Climatology and Dynamics of the Summer Etesian Winds over the Eastern Mediterranean

Abstract

he Etesians are persistent northerly winds that prevail over the eastern Mediterranean during summer. A climatology of Etesian outbreaks over the Aegean was compiled with the aid of the 40-yr ECMWF Re-Analysis (ERA-40) dataset and their vertical organization is investigated. Their variability arises from high-frequency variability originating in the midlatitudes, interannual and intraseasonal variability controlled by the South Asian monsoon, and a local diurnal cycle. Consistent with the monsoon influence, Etesian outbreaks are most frequent from mid-July to mid-August. In agreement with previous studies, a negative trend in the incidence of Etesian outbreaks is detected during the overall June?September period, which is strikingly strong for September but diminishes in June. The strengthening of the Etesians by day over the central and southern Aegean results from the deepening of the Anatolian thermal low because of the daytime sensible heating near the surface. The timing of an outbreak onset is controlled by wave disturbances originating over the North Atlantic that trigger the development of a strong ridge over the Balkans, which induces anomalously strong northerly flow and subsidence over the Aegean. During Etesian outbreaks, sharp tropopause folds and stratospheric intrusions of high potential vorticity descend deeply into the troposphere.