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Introduction

Interannual (IA) variability in the Southern Hemisphere (SH) extratropical lower stratosphere exhibits a maximum during late winter and spring (August-November) polewards of 50ºS (Shiotani et al. 1993, Randel and Newman 1997). There are many aspects of the IA variability of the SH stratosphere that are not well understood at present (see section 5 in Randel and Newman 1997).

It has also been established that it is during the austral spring when large-scale teleconnection patterns linked to the El Niño ­ Southern Oscillation phenomenon (and surface climate anomalies, particularly in South America) are present in the upper levels of the SH tropospheric circulation (Karoly 1989, Arkin 1982, Aceituno 1989). These SH upper-troposphere anomalies have been described as Rossby wave-like teleconnection patterns (Kousky and Bell 1992, Farrara et al. 1989, Mo and Ghil 1987), and referred as the Pacific South America (PSA) pattern, a wave 3 pattern, with nodes from New Zealand to Tierra del Fuego. Qualitative explanation of these type of global teleconnection patterns including their wave train nature is usually based on the theory of stationary Rossby wave dispersion on the sphere (Held 1983, Hoskins and Karoly 1981).

Lower stratosphere anomalous circulations are directly related to upper troposphere anomalies through changes in the tropopause height. Wave number 3 have been observed as a component of lower stratosphere anomalous features (ozone, wind, etc.) in the SH, and also strong wave number 1 features have been documented in observational studies, particularly for total ozone perturbations (Ambrizzi et al 1998).