Interannual Variability of the Bonin High Associated with the Propagation of Rossby Waves along the Asian Jet

Abstract

Interannual variability of the Ogasawara (Bonin) high in August is examined in relation to propagation of stationary Rossby waves along the Asian jet using monthly averages from the NCEP/NCAR reanalysis dataset for 52 years. The perturbation kinetic energy at 200 hPa is used as a measure of the activity of stationary Rossby waves along the Asian jet. Composite maps of five relatively wavy-jet years with close phases show an enhanced anticyclone over Japan. This anomalous ridge has a maximum amplitude at 250 hPa and extends throughout the troposphere with little zonal and slight northward tilts. Wave-activity and isentropic potential vorticity analyses clearly show that the ridge is created by the propagation of stationary Rossby waves to Japan. The anomalous ridge accompanies a positive temperature anomaly over Japan in the entire troposphere. A negative temperature anomaly to the east of Japan is also created in the lower troposphere by the northerly flow between the anomalous ridge and trough. By contrast, the equivalent-barotropic ridge over Japan is very weak in the zonal-jet years. Although Rossby waves are as strong as those in the wavy-jet years near the source, they are found to converge to the southeast of its source with little further downstream propagation. This contrast in the behaviour of Rossby waves is consistent with the intensity of the Asian jet to the east of 90°E. The composite analysis suggests that the enhancement of a deep ridge near Japan is regulated by the intensity of the Asian jet. The composite analysis study conducted here emphasizes the importance of the propagation of stationary Rossby waves along the Asian jet for the late summer climate in northeastern Asia.