A mechanism for African monsoon breaks: Mediterranean cold air surges
A mechanism for African monsoon breaks: Mediterranean cold air surges
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非洲季风爆发的机制：地中海冷空气涌动
DOI：
10.1029/2008jd010654
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发表时间：
2009
期刊：
影响因子：
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通讯作者：
K. Cook
中科院分区：
文献类型：
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作者：
E. Vizy;K. Cook
从地中海到北方非洲北部的夏季冷空气激增的记录，并使用热带降雨测量使命降雨估计和再分析产品的季风中断的影响进行了分析。在1998年至2006年期间，每年夏季都会出现6-10次冷空气潮，低层温度异常范围从低于-1 K到超过-6 K。合成分析表明，北方非洲上空的冷空气潮持续2-10天，以约5.5 m s-1的速度向赤道移动，比观测到的气候学低层纬向气流快0.5-1.5 m s-1。北方非洲冷涌具有与世界其他地方观察到的冷涌相似的特征，包括流体静力引起的表面压力脊和放大的对流层上部脊/槽模式。在非洲冷涌之前，通过一个短波槽和加强对流层上层副热带西风急流条纹在地中海。这些事件与地中海中部急流入口区域的汇合增加、直接次级环流增强、下沉和非洲东北部上空的低层非地转偏北气流有关。合成分析表明，冷涌的通过与阿尔及利亚南部、尼日尔西部、马里北方和毛里塔尼亚上空对流活动的增强有关，冷涌到达萨赫勒东部前2至5天（北纬17.5度），当东北气流在阿特拉斯山脉和阿哈加尔山脉之间加强并将来自地中海西部和北大西洋东部的相对潮湿的空气输送到该地区时，在20°N以北的非洲西部地区，水汽辐合增加。在苏丹东部萨赫勒和乍得东部，合成结果显示，当风暴潮到达时，对流活动中断，低层辐合减少，持续约6天。这些结果为改善北方非洲降雨的短期预测提供了很大的希望。
Surges of cold air from the Mediterranean into northern Africa during the boreal summer are documented, and their influence on monsoon breaks is analyzed using Tropical Rainfall Measuring Mission rainfall estimates and reanalysis products. Between 1998 and 2006, 6–10 cold air surges occurred each summer, with low‐level temperature anomalies ranging from less than −1 K to over −6 K. Composite analysis indicates that cold air surges over northern Africa persist for 2–10 days and travel equatorward at approximately 5.5 m s–1, which is 0.5–1.5 m s–1 faster than the observed climatological low‐level meridional flow. Northern African cold surges have characteristics similar to surges observed elsewhere in the world, including a hydrostatically induced ridge of surface pressure and an amplified upper tropospheric ridge/trough pattern. The African cold surge is preceded by the passage of a shortwave trough and an intensification of the upper tropospheric subtropical westerly jet streak over the Mediterranean Sea. These events are associated with increased confluence in the jet entrance region over the central Mediterranean, an enhanced direct secondary circulation, subsidence, and low‐level ageostrophic northerly flow over northeastern Africa. Composite analysis shows that the passage of a cold surge is associated with an enhancement in convective activity over southern Algeria, western Niger, northern Mali, and Mauritania 2 to 5 days before the surge reaches the eastern Sahel (∼17.5°N), when northeasterly flow channeled between the Atlas and Ahaggar Mountains strengthens and transports relatively moist air from the western Mediterranean and eastern North Atlantic over the region and increases moisture convergence over western Africa north of 20°N. Over the eastern Sahel of Sudan and eastern Chad, the composite results reveal a break in convective activity and decrease in low‐level convergence when the surge arrives that persists for about 6 days. These results offer great promise for improving the short‐range prediction of rainfall over northern Africa.