Analysis of the temporal and spatial trend of atmospheric circulation patterns and its effects on Iran's rainfall in the last two decades

In this research, the daily data of geopotential height of 500 hectopascals (hPa) with a spatial resolution of 1 degree from the ECMWF database for Southwest Asia and rainfall station data from the National Meteorological Organization (1979 to 2018) have been exerted. The technique used the principal component analysis and cluster analysis. With these analyses, nine circulation patterns were identified. The changes in the patterns were tested at the 95% significance level by the non-parametric Mann-Kendall test, and Sen's slope estimator was exerted to estimate the number of changes. The significance test of the trend for the winter patterns in Iran's rainy season revealed the significant trend of increasing the height of the geopotential, which has led to a decrease in the pressure gradient and a decrease in instability, and finally, a weakening of the winter precipitation patterns. Significant positive trends of geopotential height showed the continuation of these conditions for summer patterns (increasing stability, decreasing rotation, and decreasing precipitation). Of the nine known patterns, only one seasonal pattern showed a significant negative trend in the country. This pattern, with a slight increase in rainfall, indicates the formation of unstable conditions, which can lead to moderate-season rains if moisture is available. The findings showed that a rainy winter pattern had been eliminated in the last two decades, and a summer pattern had appeared instead.