Identifying and detecting causes of changes in spatial patterns of extreme rainfall in southwestern Iran

Abstract

Extreme rainfall in southwestern Iran annually leads to considerable financial and human losses. Reports over several years show different levels of damage caused by these events. This study aims to identify and investigate changes in the spatial patterns of systems that generate extreme rainfall in southwestern Iran. We utilized the maximum 24-hr precipitation from seven synoptic stations in southwestern Iran from 1971 to 2020. The principal component analysis technique was utilized on geopotential height data at 500 hpa to identify spatial patterns of precipitation. The data was collected from 5° to 60°N and 10° to 70°E that were received from the NCEP/NCAR reanalysis dataset. The study period was divided into two based on the identified change point by the Pettitt test, and according to the results, the first climatic period in southwestern Iran had nine sources or components of extreme rainfall. In comparison, in the second period, they had eight sources of extreme rainfall. The most important source of extreme rainfall for both periods and the first source of the second period are located at 15° north latitude over the Indian Ocean, the Arabian Sea, the southern Red Sea, the Gulf of Aden, and certain areas in the eastern tropical regions of Africa.

Research highlights

As a result, the statistical period was divided into two periods of 21 and 29 years. The first period, which includes 82 extreme rainfall events, spans from 1971 to 1991, and the second period, which provides 96 extreme rainfall events, spans from 1992 to 2020. The most important source of extreme rainfall for both periods and the first source of the second period are located at 15º north latitude over the Indian Ocean, the Arabian Sea, the southern Red Sea, the Gulf of Aden, and certain areas in the eastern tropical regions of Africa. This component accounts for 18.8 and 20.8% of the total variance in the data for the first and second climatic periods, respectively. It has the most significant impact on changes in the topography of the 500 hpa geopotential height. As a result, changes in the topography of the 500 hpa geopotential height in these regions have played a crucial role in forming systems that create extreme rainfall in southwestern Iran. This source facilitates the advection of moisture in the upper and middle levels of the atmosphere towards the studied area due to the circulation of high pressure at the 500 hpa level. In addition, the formation of low-pressure thermal centers and the displacement of their tongues towards the studied area are crucial in extreme rainfall in the study area. The location of sources that create extreme rainfall systems did not see significant changes between the two climatic periods, but their importance in forming these systems varied. Additionally, there were no significant spatial changes in the sources of extreme rainfall formation in southwestern Iran during the two climatic periods, but there were changes in their intensity. For instance, the seventh source exhibited a weaker influence during the first climatic period (PC7) but strengthened during the second climatic period (PC5). In this source, the presence of a ridge over Russia strengthens the Siberian high-pressure system, causing it to expand over Iran. As these high-pressure ridges expand and pass over the Oman Sea and the Persian Gulf, they transport moisture toward the study area. Conversely, the eighth source demonstrated a stronger impact during the first climatic period (PC8), whereas it exhibited a weaker effect during the second climatic period (PC4). The changes in the 500 hpa level in this geographic region are mainly associated with forming different blocking systems, accompanied by the breaking of Rossby waves. The southern branch of these systems affects the study area.