Observed trends in snow phenology and duration across Romania (1961 to 2020)

Snow cover phenology (onset, melting) and duration are expected to react to temperature trends. Quantification of snow cover changes is an essential step for further climate change impact evaluations given their multiple meteorological, hydrological, ecological, and societal implications. This study revisits the snow trends across Romania, using data from 114 weather stations (14 located in the Carpathian Mountains, at above 1,000 m), with complete and long-term series of snow observations,covering the 1-2,504 m elevation range. The trends in the dates of snow cover onset (SCO), snow cover melting (SCM) and snow cover duration (SCD) over the past 60 years (1961–2020) have been investigated over five elevation bands (<500, 501-1,000, 1,001-1,500, 1,501-2,000, and 2,001-2,500 m) for identifying hot-spots of snow cover change and retrieving evidence of elevation-dependency under climate warming. A declining SCD was systematically observed country-wide, statistically significant at only 25% of the weather stations included in this study. The decline is more accelerated in the lowlands, generallybelow 500 m. On the opposite, there is no statistically significance SCD change at above 2,000 m. Overall, SCD trends show no statistical dependency on elevation. The SCD decline is driven by the negative changes in SCM, due to the stronger warming in the late-winter and spring than in the fall and early winter. The snowmelting date advanced the most by over 7 day decade^-1at mid-to-high elevations (1,500-2,000 m) and in the lowlands (below 1,000 m). In the mountains, the most notable delays occurred in the Western and Southern Carpathians (~4-7 days decade^-1). At above 2,000 m, the negative SCM trends are weaker (~4 days decade^-1) and not statistically significant. Unlike SCM, over most parts of the country (63% of stations) SCO advanced towards winter, although only about 7% of trends were statistically significant (mostly at stations below 300 m). The SCO advance evolved at fairly comparable rates when comparing the highlands (above 1,000 m, ~4 day decade^-1) and the lowlands (below 1,000 m, 4 to 5 days decade^-1). An opposite climate signal, indicating earlier snow onsets was also observed at about 30% of stations (including the highest elevation stations, located at above 2,200 m), although not statistically significant. Overall, SCO and SCM trends show a weak-to-moderate but statistically significant relationship with elevation (r=.23-.25, p<0.05), suggesting, on the one hand, that warming effects on snow phenology are particularly strong in the lowlands and, on the other hand, that there are other driving factors influencing the snow phenology(i.e., atmospheric circulation, local factors).Our results are linked to the rising temperature, particularly strong in winter and spring, which in our case is more accelerated in the low elevation areas.

This study was funded by the Ministry of Environment, Water and Forests, in the framework of research project A.III.10 (Spatial-temporal climate variability in Romania).