بررسي پيوند نوسان‌هاي مادن‌- جوليان با النينو- نوسان‌هاي جنوبي و بازخورد آن بر بارش‌هاي پاييزه استان فارس

On the Madden-Julian Oscillations and El Niño-Southern Oscillation connection and their feedback on autumnal precipitation in Fars province, Iran

النينو- نوسان­ هاي جنوبي (ENSO) و نوسان­ هاي مادن‌- جوليان (MJO) دو پديده بزرگ­ مقياس جوي‌- اقيانوسي در پهنه­ هاي استوايي اقيانوس آرام و هند هستند كه بر نوسان­ هاي اقليمي گستره ­هاي استوايي تا جنب حاره ­اي تأثير قابل توجهي مي­ گذارند. در اين پژوهش، پس از تعريف فازهاي مثبت و منفي MJO، برهم‌كنش اين فازها با فازهاي ENSO ارزيابي و تأثير اين برهم‌كنش بر ابرناكي و روانش رطوبت در سطح خاورميانه و اقيانوس­هاي آرام و هند بررسي و بازخورد اين روانش رطوبتي بر رخداد بارش پاييزه 9 ايستگاه همديدي و باران‌سنجي استان فارس در بازه زماني 2013-1974 واكاوي شد. در دوران النينو، احتمال رخداد هريك از فازهاي مثبت يا منفي MJO تقريبا يكسان است در‌‌حالي‌كه در دوران لانينا، احتمال رخداد فاز مثبت MJO نزديك به دو برابر فاز منفي است. چيرگي فاز منفي MJO در دوران النينو با افزايش كاركردهاي همرفتي در گستره­ هاي استوايي غرب اقيانوس هند و تا اندازه ­اي در شرق اقيانوس آرام همراه است. اين همزماني، وزش بادهاي نمناك شرقي را از اقيانوس هند به سوي شبه‌جزيره عربستان، خليج فارس و جنوب ايران افزايش مي دهد. در دوران النينو، بارش و احتمال رخداد آن در فاز منفي MJO در همه ايستگاه­ هاي بررسي شده به‌گونه معني ­داري بيشتر از مقادير مشابه در دوران فاز مثبتMJO است. در دوران لانينا، چيرگي رخداد فاز مثبت يا منفي MJO، ويژگي­ هاي بارش فارس نوسان درخور‌ توجهي نداشت. با چيرگي فاز منفي MJO در دوران النينو، بيشترين و كمترين افزايش شدت بارش نسبت به فاز مثبت در اين دوران، در بخش­هاي شرقي و جنوب­شرقي استان فارس به‌ترتيب به اندازه 20-5 و 100-80 درصد است. در بيشتر گستره­هاي مركزي و جنوب­غربي استان، افزايش 60-40 درصدي و در بخش­هاي شمالي و جنوب­شرقي، افزايش 110-60 درصدي احتمال رخداد بارش فاز منفي MJO در دوران گرم ENSO نسبت به فاز مثبت آن در دوران گرم ديده مي­ شود.

The Madden-Julian Oscillation (MJO) and El Niño Southern Oscillation (ENSO) are known as the primary modes of large-scale climate variability in the equatorial parts of the Indian and Pacific Oceans, respectively. The present study has made an effort to analyze interactions of these two oscillations during boreal autumn and the feedback of these interactions on the occurrence of dry or wet periods in Fars province. Cold and warm phases of ENSO (La Niña and El Niño, respectively) are then classified according to the size of the SOI. Then positive and negative phases of the MJO were defined according to the algebraic sign of the real-time multivariate MJO series 1 (RMM1). Daily values of October-December precipitation data of 9 synoptic stations of Fars province were obtained for the 1974-2013 period. Precipitation amount and the occurrence probability of precipitation were investigated for La Niña, El Niño MJO positive phase, MJO negative phase and other four combinations of these phases. Since the impacts of ENSO or MJO phases on precipitation variability had been studied previously, the present study mainly motivated to investigate precipitation characteristics for the episodes that El Nino or La Nina events were concurrent with the MJO positive phase (El-P or La-P, respectively) or MJO negative phase (El-N and La-N, respectively). The characteristics of atmospheric circulation and vapor transport over the ocean waters and the Middle East are then investigated to justify the obtained results. Some parameters including precipitation, intensity and occurrence probability of a rainy day were computed for all phases, separately. The ratio between these parameters during opposite phases was computed and spatially mapped to evaluate the effects of the interactions. The results indicated that the frequency of the positive and negative MJO phases is significantly associated with the ENSO condition. Differences between the frequency of the positive and negative MJO phases during the El Niño events were found to be insignificant. Contrariwise, during the El Niño events, the frequency and occurrence probability of the positive MJO phase are about twice than corresponding statistics during the MJO negative phase. It was found that ENSO and MJO have a significant influence on autumnal precipitation in Fars province. During El Niño or negative MJO phase, the mean occurrence probability and intensity of precipitation were significantly greater than the corresponding values during La Nina or positive MJO in Fars. For almost entire parts and for the periods of El Nino, precipitation has significantly enhanced or suppressed during the negative and positive MJO phase, respectively. On the other hand, such differences are not significant when the opposite phases of the MJO have concurred with the La Nina events. Compare to El-P, mean precipitation during El-N has increased by about 300% in the southeast and about 70% in the eastern districts. When the El-N phase has prevailed, the autumnal precipitation intensity has increased about 40-110% and 4-40% over the southern and northern area of Fars, respectively.