Southern Hemisphere Storm Tracks, Blocking, and Low-Frequency Anomalies in a Primitive Equation Model

Abstract

The results of a study are presented that indicate three-dimensional instability theory is able to generate analogs of a wide variety of Southern Hemisphere observed fluctuations including those associated with cyclogenesis, blocking, and low-frequency teleconnection patterns. This study has been conducted with a two-level primitive equation eigenvalue model and the growing modes for both January and July averaged basic states are examined. In both seasons, the fastest-growing cyclogenesis modes have largest amplitudes in the eastern part of the hemisphere just downstream of the polar jet stream maxima. In July, there is a tendency to form elongated eddies in the region downstream of Australia where growth on both polar and subtropical jet streams occurs. These results are in general agreement with the observational studies of Southern Hemisphere storm tracks. For both January and July, larger-scale slower-propagating dipole or multipole modes are found that are consistent with blocking in some or all of the observed locations in the Australian/New Zealand region, east of the Falklands, and to the southeast of South Africa. The faster-growing low-frequency anomaly modes have their primary centers located to the south of 60°S and have features in common with either or both of high-latitude and wavenumber 3 observed Southern Hemisphere teleconnection patterns or, to a lesser extent, with the El Niño-Southern Oscillation anomaly patterns for 1982?1983. Perturbations similar to the Southern Hemisphere wavetrain pattern mode extending from Australia to southern South America and to the global 30?60-day oscillation are also found.