Snow sensitivity to climate change during compound cold-hot and wet-dry seasons in the Pyrenees

Abstract. The Mediterranean basin has experienced one of the highest warming rates on Earth over the last decades and climate projections anticipate water-scarcity future scenarios. Mid-latitude Mediterranean mountain areas such as the Pyrenees play a key role in the hydrological resources for intensely populated lowland areas. However, there are still large uncertainties about the impact of climate change on the snowpack in high mountain ranges of the Mediterranean region. Here, we provide a climate sensitivity analysis of the Pyrenean snowpack through five key snow climate indicators. Snow sensitivity is analyzed during compound temperature and precipitation extreme seasons, namely Cold-Dry (CD), Cold-Wet (CW), Warm-Dry (WD) and Warm-Wet (WW) seasons, for low (1500 m), mid (1800 m) and high (2400 m) elevation sectors of the Pyrenees. To this end, a physically-based energy and mass balance snow model (FSM2) is validated by ground-truth data, and subsequently applied to the entire range, forcing perturbed reanalysis climate data for the 1980–2019 baseline scenario. The results have shown that FSM2 successfully reproduces the observed snow depth (HS) values, reaching R² > 0.8, and relative RMSE and MAE lower than 10 % of the observed HS. Overall, climate sensitivity decreases with elevation and increases towards the eastern Pyrenees. When temperature is progressively warmed at 1 ºC intervals, the largest seasonal HS decreases from baseline climate are found at +1 ºC, reaching values of -47 %, -48 % and -25 % for low, mid and high elevations, respectively. Only an upward trend of precipitation (+10 %) could counterbalance temperature increases (<= 1 ºC) at high elevations during the coldest months of the season, since temperature is far from the isothermal 0 ºC conditions. The maximum (minimum) seasonal HS and peak HS max reductions are observed on WW (CD) seasons. During the latter seasons, the seasonal HS is expected to be reduced by -37 % (- 28 %), -34 % (- 30 %), -27 % (-22 %) per ºC, at low, mid and high elevation areas, respectively. For snow ablation climate indicators, the largest decreases are observed during WD seasons, when the peak HS date is anticipated 10 days and snow duration (ablation) decreases (increases) 12 % per ºC. The results suggest similar climate sensitivities in mid-latitude mountain areas; where significant snowpack reductions are anticipated, with relevant consequences in the ecological and socioeconomic systems.