Recent Intensification of the Western Pacific Subtropical High Associated with the East Asian Summer Monsoon

Abstract

The summer western Pacific subtropical high (WPSH) has intensified during the past three decades. However, the underlying mechanism is not yet well understood. Here, it is shown that baiu rainband activity in midsummer, which is part of the East Asian summer monsoon, plays an important role in recent intensification in the WPSH along the baiu rainband. In contrast with the WPSH, the summer Okhotsk high, which is located to the north of the baiu rainband, has weakened during the past three decades. The north–south contrasting changes between the two highs reflect a response to northward-moved and enhanced baiu heating, which intensifies the upper-tropospheric ridge, resulting in the baroclinic intensification of the WPSH. Regional climate model experiments also support the observational analysis. Therefore, baiu convective activity in midsummer can act as a major driver for the WPSH intensification. The results here suggest that the mechanism intensifying the summer North Pacific subtropical high clearly differs between the western and eastern Pacific.

An evaluation of boreal summer intra-seasonal oscillation simulated by BCC_AGCM2.2

“…The results indicate that diabatic heating plays a reinforcing role in the westward and intensified WNPSH, which are consistent with some previous studies (Zhou et al 2009;Matsumura et al 2015). However, some studies have found the WNPSH bias is also related to other bias in BCC_AGCM2.2, such as the wet bias over the Indian summer monsoon region, weak walker circulation, and lower air temperature over the Tibetan Plateau Kan et al 2015).…”

Response of the Baiu Rainband to Northwest Pacific SST Anomalies and Its Impact on Atmospheric Circulation

“…We used realistic long-term experimental results (CTRL and NOMP runs) from Matsumura et al (2015a), which were obtained using cumulus parameterization (Grell and Dévényi 2002) and cloud microphysics (Hong et al 2004) for the cloud scheme. The model domain focuses on the NW Pacific poleward of 308N with a horizontal resolution of 25 km (170 3 150 grid) and 40 vertical layers.…”

“…1) might not satisfactorily capture the midlatitude atmospheric response to oceanic forcing. Indeed, recent tripolar-like precipitation changes (Matsumura et al 2015a) are not seen in Fig. 1, and the baiu rainfall response to KOE SST forcing remains unclear.…”

“…Since the 1980s, the tropical Indian Ocean SST forcing of the PJ pattern has strengthened, resulting in increased El Niño-Southern Oscillation (ENSO) influence on baiu/EASM rainfall . Over the NW Pacific, however, baiu-rainband activity has strengthened and shifted northward in the past three decades, leading to weakening of the Okhotsk high and intensification of the western Pacific subtropical high (WPSH) (Matsumura et al 2015a). The cause of the northward shift of the baiu rainband remains unclear.…”

“…In previous studies, idealized model experiments were performed to clarify the impacts of baiu (Lu and Lin 2009;Sampe and Xie 2010) and KOE SST (Nakamura and Miyama 2014) forcings in the summer atmospheric circulation, because of the difficulty in obtaining a robust atmospheric response to those forcings in realistic model experiments, especially in global climate models. Here and in an earlier study (Matsumura et al 2015a) realistic long-term numerical experiments are performed using a regional climate model that can simulate baiu-rainband activity better than global climate models. Our model experiments can also isolate the atmospheric responses to baiu and KOE SST forcings in the realistic interannual variability.…”