Why the Western Pacific Subtropical High Has Extended Westward since the Late 1970s

Abstract

The western Pacific subtropical high (WPSH) is closely related to Asian climate. Previous examination of changes in the WPSH found a westward extension since the late 1970s, which has contributed to the interdecadal transition of East Asian climate. The reason for the westward extension is unknown, however. The present study suggests that this significant change of WPSH is partly due to the atmosphere's response to the observed Indian Ocean-western Pacific (IWP) warming. Coordinated by a European Union's Sixth Framework Programme, Understanding the Dynamics of the Coupled Climate System (DYNAMITE), five AGCMs were forced by identical idealized sea surface temperature patterns representative of the IWP warming and cooling. The results of these numerical experiments suggest that the negative heating in the central and eastern tropical Pacific and increased convective heating in the equatorial Indian Ocean/Maritime Continent associated with IWP warming are in favor of the westward extension of WPSH. The SST changes in IWP influences the Walker circulation, with a subsequent reduction of convections in the tropical central and eastern Pacific, which then forces an ENSO/Gill-type response that modulates the WPSH. The monsoon diabatic heating mechanism proposed by Rodwell and Hoskins plays a secondary reinforcing role in the westward extension of WPSH. The low-level equatorial flank of WPSH is interpreted as a Kelvin response to monsoon condensational heating, while the intensified poleward flow along the western flank of WPSH is in accord with Sverdrup vorticity balance. The IWP warming has led to an expansion of the South Asian high in the upper troposphere, as seen in the reanalysis.

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).…”

“…Next we carry out further analysis to understand the bias in simulated WNPSH. It is believed that diabatic heating associated with the Asian summer monsoon rainfall affects the summer subtropical circulation over the North Pacific (Rodwell and Hoskins 2001;Zhou et al 2009). Figure 11 shows the model bias in vertical integrated diabatic heating (shaded) from the surface to 200 and 850 hPa non-divergent flow (vector) for phase 1 and 5.…”

Drying of Indian subcontinent by rapid Indian Ocean warming and a weakening land-sea thermal gradient

“…In fact previous studies using AGCM experiments driven by recent basin-wide warming in the tropical Indian Ocean reproduced the observed zonal wind changes in the recent decades 38 . In the model sensitivity experiment, however, the positive SST offset is prescribed only over the western Indian Ocean region, where the long-term warming trend is observed in the SST records during 1901-2012.…”

Support for global climate reorganization during the “Medieval Climate Anomaly”

“…4E,F with H). Such a pattern of opposing variability in monsoon rainfall over southern and northern China is well documented and characterizes observed and simulated precipitation trends over the latter half of the twentieth century, changes apparently related to shifts in tropical Pacific and Indian Ocean temperatures (see discussions in Li et al 2008;Zhou et al 2009). …”