The Angola Low: relationship with southern African rainfall and ENSO
The Angola Low: relationship with southern African rainfall and ENSO
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安哥拉低压：与南部非洲降雨量和 ENSO 的关系
DOI：
10.1007/s00382-018-4222-3
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发表时间：
2019
期刊：
影响因子：
4.6
通讯作者：
J. Pergaud
中科院分区：
文献类型：
--
作者：
J. Crétat;B. Pohl;B. Dieppois;S. Berthou;J. Pergaud
对1980/81-2014/15年11月至3月安哥拉及周边国家700 hpa风涡度日异常特征进行聚类分析，确定了安哥拉低压的主要状态。在日时间尺度上，我们考察了AL的主要状态对南部非洲日降雨量的调节程度。在年际时间尺度上，我们评估了这些AL状态的发生与El Niño南方涛动（ENSO）之间的关系，以及AL在解释ENSO有时未能驱动南部非洲降雨方面的作用。考虑了三种重新分析，以解释由于南部非洲可用于同化的数据缺乏而引起的不确定性。确定了安哥拉低压的三种优先状态：接近其季节气候学的AL状态，具有轻微的纬向位移，异常弱AL状态和异常强AL状态，具有经向位移。这些不同的州都显著地调节着南部非洲的日降雨量。近气候性AL状态促进了南亚副热带东部降水异常和西部干旱降水异常。在近气候位置轻微西移导致降水纬向梯度逆转。其余的制度显著调节南部非洲降雨的经向梯度。异常微弱和异常偏北的AL状态促进了热带南部非洲的湿降雨异常和亚热带南部非洲的干降雨异常。在年际尺度上，ENSO显著调节了三次再分析中大多数AL状态的季节发生。在所有再分析中，异常微弱和向南的AL状态与区域降雨的相关性比ENSO更强，这表明考虑AL变率可能会改善季节性预报。对1982/83年和1997/98年两次El Niño事件的个案分析表明，1997/98年弱降水异常和强季节性AL可能是ENSO和印度洋耦合变率模式相互抵消的结果。
The main states of the Angola Low (AL) are identified using clustering analysis applied to daily anomalous patterns of 700-hPa wind vorticity over Angola and adjacent countries from November to March for the 1980/81–2014/15 period. At the daily timescale, we examine the extent to which the main states of the AL modulate daily rainfall over southern Africa. At the interannual timescale, we assess both the relationship between the occurrence of these AL states and El Niño southern oscillation (ENSO) and the role of the AL in explaining ENSO’s failure in driving southern African rainfall at times. Three reanalyses are considered to account for uncertainties induced by the scarcity of data available for assimilation over southern Africa. Three preferential states of the Angola Low are identified: AL state close to its seasonal climatology with slight zonal displacements, anomalously weak AL state and anomalously strong AL state with meridional displacements. These different states all significantly modulate daily southern African rainfall. Near-climatological AL state promotes wet rainfall anomalies over eastern subtropical southern Africa and dry rainfall anomalies over its western part. A slight westward shift in the near-climatological position of the AL leads to reversed zonal gradient in rainfall. The remaining regimes significantly modulate the meridional gradient in southern African rainfall. Anomalously weak and anomalously northward AL states promote wet rainfall anomalies over tropical southern Africa and dry rainfall anomalies over subtropical southern Africa. The reverse prevails for anomalously southward AL. At the interannual timescale, ENSO significantly modulates the seasonal occurrence of most AL states in the three reanalyses. Anomalously weak and southward AL states are more strongly correlated with regional rainfall than ENSO in all reanalyses, suggesting that accounting for AL variability may improve seasonal forecasts. Case study analysis of the major 1982/83 and 1997/98 El Niño events suggests that the weak rainfall anomalies and strong seasonal AL in 1997/98 may result from counteracting effects between ENSO and Indian Ocean coupled modes of variability.