Influence of the Anticyclonic Anomaly in the Subtropical Jet over the Western Tibetan Plateau on the Intraseasonal Variability of the Summer Asian Monsoon in Early Summer

The upper-level troposphere over the western Tibetan Plateau, where the subtropical jet is located in summer, is a region of marked intraseasonal variability in geopotential height (GPH). This study investigates the influence of an anomaly in this region on the summer Asian monsoon. To this end, the GPH index is defined as the daily geopotential height anomaly at 200 hPa over the region based on 40-yr European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-40) data. Composites with respect to strongly positive values of the GPH index are analyzed. The results indicate that the temporary anomaly in the subtropical jet influences the monsoon over South Asia, Southeast Asia, and probably also over East Asia, because of two main processes: the eastward propagation of quasi-stationary Rossby wave anomalies at upper and lower levels along the subtropical jet, and a belt of strong westerlies at 15 degrees N (Arabian Sea-Bay of Bengal-the Philippines). The two mechanisms that underlie the lower-level Rossby wave anomaly are discussed here for the first time, based on the Rossby ray-path theory, as follows: 1) anomalous descent generated by the upper-level anticyclonic anomaly over Afghanistan and the western Tibetan Plateau causes the development of a heat low over the Thar Desert and neighboring areas, and 2) an anomalous southwesterly appears over the Arabian Sea, accompanied by the heat low, and interacts with the Western Ghats, resulting in an anticyclonic anomaly over the Indian subcontinent. The anomaly then starts to propagate eastward along a Rossby waveguide.