Loss in snow storage: attribution to elevation bands and meteorological drivers in the Alps

In recent years, Alpine regions have experienced several winters with little snow. This lack of snow - also referred to as snow drought - can have serious hydrological consequences, as highlighted by the prolonged hydrological drought in northern Italy in 2022. This event illustrates the important link between a snow drought in the upper reaches and a hydrological drought in the lower reaches of the catchment. Snow storage is expected to decrease in response to rising temperatures, which may lead to a potential increase in the number and spatial extent of snow droughts . However, it is not yet clear whether a decline in snow storage and the occurrence of low-snow years are solely caused by rising temperatures or whether changes in other, dynamic mechanisms, such as weather patterns leading to persistent dry spells, also play an important role.

Here, we use two national daily gridded snow products for Switzerland and Austria to quantify (1) changes in catchment snow storage, i.e. snow water equivalent (SWE), since 1961and attributing these changes to SWE deficit contributions  from low to high elevation bands; (2) changes in seasonal accumulation and melt characteristics; and (3) the occurrence, dynamics, and meteorologic drivers of low-snow years, i.e. years with annual maximum SWE below the 30th percentile, across various elevation bands.

Our results show a median loss of total annual catchment snow storage of approx. 20 % across 251 catchments in Switzerland and Austria over the period 1962-2023 , with a marked regime shift at the end of the 1980s. All elevation ranges experience a loss in snow storage, but most of the snow loss (approx. 60%) can be attributed to the loss in middle elevation snow storage (1200-2100m). Further, we observe a clear increase in the fraction of area under low snow conditions in all elevation bands, especially since the late 1980s. Thereby, low snow years are connected to below normal winter precipitation, especially at higher elevations (>2100m). At lower and middle elevations, warm winter temperature anomalies are additionally important to explain the occurrence of low snow years. A better understanding of the observed trends and the dynamical drivers of low snow years will help us to better constrain future projections of snow storage and associated hydrological impacts.