On the link between Indian summer monsoon and the Etesian pattern over the Aegean Sea

Abstract

The Indian summer monsoon (ISM) is a key factor influencing the eastern Mediterranean climate. During early summer period, the thermal low over Pakistan and northwestern India extends towards the Mediterranean through inner Iraq and Turkey. On the other hand, the Azores high expands eastwards and meets the Balkan high pressure system, forming one common system. Greece lies at the transitional zone between these two pressure systems, where persistent northeasterly wind currents form especially over the Aegean, known as the Etesians. The objective of this study is to investigate the interrelation between the ISM and the Etesian winds over the Aegean Sea during summer for an extended period of 44 years (1958-2001) with the aid of composite and correlation analyses and ERA 40 datasets. More specifically, the study will focus on the ISM activity during and prior to or after ‘Etesian episodes’ in order to determine the extent to which one system may lead another. The present analysis reveals that during enhanced monsoon years two upper level ridges prevail over the greater area of interest, one over western Europe and northern Africa, which is part of the Azores high and a second one over Pakistan region, Persian Gulf and Middle Asia, which extends to the west and connecting to the first one forms a slight trough over Eastern Mediterranean. At lower levels, an intensification of the thermal Asian low as well as of the high pressure system prevailing over western and central Europe is apparent. Concurring with this pattern in upper and lower troposphere, anomalously strong northerlies appear over the Aegean during periods with enhanced monsoon activity, as horizontal surface wind anomalies depict. In agreement with previous studies, mid-level subsidence during excess monsoon rainfall periods is found to be more intense over the Eastern Mediterranean, Iraq, Saudi Arabia and east of the Caspian Sea.