Analysis of rainfall variability over Tanzania in late austral summer

Abstract

Based on site-observation data, NCEP–NCAR reanalysis data, and Climatic Research Unit gridded data, the rainfall variability over Tanzania during late austral summer (January–March, JFM) was analyzed for the period 1961–2011. Further, the associated atmospheric circulation and SST anomalies (SSTAs) were explored to understand the mechanisms of dry- and wet-year cases based on an interannual time scale. The correlation, Morlet wavelet power spectrum, and composite analysis methods were employed. The results showed that the JFM standardized rainfall anomaly time series exhibited significant time scales of variability at interannual (2–8 years) and quasi-decadal (8–12 years). During dry years, anomalous anticyclonic northeasterly flow originating from western tropical Indian and southeast trades from the Indian Ocean to the southeast were associated with subsiding dry air, which resulted in suppression of rainfall as observed. In the typical wet-year cases, meanwhile, anomalous westerlies from the tropical and southeast Atlantic were strengthened over the Congo basin, delivering more precipitation to the region. Significant correlation was exhibited over the western tropical and southeast Indian Ocean, as well as the southeast and tropical Atlantic Ocean. These SSTA patterns favored atmospheric general circulation anomalies that were closely related to JFM rainfall over Tanzania.摘要利用站点观测资料和再分析资料, 采用相关分析, Morlet小波功率谱分析和复合分析等方法, 研究了1961–2011年南半球夏季后期 (1–3月) 坦桑尼亚降水的年际变化特征, 并探讨了相关的大气环流和海温异常情况, 以及坦桑尼亚干,湿年发生的机制. 研究结果表明: 坦桑尼亚1–3月降水变化存在显著的2–8年的年际变化周期和8–12年准年代变化周期. 在坦桑尼亚1–3月降水异常偏少的典型干旱年, 来自热带西印度的异常反气旋的东北气流和北印度洋东南气流造成干燥空气下沉, 从而抑制坦桑尼亚地区降水; 而在典型多雨年, 来自非洲大陆热带和东南大西洋的异常西风气流在刚果盆地上空显著偏强, 从而带来更多降水. 热带印度洋和印度洋东南部, 大西洋东南部和热带大西洋均表现出显著的相关性. 此外, 热带中太平洋和南太平洋中部也存在显著的相关. 这些海温异常型与坦桑尼亚1–3月的降水及相关大气环流异常有密切的关联.