ENSO冷暖位相影响东亚冬季风与东亚夏季风联系的非对称性

东亚冬季风（East Asian Winter Monsoon,简称EAWM）和东亚夏季风（East Asian Summer Monsoon,简称EASM）作为东亚季风系统的两个组成部分,他们之间存在显著的转换关系。前人的研究表明EAWM与次年EASM的转换关系只有在ENSO事件发生时才显著,然而这些研究都是基于ENSO对大气环流的影响是对称的这一假设下进行的。本文的研究表明EAWM和次年EASM的转换关系在ENSO冷暖事件中存在着明显的不对称性。通过将EAWM分为与ENSO有关的部分（EAWM_EN）和与ENSO无关的部分（EAWM_RES）,我们发现在强EAWM_EN年（即La Ni?a年）,在西北太平洋会存在一个从冬季维持到次年夏季的气旋性环流异常（the anomalous western North Pacific Cyclone,WNPC）,从而造成EASM偏弱;而在弱EAWM_EN年（即El Ni?o年时）,在西北太平洋会存在一个从冬季维持到次年夏季的反气旋性环流异常（the anomalous western North Pacific anticyclone,WNPAC）,从而引起次年EASM偏强。比较而言,WNPAC的位置比WNPC的位置偏南,且强度更强,因而在El Ni?o年能够引起次年EASM更大幅度的增强。造成这一不对称联系的主要原因是热带太平洋和印度洋异常海温的演变差异。在强EAWM_EN年,热带太平洋的负海温异常衰减地较慢,使得在次年夏季仍然维持着显著的负异常海温;相反,在弱EAWM_EN年,热带太平洋的正海温异常衰减地较快,以至于在次年夏季的异常海温信号已经基本消失,但此时印度洋却有着显著的暖海温异常。海温演变的差异进一步造成了大气环流的差异,从而导致EAWM与次年EASM联系的不对称性。

Asymmetric Role of ENSO in the Link between the East Asian Winter Monsoon and the Following Summer Monsoon

Evidence demonstrates a link exists between the East Asian winter monsoon (EAWM) and the following East Asian summer monsoon (EASM). Previous studies have found that the EAWM and the following EASM are only closely linked in ENSO years. However, all these studies are based on the hypothesis that the atmosphere''s response to ENSO is symmetric. In fact, we demonstrate ENSO plays an asymmetric role in the EAWM-EASM link. This study divides the variability of the EAWM into an ENSO-related part (EAWM_EN) and ENSO-unrelated part (EAWM_RES). The results demonstrate that during strong EAWM_EN conditions (i.e., La Niña events), an anomalous cyclone persists from winter to summer over the western North Pacific (WNP), which can cause a weak EASM in the following summer. In contrast, during weak EAWM_EN conditions (i.e., El Niño events), an anomalous anticyclone persisting from winter to summer is observed over the WNP. However, the anomalous anticyclone''s intensity is much stronger than the counterpart and is located more southward. As a result, the intensity of EASM anomalies is stronger during weak EAWM_EN years. This asymmetry may be attributed to the different evolutions of the SST anomalies over the tropical Pacific Ocean and the North Indian Ocean. During strong EAWM_EN conditions, the SST anomalies in the tropical central-eastern Pacific decay slowly and thus significant signals still persist in the following summer, which contribute to the persistence of the WNP anomalous cyclone in the following summer. However, during weak EAWM_EN conditions, the SST anomalies decay quickly in the tropical Pacific, while the Indian Ocean warms significantly in the following summer. Thus the anomalous anticyclone in the WNP results from the significantly warmed Indian Ocean through the "capacitor mechanism".