A possible mechanism for the weakening of El Niño-monsoon relationship during the recent decade
A possible mechanism for the weakening of El Niño-monsoon relationship during the recent decade
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DOI：
10.1007/s00703-003-0608-8
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发表时间：
2004-05
影响因子：
2
通讯作者：
D. S. Pai
中科院分区：
文献类型：
--
作者：
D. S. Pai
利用观测资料探讨了地面气温异常梯度与印度夏季风降水的关系。1月份横跨欧亚大陆的赤道(极)经向温度异常梯度是随后印度夏季风降水过剩(不足)的一个很好的前兆。这种指向赤道(极)的梯度表明欧亚大陆上空低于(高于)正常的阻塞活动，这导致印度季风区上空的中纬度西风干冷气流的南下穿透比正常的少(多)，最终加强(减弱)正常的季风环流。尽管厄尔尼诺(暖ENSO)与印度夏季风降水(ISMR)有很强的基本联系，但在1921-1940年期间和最近十年(1991-1998年)，这种联系很弱。在1921-1940年和1901-1998年的厄尔尼诺期间，1月份横跨欧亚大陆的经向温度异常梯度(欧亚强迫)指向赤道。另一方面，在1901-1920年和1941-1990年的厄尔尼诺期间，这种梯度指向极点。因此，在1921-1940年和1991-1998年期间，厄尔尼诺现象对印度季风的不利影响被有利的欧亚强迫所减弱，从而导致厄尔尼诺现象与ISMR之间的弱联系。这一发现与欧亚大陆冬季变暖的假说不一致，该假说是近十年来观察到的这种关系减弱的原因。
Observational data are used to explore the relationship between surface air temperature anomaly gradients and Indian summer monsoon rainfall (ISMR). The meridional temperature anomaly gradient across Eurasia during January directed towards equator (pole) is a very good precursor of subsequent excess (deficient) Indian summer monsoon rainfall (ISMR). This gradient directed towards equator (pole) indicates below (above) normal blocking activity over Eurasia, which leads to less (more) than normal southward penetration of dry and cold mid latitude westerlies over the Indian monsoon region, which ultimately strengthens (weakens) the normal monsoon circulation. These findings suggest a mechanism for the weakening of relationship between El Niño and ISMR.Though there is a strong fundamental association between El Niño (warm ENSO) and deficient Indian summer monsoon rainfall (ISMR), this relationship was weak during the period 1921–1940 and the recent decade (1991–1998). During the El Niño years of 1921–1940 and 1901–1998, the meridional temperature anomaly gradient across Eurasia (Eurasian forcing) during January was directed towards equator. On the other hand, during the El Niño years of 1901–1920 and 1941–1990 this gradient was directed towards pole. Thus during 1921–1940 and 1991–1998, the adverse impact of El Niño on Indian monsoon was reduced by the favorable Eurasian forcing resulting in the weak association between El Niño and ISMR. This finding disagrees with the hypothesis of winter warming over the Eurasian continent as the reason for the observed weakening of this relationship during recent decade.