ATMOSPHERIC-HYDROSPHERIC MECHANISMS OF CLIMATE ANOMALIES IN THE WESTERN EQUATORIAL INDIAN-OCEAN
ATMOSPHERIC-HYDROSPHERIC MECHANISMS OF CLIMATE ANOMALIES IN THE WESTERN EQUATORIAL INDIAN-OCEAN
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DOI：
10.1029/93jc02330
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发表时间：
1993-11-15
影响因子：
3.6
通讯作者：
GREISCHAR, L
中科院分区：
文献类型：
--
作者：
HASTENRATH, S;NICKLIS, A;GREISCHAR, L
利用印度洋的长期船舶观测、表层海流测量、次表层预报、欧洲中期天气预报中心的高空分析、东非和印度的雨量计系列以及南方涛动(SO)指数，研究了东非沿岸降水异常的大气-海洋机制，其中高-SO位相由塔希提/达尔文异常的高/低气压来定义。以4-5月和10-11月为中心的两个雨季，东非沿海降水异常的原因不同，只有后者与SO有较强的相关性。在高压-SO阶段，4-5月整个印度洋区域气压较低，而在10-11月，气压西高东低。与之相伴的是，西部的表层海水异常寒冷，强烈的西风席卷了印度洋的赤道地带。东非降雨量异常是通过在东非北部秋季雨季发挥最有效作用的多种合作机制的组合来实现的。(1)赤道西风有利于赤道东非地区对流层辐散的降低，在高位相，由于东向气压梯度的异常，这种辐散加速，特别是在10-11月。(2)赤道西风驱动上水圈赤道东急流，在盆地西端产生冷水上升流，海面温度进一步静力影响纬向气压梯度，并反馈到赤道西风。(3)西印度洋的冷水异常也抑制了对流，最明显的冷水异常出现在10-11月的高SO位相。(4)在高SO位相，印度夏季风趋于强烈，留下一个异常寒冷的西印度洋，西印度洋冷到机制1-3，向东的赤道急流对东非沿海北半球秋季降水异常起到反馈机制的作用。
Atmosphere-ocean mechanisms of rainfall anomalies at the coast of eastern Africa are studied using long-term ship observations in the Indian Ocean, surface current measurements, subsurface casts, upper air analyses by the European Centre for Medium Range Weather Forecasts, rain gauge series in eastern Africa and India, and an index of the Southern Oscillation (SO), the high-SO phase being defined by anomalously high/low pressure at Tahiti/Darwin. The causalities of precipitation anomalies at the coast of eastern Africa differ for the two rainy seasons centered on April-May and October-November, and only the latter is strongly related to the SO. In the high-SO phase, April-May pressure is low over the entire Indian Ocean domain, whereas in October-November, pressure is high in the west and low in the east. Concomitantly, surface waters are anomalously cold in the west, and strong westerlies sweep the equatorial zone of the Indian Ocean. Eastern African rainfall anomalies are related to the SO through a combination of cooperative mechanisms that function most effectively in the boreal autumn rainy season of eastern Africa. (1) Equatorial westerly winds are conducive to lower tropospheric divergence over equatorial East Africa, and in the high-SO phase these are accelerated, especially in October-November, owing to the anomalous eastward pressure gradient. (2) The equatorial westerly winds drive the eastward equatorial jet in the upper hydrosphere, which entails cold-water upwelling in the western extremity of the basin where sea surface temperature further hydrostatically affects the zonal pressure gradient and thus feeds back into the equatorial westerly winds. (3) In addition, cold-water anomalies in the western Indian Ocean, most pronounced in October-November during the high-SO phase, also suppress convection. (4) In the high-SO phase, the Indian summer monsoon tends to be strong, leaving behind an anomalously cold western Indian Ocean, which in turn feeds into mechanisms 1 to 3. The eastward equatorial jet thus has a role to play in feedback mechanisms contributing to the anomalies of the boreal autumn rains at the coast of eastern Africa.