Tropospheric features associated with the onset and cessation of the rainy season in Iran

In the present study, according to a provided definition of the widespread rainy season (WRS), the onset of the WRS (OWRS) and the cessation of the WRS (CWRS) occurred in three and four phases, respectively. The phases of the OWRS (26 Sep, 5 Nov, and 15 Dec) were related to the interaction of the sub-polar low pressure (central pressure lower than 996 hpa) and the Siberian high pressure (central pressure higher than 1020 hpa) at the tropospheric lower level and the ridge of Arabia at the 500 hpa (higher than 5850 m). Thus, the eastward-moving of the Mediterranean cyclones (the number of cyclones from the first to the third phase was 264, 245, and 407, respectively) bred the positive moisture flux convergence and reduced the outgoing long waves radiation (OLR) values to about 240 W/M2. Simultaneous with the northward shift of the sub-polar low pressure and penetrating the Siberian high pressure to the west of Iran and the extension of 300-hpa jet stream, the next phase of the OWRS appeared in the southern regions. The four phases of the CWRS (6 Oct, 31 Dec, 5 Mar, and 3 Apr) commenced with the strength (central pressure higher than 1032 hpa) and expansion of the Siberian high pressure, concurrent with the formation of high pressure in the northwest of Iran, followed by the second phase on the Iranian coast of the Caspian Sea. The reduction in gradients of 500 hpa height in the latitude of 20–40 N was associated with increased zonal wind in the longitude of 40 E toward the east and the weakening of the jet stream at 300 hpa. In addition to increasing the OLR (more than 250 W/m 2 ), the thermal low pressure, positive values of meridian wind, and negative values of zonal wind over Iran led to the transfer of the characteristics of the adjacent desert, and the maximum OLR planetary belt moved toward the country. Contrary to previous studies, which attributed the onset and cessation of the rainy season to a few atmospheric features, the results of the current study suggest that the onset and cessation of the rainy season resulted from a very complex interaction of several tropospheric factors and geographical features of different regions. This interaction, primarily in geographically diverse realms (e.g., Iran), leads to complicated spatial patterns of the onset and cessation of the rainy season.