Atmospheric Rivers: An Overlooked Threat to the Marginal Snowpack of the Australian Alps

Atmospheric rivers (ARs) are tropospheric corridors that provide;90% of poleward water vapor transport. They are predicted to increase in frequency and intensity if global warming continues unabated. Here we present a case study of the first direct observations of the impact of AR rain-on-snow (RoS) events on the marginal snowpack of the Australian Alps. Reanalysis data show ARs embedded within strong northwesterly airflow extended over 4000 km from the eastern Indian Ocean to southeast Australia, where orographic processes enhanced RoS. We quantify for the first-time radiation and turbulent energy flux exchanges using eddy covariance and the contribution of rain heat flux to the snowpack during the AR RoS events. The hydrological response of an above snow line catchment that includes Australia's highest peak during the events was rapid, with discharge increasing by nearly two orders of magnitude above historical mean winter discharge. This reflects the isothermal properties of the marginal Australian snowpack, where small increases in energy from RoS can trigger rapid snowmelt leading to flash flooding. Discharge decreased quickly following the passage of the ARs and onset of cold air advection. Based on climate projections of approximate to+12.5 degrees C warming in the Australian Alps by midcentury combined with an already historically, close-to-ripe snowpack, we postulate that ARinduced RoS events will accelerate the loss of snow cover.