Seasonal snowpack is an important element of mountain cryosphere. In the New Zealand’s Southern Alps/ Kā Tiritiri o te Moana, seasonal snow cover is of socio economic importance because of its key role in energy, agriculture and tourism sectors. Despite the extensive snow cover in such alpine regions, the knowledge of the snow processes such as large snowfall and snowmelt events is limited. Large snowfall and snowmelt events are a result of atmospheric circulation patterns that influence moisture transfers and surface climate in mountain regions. However, analysis of the relationship between large snowfall and snowmelt events and atmospheric forcing in alpine regions has remained a challenge mainly due to the scarcity of climate observations and snow measurements at higher altitudes. A better understanding of the synoptic-scale weather patterns can provide an insight into the distinct characteristics of atmospheric forcing impacting snow accumulation and snowmelt processes, especially in remote mountain regions. Therefore, the primary aim of this dissertation is to improve the understanding of synoptic-scale atmospheric forcing during large snowfall and snowmelt events in alpine regions.

The New Zealand Southern Alps, surrounded by the Pacific Ocean and in the path of the westerly air flows represent a typical maritime environment, making them an ideal location for the study of alpine snow processes. To explore the synoptic climatology of large snowfall and snowmelt events, the 90th percentile value of daily snowfall (snowmelt) from three automatic weather stations (AWS) across the Southern Alps was used. A composite anomaly approach using reanalysis atmospheric data (i.e. sea level pressure, temperature and geopotential heights) was applied to characterize the main synoptic-scale hydrometerological conditions associated with these events. Additionally, an analysis of integrated vapour transport (IVT) was conducted in order to learn more about the moisture transport characteristics of precipitation during large snowfall and major snowmelt events associated with rain on snow (ROS). The application of IVT fields allowed to identify the distinct characteristics of moisture transports and the potential role of atmospheric rivers (ARs) in transferring moisture across the Tasman Sea towards the Southern Alps.

Large snowfall events were found to account for 20-40% of total annual snow accumulation. Synoptic-scale atmospheric patterns influence the variability in timing and magnitude of large snowfall and snowmelt events. Weather patterns during large snowfall events in the Southern Alps are mainly characterised by strong negative anomalies of sea level pressure (SLP) and geopotential heights at 500 hPa (Z500) located over the southwest of New Zealand’s South Island. However, over the New Zealand region, the days leading to large snowfall events experienced positive anomalies of Z500 accompanied by positive anomalies of low-tropospheric temperatures (850 hPa and 1000 hPa). These positive anomalies were associated with the passage of relatively warm airflows over the Tasman Sea and across the Southern Alps. Troughing regimes were found to account for ~78% of large snowfall events. Large snowmelt events, however, were found to take place during both high pressure systems and troughing regimes, with the majority of rapid snowmelt events (~80%) occurring during high pressure systems with anomalously high temperatures. Observations of snowmelt at Mueller Hut revealed that even though snowmelt mostly occurs during spring, considerable melt (~300 mm day⁻¹) can also occur during winter month. These significant winter-melt events were found to be associated with rain-on-snow events. Anomalies of temperature revealed rising mid- and low-tropospheric temperatures (at 500, 700 and 850 hPa) during both high-pressure and troughing systems associated with large snowmelt events.

Atmospheric rivers making landfall in the Southern Alps were found to impact the seasonal snowpacks in two ways. Firstly, they produce large snowfall events and secondly, they generate major spring- and winter-time rain-on-snow (ROS) events. While ARs accounted for majority of large snowfall events across the Southern Alps (~70%), they were also responsible for nine out of ten largest ROS events identified at Mueller Hut station near the Main Divide of the Southern Alps. Similar hydrometeorological characteristics (e.g. duration and shape) were identified for both rain-producing and snow-generating ARs; however, in terms of strength, the former were found to contain higher IVT values over the Southern Alps (up to ~822 kg m⁻¹ s⁻¹). AR-related ROS events were characterised by anomalously high temperatures, high advection of warm airflows and rising freezing level resulting in warm environments over the snowpacks, with air temperatures as high as ~10 °C, creating ideal conditions for rapid snowmelt at higher altitudes. The results of this study have improved the current knowledge of the hydrometeorological characteristics of snow processes in a mid-latitude maritime climate. Considering the high sensitivity of seasonal snowpacks in maritime environments to changes in atmospheric variables, the findings will contribute to the research into further quantifying the impacts of climate change on atmospheric circulation patterns as well as the timing and frequency of rain- and snow-producing ARs in such regions.