Barotropic Disturbances on Intraseasonal Time Scales Observed in the Midlatitudes over the Eurasian Continent during the Northern Summer

Abstract

The intraseasonal variations in the subtropical jet over the Eurasian continent and the western North Pacific in the northern summer are investigated through a statistical study using the ECMWF objective analysis data for 14 years, from 1980 to 1993. The quasi-stationary Rossby waves are evident in the climatological subtropical jet. They are characterized by well-confined zonal wavenumbers of about 5-7 and less confined spectral peaks at 30- to 45-day and 14-day periods. They are strongly phase-locked in the zonal direction.
Through a case study for a typical year (1983), evident standing wave-like disturbances with a 32-day period are found in the subtropical jet. Their propagation routes correspond to the well-defined waveguide for the quasi-stationary Rossby wave formed along the subtropical jet. However, they are not completely confined in the waveguide, but protrude slightly from the waveguide with its southern rim crossing their critical latitude, which lies along the center of the Tibetan High. The vertical structure of the disturbances shows a slight but systematic westward phase tilt, indicating a northward heat flux.
In ray-path analysis for barotropic quasi-stationary Rossby waves, temporally and meridionally smoothed basic flows are used in order to take into account temporal and zonal extensions of wave packets. This method is successfully used to diagnose the propagation routes of the observed disturbances.