Global characteristics of medium-scale tropopausal waves observed in ECMWF operational data

Global characteristics and seasonal variation of medium-scale (zonal wavelengths of 2000-3000 km) waves observed around the midlatitude tropopause are examined using 6-hourly European Centre For Medium-Range Weather Forecasts operational data over four years (1990-93), covering both hemispheres. Medium-scale waves and synoptic-scale waves are extracted using time filters and their characteristics are compared. Hovmoller diagrams indicate the existence of medium-scale waves in the Southern Hemisphere as well as in the Northern Hemisphere. The zonal wavelengths of medium-scale waves are slightly larger in the Southern Hemisphere than in the Northern Hemisphere. Medium-scale waves are mostly active in three regions: the North Atlantic in winter (Dec-Jan-Feb), the North Pacific in Spring (Mar-Apr-May, MAM), and the south Indian Ocean in autumn (MAM). These regions are roughly corresponding to and slightly downstream of storm tracks where synoptic waves are dominant. Significant differences in seasonal variation of the intensity between the two kinds of waves are also found. The maximum of wave amplitudes is seen around the tropopause at latitudes slightly higher than the jet stream axis, where the meridional gradient of the quasigeostrophic potential vorticity (QPV) is maximized. The positive large QPV gradient is attributed to the atmospheric structure around the midlatitude tropopause that is located in vertical westerly shear of the jet stream. This fact suggests that the medium-scale waves are waves trapped around the midlatitude tropopause.