کارشناس ارشد مرکز پژوهشهای علوم جوّی-اقیانوسی دانشگاه شیراز
عنوان مقاله [English]
The Madden Julian Oscillation (MJO) is known as the primary mode of large-scale inter-seasonal variability in tropical regions that affects equatorial and extra-tropical climates. It was characterized as a 45-60 day wave that develops over the tropical Indian Ocean and then travels east across the tropics at 5-10 m/s. The phenomenon has a frequency of 6-12 events per year associated with a period ranging from 30 to 60 days. In its active stage, the MJO is associated with increased convective activity over the equatorial eastern Indian and western Pacific Oceans. This study investigated the effects of the MJO on the occurrence of wet and dry events in Khouzestan Province, the south west part of Iran during November-April. Monthly precipitation data from eight stations spread over various parts of the province was analyzed during 1979-2005. By using two well-known MJO indices (MK and WH), the positive and negative phases of the MJO phases (enhanced and suppressed convective activity over the equatorial Indonesian region, respectively) were identified for monthly and seasonal periods. The MJO-precipitation composites associated with opposite phases of the oscillation were constructed on seasonal timescales. Eight sets of the seasonal MJO-precipitation composites were, therefore, constructed for every station. The non-parametric Mann-Whitney test was then applied to investigate if the precipitation mean during the MJO positive phase of each composite is significantly different from its corresponding value during the negative phase. Moreover, the ratios of , and were computed for each station for monthly and seasonal timescales. Since the ratio of is greater than one, precipitation is reduced (enhanced) in the positive (negative) MJO phase.
Another examination was also performed to investigate if the frequency of dry or wet events was significantly associated with the occurrence of the positive or negative MJO phase, respectively. For conducting this examination, the events where precipitation amount was below or above the long-term average were first counted and considered as the frequency of dry or wet incidents, respectively. These frequencies were then put in a 2 by 2 contingency table to delineate the incidents of dry or wet events during each of the MJO phases. The Fisher Exact test was then applied to the constructed contingency tables. If the computed p was less than 0.05, the frequency of wet or dry events was significantly associated with the occurrence of the negative or positive phase of the MJO, respectively.
The results were shown that, for all considered stations, seasonal precipitation during the negative MJO phase was significantly greater (from about 1.9 to 3.0 fold) than corresponding values during the positive phase. Furthermore, the applied Mann-Whitney test indicates that the mean values of precipitation during the positive phase are statistically less than the corresponding values during the negative phase. In other words, the precipitation is reduced (enhanced) in the positive (negative) MJO phase. Moreover, the applied statistical tests have proved that the frequency of wet or dry events is related to the prevalence of the negative or positive MJO phase, respectively. As the positive MJO phase was engulfed, the probability of dry events varied from 50% to 90%. On the other hand, the probability of wet events was found to vary from 55% to 80% during the MJO negative phase.