Abstract
The Indian summer monsoon (ISM) is a prominent feature of the summer circulation in the Northern Hemisphere (NH) and has been found to modulate the weather and climate conditions in many remote regions. This study investigates the most recurrent patterns of summertime midlatitude circulation, over the eastern Mediterranean (EM) and also globally, that are most associated with the ISM. Monthly data of 44 summers from the ERA40 dataset are used and two multidimensional statistical methods, the Principal Component Analysis (PCA) and Canonical Correlation Analysis (CCA), are implemented. The ISM is found to be related to subsidence anomalies in the middle and more extendedly in the upper troposphere over the central and eastern Mediterranean and with an Etesian-like pattern regarding the field of the lower troposphere winds. An equatorial Rossby wave pattern, extending westward from an ISM heat source up to EM and N. Africa, was identified to be associated with the variability of ISM. The observed relationship between the ISM and the EM circulation features can be attributed to this equatorial Rossby wave response to the monsoon forcing. CCA implementation revealed the interconnection of the aforementioned PCA results with an ISM action center over the northern Arabian Sea and the monsoon trough region.
Highlights
The Mediterranean basin lies at the boundary between two very different climate regimes, the arid climate of North Africa to the south and the wet climate of central Europe to the north
In order to demonstrate the robustness of the connection found earlier between the Indian summer monsoon (ISM) and specific circulation features, a cross-validation analysis was performed for the same period with the aid of Canonical Correlation Analysis (CCA)
CCA was applied on the main components that were derived from the application of Principal Component Analysis (PCA)
Summary
The Mediterranean basin lies at the boundary between two very different climate regimes, the arid climate of North Africa to the south (subtropical zone) and the wet climate of central Europe to the north (temperate zone). The warm thermal structure of the Rossby wave spreads westward causing significant depression of the isentropes and as the air on the southern flanks of the mid-latitude westerlies interacts with this warm structure, it slides down the sloping isentropes and it descends The localization of this adiabatic descent is attributed to the influence of local up-stream orography, the Atlas mountains in Northwest Africa for the case of eastern Sahara-eastern Mediterranean and the Zagros mountains for the case of Kyzlkum desert, to the east of the Caspian Sea, which focuses the subsidence over specific areas [6,15].
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