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

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 Niño 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.