Circulation anomalies associated with tropical-temperate troughs in southern Africa and the south west Indian Ocean
Circulation anomalies associated with tropical-temperate troughs in southern Africa and the south west Indian Ocean
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与南部非洲和西南印度洋热带温带槽相关的环流异常
DOI：
10.1007/s003820050323
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发表时间：
1999
期刊：
影响因子：
4.6
通讯作者：
R. Washington
中科院分区：
文献类型：
--
作者：
Martin C. Todd;R. Washington
摘要最近首次利用新获得的卫星产品客观地描述了南部非洲（SA）和西南印度洋（SWIO）夏季的日降雨变化。所有月份的主要变化模式是偶极子结构，降雨带从西北向东南方向穿过该地区。这些代表了与热带温带槽（TTT）有关的云带的位置。本研究客观地识别了11月至次年2月期间的主要TTT事件，并根据NCEP再分析资料的合成描述了相关的大气结构。降雨偶极子的两个阶段与明显对比的环流模式。此外，还有明显的季节内变化。初夏，温带槽和TTT云带的位置在西南印度洋和西南大西洋之间交替。在夏末，TTT主轴优先位于西南印度洋上方，与印度洋高压的东移有关。在所有的月份，积极的事件，其中TTT云带主要位于SWIO，与大规模的水汽通量异常，其中辐合通量形成一个明显的极向通量沿着云带。这表明TTT事件是能量和动量向极转移的主要机制。水汽输送沿着三条主要路径发生：（1）北方或中部印度洋（异常通量向东延伸到海洋大陆），（2）赤道以南的非洲和赤道大西洋，（3）从南部围绕热带-温带槽的气旋性气流内。（2）的相对重要性在夏末最大。因此，天气尺度的TTT事件在SA/SWIO往往是由大尺度的行星环流模式。Hovmoeller图显示，TTT的发展与10°-30 ° E之间的热带对流增强（本身表现出约5天的周期性）相一致，并且通常与40°E附近的向西传播对流的辐合相一致。200 hPa位势距平的调和分析表明，TTT特征是由一个特定的纬向非对称波型，与波5占主导地位或显着的，在所有的月份，除了2月时，准定常波1，2和3占主导地位。这些发现说明了热带和热带外的动力学在理解TTT事件的重要性。最后，它建议，在11月至1月TTT在SA/SWIO的降雨可能是在南太平洋和南大西洋辐合区观察到类似的降雨偶极子结构的相位。
Abstract Daily rainfall variability over southern Africa (SA) and the southwest Indian Ocean (SWIO) during the austral summer months has recently been described objectively for the first time, using newly derived satellite products. The principle mode of variability in all months is a dipole structure with bands of rainfall orientated northwest to southeast across the region. These represent the location of cloud bands associated with tropical temperate troughs (TTT). This study objectively identifies major TTT events during November to February, and on the basis of composites off NCEP reanalysis data describes the associated atmospheric structure. The two phases of the rainfall dipole are associated with markedly contrasting circulation patterns. There are also pronounced intra-seasonal variations. In early summer the position of the temperate trough and TTT cloud band alternates between the SWIO and southwest Atlantic. In late summer the major TTT axis lies preferentially over the SWIO, associated with an eastward displacement in the Indian Ocean high. In all months, positive events, in which the TTT cloud band lies primarily over the SWIO, are associated with large-scale moisture flux anomalies, in which convergent fluxes form a pronounced poleward flux along the cloud band. This suggests that TTT events are a major mechanism of poleward transfer of energy and momentum. Moisture transport occurs along three principle paths: (1) the northern or central Indian Ocean (where anomalous fluxes extend eastward to the Maritime Continent), (2) south equatorial Africa and the equatorial Atlantic, (3) from the south within a cyclonic flow around the tropical-temperate trough. The relative importance of (2) is greatest in late summer. Thus, synoptic scale TTT events over SA/SWIO often result from large-scale planetary circulation patterns. Hovmoeller plots show that TTT development coincides with enhanced tropical convection between 10°–30°E (itself exhibiting periodicity of around 5 days), and often with convergence of eastward and westward propagating convection around 40°E. Harmonic analysis of 200 hPa geopotential anomalies show that TTT features are forced by a specific zonally asymmetric wave pattern, with wave 5 dominant or significant in all months except February when quasi-stationary waves 1, 2 and 3 dominate. These findings illustrate the importance of tropical and extratropical dynamics in understanding TTT events. Finally, it is suggested that in November–Januar TTT rainfall over SA/SWIO may be in phase with similar rainfall dipole structures observed in the South Pacific and South Atlantic convergence zones.