The Westward, Northward Advance of the Subtropical High over the West Pacific in Summer

The subtropical high over the West Pacific in summer is one of the most important atmospheric circulation components which influences the weather and climate of China. Using the daily averaged data derived from NCEP/NCAR reanalysis dataset and interpolated outgoing longwave radiation (OLR) data provided by the National Oceanic and Atmospheric Administration (NOAA), the variation of the position of the summer rainfall belt in eastern China is diagnosed for the selected years of 2005, 2003 and 1998. The work analyzes the nonoccurrence of Meiyu in the mid and lower reaches of the Yangtze River during the summer of 2005, the heavy flood in the Yangtze River basin during the summer of 1998, as well as the prolonged heat wave south of the Yangtze River during the second half of July in 2003. Particular attention is devoted to the study on the correlation of these abnormal weather phenomena with the westward, northward advance or southward, eastward retreat of the subtropical high in the West Pacific.Evidence shows that the advance (retreat) of the subtropical high over the West Pacific in summer can modulate the positions of the heavy rainfall belt in eastern China. During the westward, northward advance (southward, eastward retreat) of the subtropical high, the heavy rainfall belt moves northward (southward). During the persistent advance of the subtropical high, a prolonged heat wave occurs in the mid and lower reaches of the Yangtze River. The process and mechanism of the westward, northward advance (southward, eastward retreat) of the subtropical high are revealed.Hoskins et al. show that the Asian Jet can act as a waveguide in the northern summer. Chang shows that the upper tropospheric waves over the mid-latitudes generally reveal the characteristics of downstream development in summer. Enomoto et al. study the relationship between the formation of the Bonin high in August and the energy propagation of stationary Rossby waves along the Asian Jet. Based on the results from previous studies, the work clearly demonstrates through the isentropic potential vorticity analyses that the advance (retreat) of the subtropical high is caused by the propagation of the stationary Rossby waves along the Asian Jet in the upper troposphere, forming a longwave ridge (trough) along the coast of China (115°—130°E); at the same time the subtropical high advances north westward (retreats southeastward). This anomalous ridge (trough) extends throughout the troposphere with the structure of the equivalent-barotropic ridge (trough). In summer when there is a persistent longwave ridge along the coast, there will be a prolonged heat wave in the mid and lower reaches of the Yangtze River.Based on the summer forecasting experiences, it is found that the medium range forecast issued by the ECMWF may better predict the westward, northward advance of the subtropical high of the West Pacific in summer.