INTERANNUAL VARIABILITY OF SOUTH‐EASTERN AFRICAN SUMMER RAINFALL. PART 1: RELATIONSHIPS WITH AIR–SEA INTERACTION PROCESSES
INTERANNUAL VARIABILITY OF SOUTH‐EASTERN AFRICAN SUMMER RAINFALL. PART 1: RELATIONSHIPS WITH AIR–SEA INTERACTION PROCESSES
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DOI：
10.1002/(sici)1097-0088(19970315)17:3
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发表时间：
1997-03
期刊：
影响因子：
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通讯作者：
A. Rocha;I. Simmonds
中科院分区：
文献类型：
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作者：
A. Rocha;I. Simmonds
本文研究了海气相互作用过程在非洲东南部夏季降雨年际变化中可能发挥的作用。首先利用主成分分析确定了非洲东南部夏季降雨的主要空间模式。保留四种模式。我们发现最重要的变化模式代表了大部分地区的降雨量变化，特别是在南部地区。针对不同的 SOI 导程，详细研究了 ENSO（由 SOI 测量）对夏季降雨的影响。这种关系使得在 ENSO 事件发生后的夏季，非洲东南部往往会经历干燥的天气。与 SOI 领先降雨量约 3 至 6 个月的关系最强。第二个指数是布兰登-马里恩指数 (BMI)，它表明印度洋上空气压场的变化，与降雨量的相关性比 SOI 更好。当该指数领先降雨量约 1 至 3 个月时，相关性最强。更重要的是，偏相关分析表明，BMI 对降雨的影响与 ENSO 无关。 SOI 和 BMI 都是夏季降雨量的潜在预测因子。对降雨与全球海温异常关联的调查揭示了热带印度洋和太平洋地区与次大陆降雨变化有关。这种关系使得非洲东南部大部分地区在异常温暖之后往往会出现干旱。当海表温度领先降雨季节约 1 至 3 个月时，相关性最强。在非洲东南部干燥的夏季，可以发现明确的大气异常现象。其中包括对流层异常温暖的温度和印度洋中部上空明显的低层气旋环流异常，这些异常在非洲东南海岸的大部分地区产生异常微弱的东风。这些对低层水流的扰动会转移大陆上的水分，导致降水减少。一个重要且相关的发现是，在消除 ENSO 影响后，印度洋上空的海温与降雨的联系仍然很强，这表明在干燥的夏季非洲东南部上空观测到的大气环流异常主要与印度洋上空的海温异常有关。这一假设将在另一篇论文中通过一系列 GCM 模拟进行检验。
This paper investigates the role that air‐sea interaction processes may play in interannual variability of south-eastern African summer rainfall. The principal spatial modes of south-eastern African summer rainfall are first identified using principal component analysis. Four modes are retained. The most important mode of variability is found to represent rainfall variability over most of the domain, particularly in the regions to the south. The influence of ENSO (as measured by the SOI) on summer rainfall is investigated in detail for different SOI leads. The relationship is such that during the summer following the onset of an ENSO event, south-eastern Africa tends to experience dry conditions. Strongest relationships are found with the SOI leading rainfall by about 3 to 6 months. A second index, the Brandon‐Marion Index (BMI) which is indicative of changes in the pressure field over the Indian Ocean correlates with rainfall better than the SOI. Strongest correlations are found when this index leads rainfall by about 1 to 3 months. More importantly, a partial correlation analysis reveals that the BMI influences rainfall independently of ENSO. Both the SOI and the BMI are potential predictors of summer rainfall. An investigation of rainfall associations with global SST anomalies reveals areas in the tropical Indian and Pacific Oceans that are linked with rainfall changes over the subcontinent. The relationship is such that warm anomalies tend to be followed by dry conditions over much of south-eastern Africa. Strongest relationships are found when SSTs lead the rainfall season by about 1 to 3 months. Well-defined atmospheric anomalies are identified during dry south-eastern African summers. These include, amongst others, anomalously warm tropospheric temperatures and marked low-level cyclonic circulation anomalies over the central Indian Ocean, which generate abnormally weak easterly winds along much of the south-eastern coast of Africa. These perturbations to the low-level flow divert moisture from the continent and result in precipitation decreases. An important and related finding is the fact that the SST‐rainfall link over the Indian Ocean remains strong after the ENSO effects have been removed, suggesting that the atmospheric circulation anomalies observed over south-eastern Africa during dry summers, are linked mainly to SST anomalies over the Indian Ocean. This hypothesis will be tested in a companion paper through a series of GCM simulations.