The seasonal evolution of planetary waves in the southern hemisphere stratosphere and troposphere

Abstract

AbstractSeasonality in planetary wave structure in the southern hemisphere stratosphere and troposphere (1000‐1 mb) is studied based on eight years (1979–1986) of daily geopotential data. Emphasis is placed on the distinctive seasonality in the stratosphere, along with tracing the stratospheric fluctuations into the troposphere. Time filtering is used to separate stationary and transient components. Maximum geopotential height variance is observed in the middle to upper stratosphere in late winter‐spring, with a distinct secondary, smaller maximum in late fall‐early winter; these maxima sandwich the strongest mid‐winter zonal winds. This sequence occurs approximately one month earlier in the upper stratosphere. Two corresponding peaks are observed in stratospheric zonal mean wind and temperature fluctuations. The two maxima in stratospheric wave variance are not in agreement with observed seasonal changes in quasi‐geostrophic refractive index of the zonal mean flow.The stratospheric geopotential variance maxima result from nearly equal amounts of stationary and transient zonal wave‐1 fluctuations, with transient wave 2 also contributing during late winter. Stationary wave 1 shows intriguing seasonal evolution: the early winter maximum is predominantly equivalent barotropic, whereas that of late winter‐spring is highly baroclinic in the stratosphere. Stratospheric poleward fluxes of heat and momentum both exhibit pronounced maxima during September‐October; these result primarily from the stationary wave‐1 contributions, and the transients assume a secondary role. Although not as pronounced as the stationary wave seasonality, seasonal differences in transient wave behaviour are also documented, including three‐dimensional propagation characteristics and wave‐mean flow interaction diagnostics.