Multiple Planetary Flow Regimes in the Southern Hemisphere

Abstract

An observational study is made of low-frequency variations of the general circulation in the Southern Hemisphere troposphere with the aid of daily data analyzed at the National Meteorological Center (NMC).
In the winter of 1983 there exist two typical patterns of the zonal mean geostrophic wind in the meridional plane. We name them the single jet regime and the double jet regime, each of which persisted with a characteristic duration of a month and appeared twice alternatively. In the single jet regime, the subtropical jet at the tropopause level is strong and the stationary wave of zonal wavenumber (WN=)3 has a larger amplitude than WN=2. On the other hand, in the double jet regime the subtropical jet is weak and the polar night jet extends down to the surface at about 55°S. Dominant stationary planetary waves are WN=1 and 2. These two regimes are also observed in the three years 1980-1982.
In terms of the quasi-geostrophic Eliassen-Palm (E-P) flux diagnostics, the vertical component of the E-P flux, which is proportional to the horizontal eddy heat flux, is larger around the mid-latitude troposphere in the single jet regime than in the double jet regime. Consequently, the convergence of the E-P flux, which acts as a wave drag on the mean westerly wind, is larger around the mid- and high latitude tropopause level in the single jet regime than in the double jet regime. The larger wave drag tends to keep the weaker westerlies in the single jet regime.
An empirical orthogonal function (EOF) analysis is made to define an index of the zonal mean wind of two dimension The first component of EOF, which represents over 40% of the total variance, corresponds to the variations between the two regimes. The amplitude of this component is used as a kind of 'zonal index'. Time variation of the zonal index is not considered an internal oscillation with an intrinsic frequency because the 'period', which is not an appropriate concept for the present phenomenon, is different in each year. The transition between the regimes is rather rapid compared with the duration of each regime. No clear relation is found between the regime transitions and the annual variation. These observational evidences support an interpretation that this low-frequency variation of the zonal index is the transition between multiple planetary flow regimes in the Southern Hemisphere.