Institute of Atmospheric Physics Chinese Academy of Sciences
本文分析了El Ni?o 衰减年夏季西太平洋副热带高压(副高)的季节内变化,发现其季节内变化存在两种模态,一种是6~8月的一致偏西,另一种是6~7月偏西,而8月逆转为偏东,其中偏西模态的异常要远大于偏东模态。对偏西模态而言,由于热带北大西洋海温正异常的强迫影响,激发出一个从北大西洋经过欧亚大陆高纬度到东亚的遥相关,抑制了暖池地区的对流,东亚地区位势高度增加,从而导致副高加强西伸。在偏东模态下,热带印度洋海温异常演变与偏西模态类似,但强度偏低,同时热带北大西洋海温正异常在４月达到峰值后衰减,导致两大洋对8月暖池地区对流的抑制作用减弱。此外,由于6~7月暖池海温持续升高,在局地海气相互作用下,8月暖池对流发展,位势高度场降低,从而造成副高减弱东退。因此,副高8月异常主要取决于热带北大西洋海温异常。在预测El Ni?o 衰减年副高异常变化时,要综合考虑两大洋海温异常的影响。
In this study, we analyzed the intraseasonal variation of the western Pacific subtropical high (WPSH) in El Ni?o decaying summers and found two distinct modes, i.e., the west mode and the east mode. The west mode exhibits a consistent westward extension from June to August. The east mode shows a westward extension during June and July, but shifts to an eastward retreat in August. The anomaly in the west mode is much larger than that in the east mode. For the west mode, due to the forcing from a positive sea surface temperature (SST) anomaly in the tropical North Atlantic (TNA), there is a teleconnection pattern from the TNA via high latitude of Eurasia to East Asia. As a result, geopotential high in East Asia tends to increase associated with suppressed convection in the warm pool region, inducing the westward extension of the WPSH with strong intensity. The SST anomaly in the tropical Indian Ocean for the east mode is similar to that for the west mode but with a relatively smaller magnitude. Besides, the positive SST anomaly in the TNA reaches a maximum in April and tends to decay afterwards. Therefore, the warm pool convection in August cannot be suppressed sufficiently by the SST forcing over the two oceans. On the other hand, the warm pool convection begins to develop due to the local air-sea interaction associated with the SST rise from June to July. In this case, the WPSH tends to retreat eastward with reduced geopotential height. The result indicates that the SST anomaly in the TNA plays a major role in the WPSH anomaly in August. When predicting the WPSH in El Ni?o decaying summer, we must consider the influence of the SST anomaly over the two oceans.