On the relationship between Iran surface temperature and northwest India summer monsoon rainfall

This study examines the role of Iran surface temperature in modulating rainfall over northwest (NW) India during summer season spanning from June through September. The analysis is based on trend analysis, correlations and regressions performed using ERA-Interim data and India Meteorological Department rainfall records based on station data from 1979 to 2013. The summer season shows increasing trends in surface temperature and rainfall over Iran and NW India, respectively. The increase of surface temperature over Iran is the consequence of mid-latitude wave train that propagates from the northeastern Atlantic/NW Europe and traverses through Eurasian region to Iran. This increases the geopotential height (GPH) anomaly at upper to middle troposphere over Iran, which increases the tropospheric temperature along the vertical cross-section of atmospheric column with maximum warming at the lower troposphere due to the strong descent at the middle tropospheric level. This increase in surface temperature intensifies the surface low-pressure area over Iran and anomalous cyclonic circulation with northerlies and westerlies over Persian Gulf and north Arabian Sea, respectively. These winds converge towards NW India with abundant of moisture supply, favouring deep convection. The increase of GPH anomaly at upper to middle troposphere over Iran also intensifies the Tibetan High westward, accordingly shifting the monsoon rainfall westward. In most of the cases, it dynamically develops into blocking ridges over East and West Asia and an anomalous high-amplitude mid-latitude trough intrusion into NW India favouring very heavy rainfall events.