Has the Extent of Summer Monsoon Rainfall Activity Increased in Iran? A Long-Term Analysis of Trends and Connection With Monsoon Indices

Abstract

In the summer of 2022, Iran experienced severe flooding that caused extensive damage to infrastructure, agriculture, and residential areas. This event prompted an investigation into whether the frequency and extent of severe summer flooding in Iran have increased due to global warming. This study investigates long-term trends in Widespread Rainy Days (WRDs) in Iran during the summer season (June to September) and analyses their correlation with Indian summer monsoon indices, specifically the Convective Index (CI) and U-wind Index (UI). Reanalysis data, including precipitation, Sea Level Pressure (SLP), and Geopotential Height (GPH) from 1836 to 2015, were used for the analysis. WRDs in Iran were calculated for each year using the connected components labelling method. The results indicate a positive correlation between precipitation and WRDs with the CI, implying that an increase in Outgoing Longwave Radiation (OLR) in the Bay of Bengal leads to higher rainfall and more WRDs in Iran. Amongst all observed correlations, the highest correlation (r = 0.58) was found in southeastern Iran. On days with the highest CI values, negative anomalies in sea level pressure (SLP) and 850-hPa geopotential height (GPH) are observed. These anomalies, along with increased humidity advection, contribute to the occurrence of WRDs in Iran. Conversely, Iran's summer rainfall shows a negative correlation with the UI. On days with the highest UI values, no negative anomalies were observed in SLP and 850-hPa GPH, and reduced humidity advection resulted in the absence of WRDs in Iran. The study also reveals that the majority of WRDs occur in June, with the highest density observed in the southeast region of Iran. The Mann–Kendall test indicates a decrease in WRDs in Iran, particularly in June (R² = 17%) and September (R² = 24%). Whilst July and August also exhibit a decline, the trend is relatively weaker (R² = 2%). These findings demonstrate a statistically significant decline in WRDs over the 180-year period, contradicting the hypothesis of increased monsoon rains in Iran due to global warming.