The influence of atmospheric circulation patterns during large snowfall events in New Zealand's Southern Alps

Large snowfall events contribute significantly to total annual snow accumulation across the maritime Southern Alps. However, the knowledge about atmospheric circulation patterns associated with large snowfall events over the New Zealand Southern Alps is very limited. Daily snow observation data from three automatic weather stations and ERA-Interim reanalysis data were used to investigate the relationship between atmospheric forcing and large snowfall events across the Southern Alps. To do so, analysis of composite anomaly maps during large snowfall events were carried out to identify the common features of the days with heavy snow accumulation. Large snowfall across the Southern Alps are mainly associated with strong negative anomalies of sea level pressure (SLP) located over the southwest of New Zealand's South Island. These conditions are concurrent with negative anomalies of geopotential heights at 500 (Z500) located in the centre of low-pressure systems. However, over New Zealand, days leading to large snowfall events experience positive anomalies of Z500 showing a relatively warm environment during such events in the maritime Southern Alps. Positive anomalies of low-tropospheric temperatures (850 and 1,000 hPa) over the Tasman Sea and across the Southern Alps, strong values of integrated vapour transport (IVT) as well as high frequency of local synoptic patterns associated with troughing regimes (similar to 78%) during large snowfall events provide more evidence of the important contribution of warm air flows.