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

Abstract

The Pacific-Japan (PJ) teleconnection pattern leads to a meridional precipitation dipole between the subtropics and East Asia in the summer of El Niño decaying years. However, observational analysis and model experiments indicate that increased sea surface temperature (SST) in the Kuroshio-Oyashio Extension (KOE) region, where a strong northward-decreasing SST gradient occurs, induces a northward shift of baiu rainfall with the subtropical jet, forming a tripolar precipitation anomaly pattern over the northwest Pacific. In July, the leading empirical orthogonal function (EOF) mode for precipitation represents the meridional dipole between subtropical and baiu rainfalls, while EOF-2 features the northward-shifted baiu rainband. The PJ atmospheric forcing increases KOE SST, which enhances northward shifts in the subtropical jet and baiu rainband, resulting in oceanic forcing over the KOE. In experiments using a regional climate model the subtropical jet response to KOE SST forcing explains about 20% of the anomalies in the control experiment. The northward-shifted baiu condensational heating also enhances the northward shift of the subtropical jet with a northeastward extension. In August when the baiu rainband disappears, the northward shift of the subtropical jet is enhanced mainly by KOE SST variability. During the summer, as a result of increased KOE SST and the northward-shifted baiu heating, cyclonic anomalies intensify over the Sea of Okhotsk and anticyclonic anomalies intensify over the northwest Pacific, affecting the western Pacific subtropical high. The results suggest that air-sea interaction over the KOE must be considered to better understand the summer northwest Pacific climate.

Jet–Precipitation Relation and Future Change of the Mei-Yu–Baiu Rainband and Subtropical Jet in CMIP5 Coupled GCM Simulations

“…A number of factors can affect the climatological latitude of the subtropical jet axis. Influences of the SST front associated with Kuroshio and Oyashio Extensions on the atmospheric circulation and jet have been shown in preceding studies (e.g., Frankignoul et al 2011;Nakamura and Miyama 2014;Matsumura et al 2016;Matsumura and Horinouchi 2016). Tropical SST can also have an impact (e.g., Inatsu et al 2002;Shaw and Voigt 2015).…”

Can coupled models perform better in the simulation of sub‐seasonal evolution of the western North Pacific subtropical high than atmospheric models in boreal summer?

“…This WNPSH intensification can be explained by the subtropical jet response to the Kuroshio and Oyashio Extension (KOE) SST. Higher KOE SST (e.g., Figure ) intensifies WNPSH by shifting the jet poleward (Matsumura et al ., ; ; Matsumura and Horinouchi, ). Thus, AGCM models may overestimate the atmospheric response to SST forcing because of no air–sea coupling.…”

Pacific Ocean decadal forcing of long-term changes in the western Pacific subtropical high

“…Over the NW Pacific, ENSO is not a major driver of the variability of the subtropical jet, but the PDO accelerates the westerlies to the south side of the climatological jet and decelerates the westerlies to its north side. Consequently, the positive (negative) phase of the PDO enhances the southward (northward) shift of the subtropical jet through a response to the decreased (increased) KOE SST (i.e., by an SST frontal shift)25, which in turn intensifies the cyclonic (anticyclonic) circulation over the NW Pacific and the anticyclonic (cyclonic) circulation over the Sea of Okhotsk8. The contrasting north–south circulation anomalies caused by the KOE SST frontal shift appear even if ENSO signal is removed26.…”

“…Over the past three decades, the EASM rainband over the NW Pacific has shifted northwards, contributing to the weakening of the Okhotsk high and intensification of the WPSH via the response to the EASM condensation heating7. The EASM rainband is shifted northwards with the subtropical jet as a result of increased sea surface temperatures (SSTs) in the Kuroshio and Oyashio Extension (KOE) region8, where a strong northward-decreasing SST gradient develops.…”