Heavy precipitation in the southwest of Iran: association with the Madden–Julian Oscillation and synoptic scale analysis

Some important characteristics of the November–April heavy precipitation in southwestern parts of Iran and their linkages to the Madden–Julian Oscillation (MJO) were assessed for the period of 1975–2011. Daily precipitation data in nine meteorological stations spread in various parts of the study area and the corresponding MJO indices were analyzed. For each station, precipitation data were sorted in descending order and those values that fell within 5% of the highest records were categorized as the heavy precipitation. Besides this, the 10% threshold was also analyzed as an axillary assessment. The considered heavy precipitation data (5% threshold) accounted from about 26–35% of total annual precipitation. About half of the heavy precipitation occurred during December–January period and the other half distributed within the months of March, February, November and April by about 17, 14, 13and 6%, respectively. The highest frequency of heavy precipitation was related to the MJO phase 8. After this, the more frequent precipitation events were respectively associated to the phases 2, 7, 1, 6, 5 and 4 of the MJO. For the phases 1, 2, 7 and 8 frequency of the heavy precipitation statistically increased when the MJO amplitude was greater than unity. In contrast, for phases 4 and 5, heavy precipitation was generally linked to the spells that the amplitude size was lower than unity. Formation of a strong north–south oriented cold front mainly in Saudi Arabia and west-east oriented warm fronts in the southwest of Iran were realized as the key elements for initiating heavy precipitation over the study area. Although development of the Mediterranean-based cyclonic circulation is essential for the formation of these fronts, moisture transport mostly originates from northern parts of the Arabian Sea, southern parts of the Red Sea and the Persian Gulf.