مطالعه بي هنجاري گردش هاي جوي همراه شده با فعاليت نوسان مادن-جولين و بارش زمستانه ايران

A study of the impacts of the MJO on atmospheric circulations and winter precipitation in Iran

در اين مطالعه بي­هنجاري الگوهاي جوي همراه شده با فازهاي مختلف شاخص همرفت حاره‌اي (MJO) و بارش رخ داده در هريك از اين الگوها بررسي شده است. براي اين منظور، از داده‌هاي روزانه ميدان‌هاي فشار، ارتفاع ژئوپتانسيلي، سمت و سرعت باد از مركز NCEP/NCAR و داده‌هاي روزانه بارش 47 ايستگاه همديدي كشور براي ماه‌هاي دسامبر، ژانويه و فوريه استفاده شد. بي­هنجاري‌هاي بارش و الگوهاي گردش جوي همراه شده با هريك از فازهاي هشت‌گانه MJO مطالعه شد. ﻧﺘﺎﯾﺞ ﻧﺸﺎن داد كه بي­هنجاري الگوهاي فشاري، مقدار و توزيع بارش كشور در هر يك از فازهاي MJO متفاوت است. بيشينه بي‌هنجاري منفي بارش زمستانه كشور، همزمان با فعاليت فازهاي چهار و پنج MJO،كه هسته همرفت حاره‌اي در بخش شرقي اقيانوس هند قرار دارد، رخ مي‌دهد كه با بي­ هنجاري‌هاي مثبت ارتفاعي قوي بر روي بخش زيادي از اروپا و نواحي شرقي مديترانه همراه مي‌باشد. ميزان كاهش بارش زمستانه همراه شده با اين فازها در اكثر مناطق كشور، بين 30 تا 100 درصد است. در فازهاي شش و هشت كه هسته فعال همرفت در نواحي مركزي آرام استوايي قرار مي‌گيرد و پديده همرفت در شرق اقيانوس هند سركوب مي‌شود، با بي‌هنجاري منفي قابل ملاحظه ارتفاع در بخش شرقي مديترانه و اروپا و بي‌هنجاري مثبت بارش زمستانه بر روي ايران، بويژه در نيمه غربي كشور، همراه مي‌باشد. در فاز شش بي‌هنجاري مثبت بارش (حدود 30 تا 150 درصد) در بخش عمده نيمه غربي و دامنه‌هاي جنوبي البرز و در فاز هشت علاوه بر نيمه غربي در بخش‌هاي شرقي كشور رخ مي‌دهد.

The Madden-Julian Oscillation (MJO) is the dominant mode of tropical intraseasonal variability, characterized by its planetary spatial scale, 30–60 day period, and eastward propagation. The extra-tropical links with the MJO are well established (Barlow et al., 2005). Such oscillation has an evident seasonality, with larger amplitude during boreal winter and spring than summer (Zhou et al., 2012). MJO can significantly modulate variations in weather and climate in the far-reaching subtropics and midlatitudes (Handerson et al., 2016). In this study, we examined the influence of the MJO on precipitation and large-scale circulation anomalies based on a 10-years daily data from 2000–2010. We further analyzed the atmospheric circulations, Iran precipitation data and their anomalies with respect to the real-time multivariate (RMM) Index phases of the MJO whose employed index was that developed by Wheeler and Hendon (2004), downloaded from the website of the Australia Meteorological Bureau. The MJO index data used in this research, considering that at least five consecutive days in a phase remained constant and its value was equal or more than 1, was extended from December 2000 to February 2010. Daily precipitation data were obtained from 47 meteorological synoptic stations in Iran from December 2000 to February 2010.Furthermore, the grid point data were extracted from NCEP-NCAR reanalysis dataset. Daily anomalies of precipitation and other variables were calculated by subtracting their 11-year means from the original data. Composites of mean daily anomalies were computed for each of the eight phases of the MJO on the basis of the RMM index with the same MJO phases in boreal winter during 2000-2010. Composites of the regional flow associated with the MJO phases during winter seasons were calculated by averaging fields of data over lists of dates obtained from the analysis of the MJO indices. Figures 2 to 6 display the composites of large-scale atmospheric circulations and precipitation anomalies for the eight MJO phases. It is very clear that winter large-scale atmospheric circulations and precipitation anomalies in Iran show meaningful and significant variations when the MJO propagates from the western Indian Ocean (phase 1) into the central Pacific Ocean (phase 8). As is observed in the figures, winter precipitation in certain parts of Iran is higher than the 1981-2010 climate normals when the MJO is in phase 2, which is close to the mean; positive anomalies weaken over the country as we get closer to phase 3. When the MJO is in phases 4 and 5, precipitation anomalies are less than normal, with the maximum negative anomalies reaching around 100% relative to the mean. During such phases (4 and 5), associated with positive geopotential height anomalies over the eastern Mediterranean Sea, precipitation negative anomalies occur in the country. Winter precipitation in certain regions is much higher than the climate normal when the MJO is in phases 6 and 8. Further observed in these phases were the precipitation positive anomalies over the vast parts of western and southern slopes of the Alborz (from 30 to 150%). So, in these situations which associated with zonal dipole of geopotential heights anomalies over Eurasia where strong negative anomalies of geopotential heights were located over the eastern Mediterranean Sea and its neighborhoods areas, the precipitations over the most part of Iran are more than the climate normals. Extrapolated from the foregoing data is the fact that the MJO influence on Iran precipitation is significant during northern winter season. When the MJO is in phases 6 and 8 (the convection increases over central and eastern Pacific Ocean), more precipitation is observed in the western regions of Iran. On the other hand, as enhanced tropical convection shifts over the Maritime continent, less precipitation is seen over the country. Therefore, owing to the broad tropical and extratropical impacts of the MJO on interaseasonal timescales, a better understanding of the MJO is potentially conducive to the amelioration of the extended range forecast of week-two and beyond, practically when there is an on-going MJO event. Numerical and empirical model experiments have shown the potential predictability of MJO up to 4 weeks. In this study, the objective was to develop composites in order to provide a compendious, large-scale overview of MJO impact on the winter season circulations and precipitation in Iran.