Intra-seasonal flow variability and a probable long-term slowdown of Universidad Glacier in the Central Andes of Chile (34° S)

Abstract. Understanding glacier dynamics in semiarid mountain regions is essential for assessing water availability and anticipating the impacts of climate variability. Here, we present the first characterization of surface velocity dynamics at Universidad Glacier, a 24.91 km² valley glacier in the semiarid Central Andes of Chile, using Sentinel-1 SAR offset tracking during two consecutive ablation seasons (2023–2024 and 2024–2025), compared with in situ GNSS stake measurements. Multi-temporal velocity fields and along-centerline profiles reveal a stable spatial flow structure: velocities peak in the mid-glacier icefall zone (3,000–3,500 m a.s.l.; up to 0.3–0.6 m d⁻¹ in individual 12-day fields, and 0.16 and 0.08 m d⁻¹ as season means in 2023–2024 and 2024–2025, respectively) and decrease both upstream, where values are indistinguishable from the off-glacier noise level (~0.10 m d⁻¹), and downstream along the debris-covered lower tongue. Superimposed on this structure, the icefall velocity in the January–February pairs of both seasons (0.03–0.06 m d⁻¹) is at the stable-terrain noise floor, whereas the March pairs show a clear increase (to 0.10 and 0.06 m d⁻¹; 2–4 times the noise level). The higher December 2023 values and the April values of both seasons coincide with elevated velocities across the entire glacier, including the accumulation area, and cannot yet be separated from decorrelation artifacts. A first-order comparison with in situ and ERA5-Land air temperature shows that total thermal forcing was nearly identical between seasons and that velocity was not in phase with air temperature: in 2024–2025 the March increase followed the seasonal temperature maximum by 4–5 weeks. A lagged subglacial-drainage response is discussed as a working hypothesis alongside methodological alternatives. Comparison with GNSS stakes (n = 11; RMSE = 0.022 m d⁻¹, bias = −0.011 m d⁻¹, regression slope = 0.51) indicates good agreement in the spatial ranking of velocities but systematic underestimation of the fastest stake, over a validated range of 0.01–0.16 m d⁻¹. Season-mean velocities are 4–8 times lower than the ITS_LIVE 1986–2025 median and 2–4 times lower than the ITS_LIVE 2020–2025 median; because a factor of 2–4 is attributable to the spatial smoothing of our offset-tracking configuration, the multidecadal deceleration is considered likely but not yet quantified. These results establish a high-temporal-resolution velocity baseline for Universidad Glacier and identify the processing and observational requirements needed to detect melt-driven dynamic variability that multi-annual velocity products cannot resolve.