The low latitude influence on winter rainfall in Victoria, South‐Eastern Australia—II. Relationships with the southern oscillation and Australian region circulation

Abstract

AbstractIn this companion to Part I (Wright, 1988a) of the study on low latitude influences on Victorian rainfall, northern Victorian winter rainfall variability is shown to be dominated by ‘interacting’ (I) fronts, and both parameters are then related to atmospheric circulation anomalies in the Australian region and the Southern Oscillation. The major circulation features accompanying extreme wet and dry winter months in northern Victoria are identified from composited sea level pressure, 500 h Pa height, and 200 h Pa wind fields. Wet winters are characterized by a vigorous subtropical westerly circulation at middle and upper tropospheric levels, a relatively weak subtropical high pressure belt over Australia, and above normal amplitude of the climatological mean long‐wave trough in western Australian longitudes. The intensity and frequency of I frontal systems influencing southeastern Australia is shown to be strongly related to these features, and to cloudiness over the tropical Indian Ocean northwest of Australia, a major source‐region for cloudbands involved in the interactions.In turn, above normal trough amplitude and cloudiness, a weak subtropical high pressure belt, and frequent and relatively well‐developed interactions, are all shown to be characteristic of winters in which the Tahiti‐Darwin Southern Oscillation Index (SOI) is positive. Anomalies tend to be in the opposite sense during negative SOI winters, which feature infrequent, generally poorly developed, interactions, and much lighter rainfall. The dual association of these circulation and cloudiness features with both the SOI and northern Victorian rainfall strongly suggests that I fronts provide the well‐known link between the SOI and northern Victorian rainfall. The current results are discussed in relation to results from other recent studies on synoptic associations with anomalous heat‐sources.