The Madden-Julian Oscillation (MJO), as the most prominent mode of tropical intraseasonal atmospheric variability, plays a pivotal role in reorganizing the general circulation patterns and modulating precipitation systems at the global scale. Through the eastward propagation of its active convective core across the eight phases of this oscillation, significant dynamical anomalies emerge in the pressure, wind, and precipitation fields across tropical, subtropical, and mid-latitude regions. These anomalies effectively modulate mid-latitude atmospheric circulations via poleward Rossby wave propagation and can exert substantial control over extreme precipitation events in semi-arid regions such as Iran. The primary objective of this study is to quantitatively assess the frequency, amplitude, and intensity of MJO activity in relation to extreme dry and wet periods during December–March over Iran across the 1991–2020 climatological period, and to analyze the underlying synoptic-dynamic mechanisms. To this end, for each of the four principal precipitation months of the cold season, ten extreme dry and ten extreme wet months were identified based on the percentage deviation of precipitation from the long-term climatological mean. The Real-time Multivariate MJO (RMM) index was employed to determine the daily phase and amplitude of MJO activity, and synoptic patterns were derived from NCEP/NCAR reanalysis data.
The results reveal that cold-season precipitation over Iran exhibits a statistically significant decreasing trend at the 90% confidence level in December, January, and March, with the amplitude of precipitation variability in extreme periods ranging from −86.6% to +135.8% relative to the long-term mean. Key findings indicate that MJO phases 4 through 6, characterized by active convection positioned over the Maritime Continent and the western Pacific Ocean, are recurrently associated with extreme dry periods, whereas phases 8, 1, and 2 show the strongest association with extreme wet periods. This influence operates through two primary dynamical pathways: (1) direct moisture transport from the Indian Ocean via modulation of equatorial easterlies, and (2) remote teleconnection through Rossby waves that deepen or weaken Mediterranean troughs.
During extreme dry periods, MJO amplitude in phases 4–6 reaches Am ≥ 2, indicative of strong and persistent oscillatory activity. This atmospheric configuration is accompanied by an intensified tropical low-pressure system over the Indian Ocean, expansion of subtropical ridges, suppression of the Mediterranean trough, and disruption of moisture transport pathways from the northwestern Indian Ocean, the Arabian Sea, the Sea of Oman, and the Red Sea toward Iran.
Conversely, MJO phases 8, 1, and 2 were dominant during the majority of extreme wet months. Poleward Rossby wave propagation, induced by suppressed convection over the Pacific Ocean, deepens the Mediterranean trough through negative geopotential height anomalies at the 500hPa level, reaching approximately 50 to 110 metres. Simultaneously, the reorganization of equatorial winds reinforces moisture transport from southern oceanic sources toward Iran. Overall, the findings demonstrate that the MJO exerts a determining influence on extreme precipitation events in Iran through two complementary mechanisms, direct modulation of equatorial circulations and moisture transport, and indirect modulation of mid-latitude circulations via Rossby wave dynamics, and offers considerable potential for advancing subseasonal precipitation forecasting.