Low-Frequency Variability of the Large-Scale Atmospheric Circulation in the Southern Hemisphere

Abstract

The low-frequency variability of the Southern Hemisphere circulation is studied using a 10-yr (1985-95) record of ECMWF analyses. Various statistical techniques are used to detect the dominant modes of anomalous variability and to document their spatial and temporal characteristics. The physical mechanisms that are responsible for maintaining these modes in their extreme phases are then investigated using a number of useful diagnostics. The hemispheric distributions of the nonseasonal variability of the 200-hPa streamfunction show large values over the subtropics as well as over the extratropics. The prominent maxima are found to occur at or near the locations of the tropospheric jet streams. The largest contribution to the nonseasonal variability comes from the intraseasonal anomalies (with periods 8-90 days). An empirical orthogonal function (EOF) analysis of the zonal wind anomalies is carried out to determine the dominant modes of low-frequency variability. The two most important modes are investigated in detail. The high-latitude mode (EOF1) shows a seesaw variation of the zonally-averaged zonal winds, with the centres of variability at 62.5˚S (the latitude of the wintertime subpolar jet) and at 40˚S. Similarly, the low-latitude mode (EOF2) is found to be associated with an out-of-phase variation of the zonal winds at 27.5˚S (the latitude of the wintertime subtropical jet) and those at 50˚S. The temporal evolution of the high-latitude mode is characterised by the occurrence of spells of well-developed anomalies with preferred durations of about 5 days and 14 days. The temporal evolution of the low-latitude mode is found to be closely related to the El Niño-Southern Oscillation phenomenon. The hemispheric distributions of the anomalous E-P flux divergence contributed by the high-frequency eddies (with periods 2-8 days) show a meridional structure closely resembling that of the low-frequency zonal wind anomalies associated with the high-latitude mode. Large contributions to the maintenance of the anomalies by the high-frequency eddies are clearly seen in the Atlantic-Indian Ocean sector. However, the low-frequency anomalies in the Pacific sector appear not to be maintained by the high-frequency eddies. The contribution of the horizontal (vertical) E-P flux divergence to the net anomalous eddy forcing on the zonal flow tends to be larger than that of the vertical (horizontal) E-P flux divergence during winter (summer). With regard to their spatial structure, the horizontal E-P flux divergence anomalies resemble the zonal wind anomalies more closely than the 3-d E-P flux divergence anomalies do. The distributions of the anomalous E-P flux divergence contributed by the medium-frequency eddies are characterised by smaller-scale, wave-like features, with little or no resemblance with the corresponding distributions of the low-frequency anomalies. This suggests that the medium-frequency eddies play no significant role in maintaining the low-frequency zonal wind anomalies associated with the high-latitude mode. For the maintenance of the low-latitude mode, the roles of the Coriolis accelerations induced by the meridional component of anomalous ageostrophic and divergent winds are also examined in addition to eddy forcing. The contributions from the ageostrophic and divergent winds are consistent with regard to their spatial structure, but the former shows somewhat larger magnitudes than the latter. The combined effect of the anomalous Coriolis accelerations and the anomalous transport of momentum by the high-frequency eddies is found to be important for maintaining the subtropical limb of the low-latitude mode during winter. A stronger (weaker) subtropical jet stream is accompanied by an increase (decrease) in the Coriolis acceleration and a decrease (increase) in the poleward momentum flux. The fluctuating momentum fluxes, at the same time, induce out-of-phase variations in the midlatitude zonal wind through anomalous divergence or convergence of the E-P flux. The medium-frequency eddies are, again, found not to be important for the maintenance of the low-latitude mode.