Synoptic conditions leading to extremely warm periods in Western Iran

The purpose of this study is the characterization of the synoptic circulation patterns leading to extremely warm periods (EWPs) over the Kurdistan province of Iran. Daily maximum temperature data from 27 meteorological stations were used for the period 1962–2012. Data were interpolated to a 6 × 6 km grid using the geospatial Kriging method. EWPs were defined as a period of at least 6 consecutive days with maximum temperature above the local daily‐based 95th percentile of the 1971–2000 period, with additional criteria for spatial extension. The identified EWPs are distributed through the whole year, with maximum frequencies in winter and summer. Interestingly, the decade 1988–1997 did not record any extreme episode and summer extremes were recorded only before 1983. The associated synoptic patterns were classified into four clusters by applying the Ward's hierarchical clustering method to sea level pressure and Z500 data. Four clusters are identified: two represent warm periods in winter, one those occurring spring, especially in March, and another one mainly reflecting summer heat waves. In general, it is found that EWPs are associated with statistically significant anomalies in regional circulation. A deep trough of geopotential height in 500 hPa over North Africa and a ridge directly over the study area were the more important causes for the occurrence of EWPs. Other synoptic configurations that favour extreme periods in winter and summer include, respectively, the northward expansion of the Sudan low pressure with an associated southerly wind advection and the persistence of anticyclonic circulation at all tropospheric levels enhancing subsidence and adiabatic heating. The spring pattern is the most frequent one (nearly 48% of all cases) and reflects a southeastward extension of the subpolar low pressure that leads to the displacement of the Siberian high to south and southwestern Asia and the accompanied northward expansion of the Sudan low.