无显著海温异常强迫下西太平洋副热带高压的异常变动:1980年和1981年的对比分析

Abnormal Activities of the Western Pacific Subtropical High without Remarkable SST Anomaly Forcing:A Comparison between 1980 and 1981

1980年和1981年夏季及其前期冬春季太平洋和印度洋海温均未出现显著异常。然而,这两年东亚夏季风环流的季节内变化却呈现显著异常,且截然不同,具体表征为:1980年西太平洋副热带高压(副高)第一次北跳异常偏早,第二次北跳异常偏晚,而1981年则相反,第一次北跳接近气候态,第二次北跳却异常偏早。就副高两次北跳过程而言,其直接原因也有显著差异:1980年副高两次北跳主要受热带西太平洋对流增强的影响,而1981年两次北跳则是由于热带西太平洋对流增强后所激发的极向传播的Rossby波列与中高纬度东传的Rossby波的锁相作用造成的。与北跳过程相比,副高北跳前后环流稳定维持的时间长短显得更为重要。研究表明,1980年夏季副高异常程度之所以堪比1983年和1998年强El Ni?o衰减年,主要是由于不同阶段南半球环流和北半球中高纬度环流的相互配合与接力,其中,6月和8月副高异常偏强对夏季平均副高异常偏强起到主要贡献,但二者的影响因子不同:6月主要受马斯克林高压(马高)偏强的影响,而8月则与澳大利亚高压(澳高)异常偏强有关。此外,7月和8月副高异常偏南是因为鄂霍茨克海阻塞高压长期维持。与1980年相比,1981年夏季马高和澳高均异常偏弱,因而南半球环流对副高异常的影响有限。北半球中高纬度环流的季节内变化对该年夏季副高的快速北进和南退起主导作用,特别是8月中高纬度盛行强烈的经向环流,使得副高异常偏东偏弱,从而导致夏季平均副高异常偏东偏弱。本文的个例分析表明,在无显著海温异常强迫的年份需要特别关注南半球环流和北半球中高纬度环流对副高及与之相关的东亚夏季风环流的季节内演变的影响,但是这些环流因子持续性较差,难以用于跨季度预测。

Evident sea surface temperature anomalies(SSTAs) were not observed over the Pacific and Indian Oceans in the summer and the preceding winter and spring of 1980 and 1981.Yet intraseasonal variation of the East Asian summer monsoon(EASM) circulation exhibited significant anomalies in both years with large differences.The western Pacific subtropical high(WPSH) experienced a much earlier first jump and an obviously later second jump in 1980,while a near-normal first jump and a notably earlier second jump were found in 1981.It was also noted that the jump processes in both years were influenced by different factors.In 1980,both jumps were induced by the enhancement of tropical western Pacific convection.In 1981,however,both jumps were attributed to the phase-locking of the poleward propagation of Rossby wave trains induced by the intensified convection in the tropical western Pacific and the eastward propagation of Rossby waves in the middle and high latitudes.Compared with the jump processes,the maintenance of stable circulations during pre-and post-jumps was more important.Due to the cooperation of the southern hemispheric circulation and circulation in the middle and high latitudes of the Northern Hemisphere at various periods,the extent of the WPSH anomaly in the summer of 1980 was comparable to that in strong El Ni?o decay years like 1983 and 1998.Specifically,the abnormally strong WPSH in June and August played a much more important role in the remarkable anomaly of summer-mean WPSH.Note that the stronger than normal WPSH in June and August was related to the enhancements of the Mascarene high(MH) and the Australian high(AH),respectively.In addition,the WPSH tended to shift southward in July and August since the blocking high in the Okhotsk Sea persisted for a long time.In contrast,the MH and AH in the summer of 1981 were relatively weak,resulting in a weak influence on the WPSH.Instead,the intraseasonal variation of circulation in the middle and high latitudes played a leading role in the rapid northward advance and subsequent southward retreat of WPSH.In particular,the prevalence of meridional circulation in August led to a weaker WPSH that shifted eastward.As a result,the whole summer-mean WPSH tended to be weaker than normal as well.The case study showed that in the absence of SSTA forcing,special attention should be paid to the influence of the southern hemispheric circulation and circulation in the middle and high latitudes of the Northern Hemisphere on the intraseasonal evolution of the WPSH and associated EASM circulation.On the other hand,both factors are difficult to be used in the seasonal prediction due to their relatively short periods of maintenance.