Drying of the Mediterranean and the Role of the Atlantic Storm Tracks: Responses to Greenhouse Gases and the North Atlantic Oscillation

Wednesday, May 14, 2014 - 07:00
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The Mediterranean region has experienced persistent drying since the middle of the 20th Century and global climate models project further drying in the future as a consequence of increasing greenhouse gases. The Mediterranean region is also known to oscillate between decades of relatively wet and dry conditions due to the strong influence of multidecadal North Atlantic Oscillation (NAO). Both the forced long-term drying resulting from human influences and those due to NAO are closely related to the change in position and strength of the North Atlantic Storm tracks. We explored the detailed mechanisms leading to the greenhouse-gas induced and NAO-associated precipitation changes in terms of moisture budget analysis in this study, focusing on the role of the change in mean circulation versus that of the storm tracks in the regional moisture budget. We employed 15 CMIP5 models and the ERA-Interim Reanalysis to investigate the various moisture budget terms for the six-month winter and summer associated with NAO and the greenhouse gas forcing. Our results indicated that the shift in the midlatitude transient eddies induced modest moisture convergence, rather than divergence, over the Mediterranean under a positive NAO. The reduction in precipitation in this region during a positive NAO was dominated by the mean moisture divergence, which opposed the transient contribution. There were significant differences between the patterns of NAO-induced moisture budget anomaly and changes due to external radiative forcing. The models predict reduced PÕE over the Mediterranean region in response to future greenhouse gas increases. For both land areas and Sea a common cause of drying is increased mean flow moisture divergence. Changes in transient eddy moisture fluxes act to diffuse changes in the mean humidity field and cause drying over the Sea, and moistening over many land areas to the north, in winter, and dry western land areas and moisten the eastern Sea in summer. Under a positive phase of the NAO, on the other hand, there was modest reduction in evaporation and wetting by the storms over the Mediterranean, and drying over northern Europe.