Important Factors for the Development of the Asian–Northwest Pacific Summer Monsoon*
Important Factors for the Development of the Asian–Northwest Pacific Summer Monsoon*
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DOI：
10.1175/2008jcli2341.1
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发表时间：
2009-02
影响因子：
4.9
通讯作者：
H. Ueda;M. Ohba;S. Xie
中科院分区：
文献类型：
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作者：
H. Ueda;M. Ohba;S. Xie
亚洲和西北太平洋夏季风呈现阶段性转换，从5月中旬到7月中旬，降水以大约1个月的间隔快速变化。发展了一种新的方法来评估海表面温度(SST)和其他变化对这些快速季风转换的影响。后一种变化包括太阳辐射、陆地记忆和大气瞬变(SLAT)效应。该方法比较了两组大气环流模式(GCM)的模拟结果，分别用观测的季节变化和分段恒定的海温强迫。结果表明，除6月中旬强季风西风引起的海温降温是阻挡季风对流加强和向北推进的显著负反馈外，其他主要转折均以板条效应为主。季风系统的最后一次区域性爆发将于7月中旬在副热带西北太平洋上空发生，其特征是那里的深对流突然加强。尽管GCM的评估对海温的影响很弱，但在7月中旬的过渡期间，对流和环流的主要变化仅限于菲律宾以东的洋面，这表明了瞬时大气调整的重要性。6月期间，其他地区的强烈对流导致副热带西北太平洋下沉，导致那里的延迟爆发。卫星观测显示，西北太平洋自由对流层湿度缓慢积累，导致7月中旬对流降水突然加强，这表明对流层逐渐增湿最终触发阈值转变的可能性。
The Asian and northwest (NW) Pacific summer monsoons exhibit stepwise transitions with rapid changes in precipitation at intervals of roughly 1 month from mid-May through mid-July. A new method is developed to evaluate the effects of sea surface temperature (SST) and other changes on these rapid monsoon transitions. The latter changes include solar radiation, land memory, and atmospheric transient (SLAT) effects. The method compares two sets of atmospheric general circulation model (GCM) simulations, forced with observed seasonally varying and piecewise constant SST, respectively. The results indicate that the SLAT effects dominate all of the major transitions, except during mid-June when the SST cooling induced by the strong monsoon westerlies is a significant negative feedback resisting the intensification and northward advance of monsoon convection. The final regional onset of the monsoon system takes place in mid-July over the subtropical NW Pacific characterized by the abrupt enhancement of deep convection there. Despite a weak SST effect from the GCM assessment herein, major changes in convection and circulation are confined to the ocean east of the Philippines during the mid-July transition, suggesting the importance of transient atmospheric adjustments. Intense convection over other regions induces subsidence over the subtropical northwest Pacific during June, contributing to the delayed onset there. Satellite observations reveal a slow buildup of free-tropospheric moisture over the NW Pacific, leading to an abrupt intensification of convective precipitation in mid-July, suggesting a possibility that the gradual tropospheric moistening eventually triggers a threshold transition.