Temporal Variability in the Characteristics of the Canterbury Nor’wester and Relationships to Modes of Atmospheric Variability

Abstract

Warm, strong north-westerly foehn winds regularly influence the Canterbury Plains in the lee of the Southern Alps of New Zealand. This wind is known locally as the Canterbury nor’wester. Globally, foehn winds attract considerable attention on account of their peculiar meteorological characteristics and effects. The biometeorological significance of the Canterbury nor’wester is well-recognised, regarding general effects of foehn winds on air quality, human thermal exchange, and mental health. Additionally, the significance of the Canterbury nor’wester to hydrology and the forecast and management of natural hazards is recognised. Such effects highlight the importance of comprehensively understanding the meteorology and temporal variability of the nor’wester. However, inter-annual and long term (inter-decadal) scale variability of the nor’wester is generally overlooked. The emphasis in this study on the inter-annual and inter-decadal timescales enables investigation of potential meteorological and climatological drivers of nor’wester variability. A 53-year (1960 to 2012) climatology of the Canterbury nor’wester is constructed from surface meteorological observations, with events diagnosed according to criteria based on surface wind speed (> 5 ms-1) and direction (within 25° of normal to the bearing of the Southern Alps). The climatology is presented in terms of various ‘metrics of nor’wester variability’ intended to characterise nor’wester events in terms of occurrence, duration, wind intensity, and thermal effects. The association of the nor’wester with synoptic conditions, characterised by Kidson’s (2000) synoptic types is investigated, in addition to associations with the Southern Annular Mode, El Niño Southern Oscillation, and Interdecadal Pacific Oscillation, which are investigated based on recognition of their effects on synoptic conditions (particularly geostrophic flow direction) over New Zealand. A consistent seasonal component is found in most metrics of nor’wester variability. There is some persistence and cyclicity at the inter-annual timescale in some metrics. No compelling patterns in long term behaviour are observed. Differences in the behaviour of nor’wester events defined according to information at different sites underscore the significance of orographic blocking and the nesting of local scale processes within mesoscale and synoptic conditions. It is found that synoptic conditions associated with increased south-westerly geostrophic flow over New Zealand (indicating a cross-barrier pressure gradient), in addition to positive zonal flow anomalies, are associated with increases in nor’wester event frequency and intensity.