The El Nio effect on Ethiopian summer rainfall
The El Nio effect on Ethiopian summer rainfall
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DOI：
10.1007/s00382-016-3421-z
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发表时间：
2017-09-01
期刊：
影响因子：
4.6
通讯作者：
Korecha, Diriba
中科院分区：
文献类型：
--
作者：
Gleixner, Stephanie;Keenlyside, Noel;Korecha, Diriba
虽然人们知道厄尔尼诺现象会导致埃塞俄比亚北部夏季降水减少，但其机制尚不完全清楚。本文利用ECHAM5大气环流模式研究了太平洋海表温度异常与kremt降水之间的物理联系。我们将ECHAM5模拟与重建的海温数据、基于测量的降雨观测和1961-2009年期间的大气再分析进行了比较。我们仅在赤道太平洋海温异常驱动下进行了复合分析和灵敏度实验。我们的研究结果表明，赤道太平洋温暖的海温异常驱动了相应的大尺度环流异常，并在干旱季节在埃塞俄比亚上空下沉。水平风场显示整个印度季风系统在这些夏季减弱，热带东风急流减弱，东非低空急流减弱。这些变化可以看作是北非的一个异常环流单体，在100-200 hPa有西风带，在500 hPa以下有东风带。地面的东风可能会减少从大西洋和刚果盆地流入埃塞俄比亚的水分。这一点和该地区的普遍沉降可以解释克里姆特降雨量的减少。我们的结果表明，高达50%的kremt降水异常是由赤道太平洋海温变率驱动的。
While El Nio is known to cause failure of Kiremt (boreal summer) rainfall in Ethiopia, the mechanisms are not fully understood. Here we use the ECHAM5 Atmospheric General Circulation Model to investigate the physical link between Pacific sea surface temperature (SST) anomalies and Kiremt rainfall. We compare ECHAM5 simulations forced with reconstructed SST data, to gauge-based rainfall observations and atmospheric reanalysis for the time period of 1961-2009. We perform composite analysis and sensitivity experiments driven only with equatorial Pacific SST anomalies. Our results show warm SST anomalies in the equatorial Pacific drive a corresponding large-scale circulation anomaly with subsidence over Ethiopia in dry Kiremt seasons. Horizontal wind fields show a slow-down of the whole Indian monsoon system with a weaker Tropical Easterly Jet and a weaker East African Low-Level Jet in these summers. These changes can be seen as an anomalous circulation cell over northern Africa with westerlies at 100-200 hPa and easterlies below 500 hPa. Surface easterlies might reduce the moisture inflow from the Atlantic and Congo basin into Ethiopia. This and the general subsidence over the region could explain the reduction in Kiremt rainfall. Our results suggest up to 50% of the Kiremt rainfall anomalies is driven by equatorial Pacific SST variability.