Links between African easterly waves, midlatitude circulation and intraseasonal pulsations of the West African heat low
Links between African easterly waves, midlatitude circulation and intraseasonal pulsations of the West African heat low
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非洲东波、中纬度环流和西非低热季节内脉动之间的联系
DOI：
10.1002/qj.555
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发表时间：
2010
影响因子：
8.9
通讯作者：
P. Knippertz
中科院分区：
文献类型：
--
作者：
C. Lavaysse;C. Flamant;S. Janicot;P. Knippertz
根据欧洲中期天气预报中心的分析，2006 年夏季，撒哈拉沙漠上空的热低压强度在 9 月的前几周显着减弱。与此同时，萨赫勒地区上空检测到广泛的对流活动。本研究的目的是确定西非高温低压（WAHL）活动减少的原因，讨论与对流的可能关系，并评估此类事件在气候时间尺度上的代表性。通过对 2006 年夏季每日 WAHL 厚度的光谱分析，确定了两个显着季节内脉动的周期带，并使用 ERA-40 再分析进行了确认，即 3-10 d 和 10-30 d。在 2006 年的案例研究和气候综合研究中，我们发现 3-10 d 周期内的不利条件与对流层低层潮湿和凉爽的平流有关，这些平流与非洲东风波 (AEW) 南段相关，从而增加了萨赫勒地区的对流活动。当AEW与中纬度低压相互作用时，这些来自南方的潮湿和寒冷的平流会更加明显。在10~30 d周期内，中纬度环流的影响得到体现。在 WAHL 厚度 10-30 d 脉动崩溃期间，西非北部上空出现上层低压槽。这种情况导致700 hPa的冷空气从利比亚涌入撒哈拉，925 hPa的反气旋环流增加，萨赫勒地区的对流活动显着增加。版权所有 © 2010 英国皇家气象学会
During summer 2006, the intensity of the thermal depression over the Sahara, derived from European Centre for Medium‐range Weather Forecast analyses, exhibited a strong decrease during the first couple of weeks of September. Simultaneously, widespread convective activity over the Sahel was detected. The aims of this study are to identify the reasons for this decrease of the West African Heat Low (WAHL) activity, to discuss the possible relationship with convection and to assess the representativity of such an event at the climatological time‐scale. From spectral analysis of the daily WAHL thickness during summer 2006, two period bands of significant intraseasonal pulsations were identified, and confirmed using the ERA‐40 reanalysis, namely 3–10 d and 10–30 d. In both the 2006 case‐study and the climatological composite study, we find that detrimental conditions in the 3–10 d period band are associated with moist and cool advections in the lower troposphere linked to the southerly sector of African easterly waves (AEWs) which increase convective activity over the Sahel. These humid and cold advections from the south are more pronounced when the AEW interacts with a midlatitude depression. In the 10–30 d period band, the impact of midlatitude circulation is demonstrated. During the collapsing period of the 10–30 d pulsations of the WAHL thickness, an upper‐level trough is seen over northern West Africa. This situation generates a surge of 700 hPa cold air from Libya into the Sahara, an increase of the 925 hPa anticyclonic circulation and a significant increase of convective activity over the Sahel. Copyright © 2010 Royal Meteorological Society