A study on seasonal variation of upper tropospheric medium-scale waves over East Asia based on regional climate model data

The seasonal variation of medium-scale waves (whose wavelengths are 1000-3000 km) in the upper troposphere is examined using regional climate model data provided by the Meteorological Research Institute. Medium-scale waves in the upper troposphere are observed at mid-latitudes over East Asia and the western Pacific throughout one year. The waves are most active in the spring season. The phases of the waves begin to propagate eastward at about 100 degrees E. The latitude where the medium-scale waves are dominant follows the meridional shift of the subtropical westerly jet. By using a lag-correlation analysis the eastward phase velocities are estimated at about 24 ms(-1) in the winter and about 17 ms(-1) in the summer, which are always larger than those of the synoptic-scale waves. The difference of the phase velocities of the two waves is consistent with the results shown by Sate ct al. (1993) for the spring of 1990. The vertical structure of the medium-scale waves is also examined. Their variance has two maxima in the vertical: one is located at 300 hPa corresponding to the tropopause, and the other is located near the surface. The phases of the lower waves propagate more slowly than those of the upper waves, in contrast to the synoptic-scale waves having phase velocities almost independent of height. By a composite analysis it is shown that the amplitude of the upper waves is maximized at the 300 hPa level and is confined near this level. Vertical tilting of the phase line of the upper wave is not observed. These results indicate that the appearance of the upper tropospheric medium-scale waves is strictly controlled by the jet both in vertical and horizontal directions.