A synoptic-scale analysis of the episodes with and without precipitation in the southwest of Iran during the phase 1 of the MJO
The Madden–Julian oscillation (MJO) is a large-scale coupling between atmospheric circulation and tropical deep convection explaining a large part of the intra-seasonal (30–90 day) variability in the tropical and extra-tropical atmospheres. Rather than being a standing pattern like the El Niño–Southern Oscillation (ENSO), the MJO is an oceanic–atmospheric moving pattern that propagates eastward at approximately 4 to 8 ms−1, through the atmosphere above the warm parts of the Indian and Pacific oceans. Due to its quasi-cyclic pattern, the MJO is known as the 30–60 day oscillation, 30–60 day wave, or intra-seasonal oscillation. The oscillation is characterized by an eastward progression of large regions of both enhanced and suppressed tropical precipitation, observed mainly over equatorial parts of the Indian and Pacific Oceans. The anomalous convective precipitation is firstly triggered over the western Indian Ocean, and persists as it propagates over the warm ocean waters of the western and central tropical Pacific. Wheeler and Hendon (2004) categorized the whole cycle of the MJO into 8 phases namely phase 1 to 8. Phase 1 of the MJO that is the theme of this study, signifies the spells for which the convective activity is mainly centered over the western parts of the Indian Ocean equator. It has been previously reported that the occurrence of the MJO phase 1 improves the probability of precipitation event in southwestern Iran (Nazemosadat and Ghaedamini, 2010). In spite of these findings, the reasons of frequent clear sky and shiny days during this phase were not yet resolved. The aim of this study was to compare the oceanic and atmospheric features of the phase 1 for the spells that the incidence of this phase is concurrence with or without precipitation in the southwest of Iran. This analysis is beneficial for improving the MJO-based precipitation and climate forecast over this area. To accomplish this task, the MJO amplitudes during phase 1 were extracted for all days during the 1975–2012 period. Among the selected dates, those days with amplitudes greater than 1 were assigned as the strong MJO events. These MJO events were then divided into two different parts comprising the events with and without pervasive precipitation in southwestern Iran. According to the adopted definition, the pervasive precipitation occurred when at least five out of nine considered stations over the study area received more than 1.0 mm precipitation. For the pervasive precipitating dates, the MJO-precipitation composites were constructed for each individual station. Similar composites were also constructed for precipitation, vector wind, outgoing long-wave radiation (OLR) and vapor flux over the Middle Eastern region and tropical parts of the Indian and Pacific Oceans using the ESRL–NOAA composite facilities. Similar compositing procedure was also performed for the no-precipitating dates to investigate atmospheric condition during such spells.From about 8% to 15% of the November–April precipitation in the southwest of Iran was found to be associated with the periods that the MJO was centered in its phase1. It was concluded that, for improving the MJO-based precipitation forecast in southwest of Iran, not only the phase number and amplitude size of the MJO index, but the position and intensity of convective activities as well as the atmospheric circulations over the Indian and the Pacific Oceans should also be analyzed. In phase 1, precipitation event in southwest of Iran is usually associated with the state of convective activities and their relevant airflows over some areas extended from equatorial parts of east Africa up to the equatorial areas of the Indian Ocean around 90o E. In general, precipitation occurs over the study area if the core center of these activities locate around 60o E. For such situation precipitation anomaly over this area is greater than 1.5 mm/day and the OLR anomalies are less than −20Wm−2or lower. Coincidence with the Indian Ocean equatorial area, precipitation event over the study area in Iran is harmonized with the enhancement of convective activities over tropical parts of the Pacific Ocean for the areas between 160o W to 140o W and 10o S to 20o S. Compared to non-precipitating periods of phase 1, easterly or southerly wind enhances by about four times over the eastern parts of the Indian Ocean, tropical parts of North Africa and the Arabian Peninsula during the precipitating spells. The Persian Gulf was found to play an influential role for re-moisturizing the southerly airflows crossing this water body during the precipitating dates.
Iranian Journal of Geophysics, Volume:10 Issue:1, 2016
73 - 87
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