latitudes because of positive phase of AO changes circulation patterns and cause drier conditions over the Eastern Mediterranean (Figure e,f). The relative strengths and positions of these large-scale systems cause Eastern and Western patterns of the Mediterranean climate variability.

Figure Captions

Figure a,b:Geographical distribution patterns of 3-month SPI values calculated for the Eastern Mediterranean Basin for the selected two winters during the period 1949-2010.

Figure c,d:Composite anomalies map of 10 driest January anomalies with respect to the January climatology calculated for a) Western Mediterranean and b) Eastern Mediterranean. Vertically integrated atmospheric moisture flux (AMF) (vectors, in kg•(ms)-1) and AMF convergence (shaded and contours, in mm month^-1)

Figure e,f: Composite map of Monthly mean wind (m.s-1) and Sea Level Pressure (hPa) for January between the period 1949-2010.tiff