Abstract. A 16-year series of daily snow-covered area (SCA) for 2000–2016
is derived from MODIS imagery to produce a regional-scale snow cover
climatology for New Zealand's largest catchment, the Clutha Catchment.
Filling a geographic gap in observations of seasonal snow, this record
provides a basis for understanding spatio-temporal variability in
seasonal snow cover and, combined with climatic data, provides insight
into controls on variability. Seasonal snow cover metrics including daily SCA, mean snow
cover duration (SCD), annual SCD anomaly and
daily snowline elevation (SLE) were derived and assessed for temporal
trends. Modes of spatial variability were characterised, whilst also preserving
temporal signals by applying raster principal component analysis (rPCA) to
maps of annual SCD anomaly. Sensitivity of SCD to temperature and precipitation variability
was assessed in a semi-distributed way for mountain ranges across the catchment.
The influence of anomalous winter air flow, as characterised by HYSPLIT back-trajectories,
on SCD variability was also assessed. On average, SCA peaks in late
June, at around 30 % of the catchment area, with 10 % of the catchment
area sustaining snow cover for > 120 d yr−1. A persistent mid-winter reduction in
SCA, prior to a second peak in August, is attributed to the prevalence of winter
blocking highs in the New Zealand region. In contrast to other regions
globally, no significant decrease in SCD was observed, but substantial spatial
and temporal variability was present. rPCA identified
six distinct modes of spatial variability, characterising 77 % of
the observed variability in SCD. This analysis
of SCD anomalies revealed strong spatio-temporal variability beyond
that associated with topographic controls, which can result in snow
cover conditions being out of phase across the catchment. Furthermore,
it is demonstrated that the sensitivity of SCD to temperature and
precipitation variability varies significantly across the catchment.
While two large-scale climate modes, the SOI and SAM, fail to explain
observed variability, specific spatial modes of SCD are favoured by
anomalous airflow from the NE, E and SE. These findings illustrate
the complexity of atmospheric controls on SCD within the catchment
and support the need to incorporate atmospheric processes that govern
variability of the energy balance, as well as the re-distribution of
snow by wind in order to improve the modelling of future changes in
seasonal snow.
A spatio-temporal analysis of trends in Northern Hemisphere seasonal snow-cover, 1971-2017
