Planetary Wave Amplitude Bimodality in the Southern Hemisphere

Abstract

A preliminary study of the probability density distribution of the wavenumber 3 amplitude in midlatitudes of the Southern Hemisphere is undertaken with 4.25 years of 500 mb height data compiled by the European Centre for Medium-Range Weather Forecasts. The wavenumber 3 amplitude probability density appears to be bimodal during winter. Analogous results during the Southern Hemisphere summer reveal a unimodal wavenumber 3 amplitude probability density distribution with the single summer mode corresponding in value to the winter low amplitude mode. The physical implications of the winter bimodality are examined to gain confidence in the result. Partitioning the data based on the two modes leads to a consistent picture of the large amplitude events in physical space. Synoptically, the large amplitude mode corresponds to “amplified waves” of broad extent. It is suggested that the SH winter time-mean eddies for wavenumbers greater than one are the statistical residue of the intermittent large-amplitude events. Individual Southern Hemisphere large amplitude events exhibit grid point height departures from zonal symmetry comparable to their Northern Hemisphere counterparts. In addition, the summer mean eddy field is very similar to the winter low amplitude mode's mean eddy field. Examination of the time series of the wave 3 amplitude and phase during winter reveals that the more persistent of the individual large amplitude events exhibit relatively stationary phase lines, although different large amplitude events may have different phases. A remarkable feature of these time series is that certain of the more persistent events exhibit a sudden phase shift while the amplitude remains large with the phase being steady both before and after the shift.