Atmospheric Rivers: An overlooked threat to the Australian snowpack in a
warming World
Abstract
The Australian seasonal snowpack can be classified as a marginal
maritime snowpack with a temperature near 0 °C throughout the snow
season. Subtle changes in atmosphere – snow energetics therefore result
in rapid change in snowpack properties, which occur against a background
of a warming climate. This has been attributed to a 40% decline in
spring snow depths in the past 40 yrs. and geologic records suggest the
seasonal snowpack is now near a 2000 yr. minimum. Modelled future snow
cover predicts further decline by 57 % to 78 % of current maximum snow
depth by the 2040s. Such research primarily attributes this decline in
snow cover to global warming. However, the past decline in Australian
snow cover can also be attributed to change in synoptic wintertime
precipitation patterns that have resulted in a dramatic increase in
proportional winter and spring precipitation of tropical origin since
the 1950s. Tropical moisture is predominantly transported into southeast
Australia during negative phases of the Indian Ocean Dipole (IOD) by
northwest cloud bands – visible expressions of atmospheric rivers
coupling tropical moisture sources northwest of Australia to the
Australian Alps. Here we present a case study of one such event that
occurred from the 21 to 23 July 2016 when 118 mm of rain-on-snow over a
12 hr period led to near complete ablation of the snowpack. While
predictions of future variability of the IOD due to global warming
remain uncertain, we suggest that warming atmospheric temperatures
increase the risk of such extreme rain–on-snow events during negative
IOD events. Combined with reduced snow cover in response to warmer
ambient wintertime temperatures, such rain-on-snow events may further
accelerate the reduction in seasonal snow cover in the Australian Alps,
possibly on occasions after which the snowpack does not recover before
spring. These conditions would present significant challenges to the
Australian snow sports industry which is worth $2 billion annually and
lead to change in snow dependent ecosystems and alpine hydrology.