Evaluation of meltwater-over-accumulation ratio as a predictor of surface ponding on Antarctic ice shelves

Ponding of surface meltwater on Antarctic Peninsula ice shelves has led to hydrofracture-driven calving and grounding line retreat, and other regions could become vulnerable to increased ponding as the climate continues to warm. Theory and qualitative observations suggest that ponding initiates when the meltwater-over-accumulation ratio (MOA) reaches 0.7. Here, we use present-day satellite-derived Antarctic meltwater products and RACMO climate model outputs to calibrate predictive thresholds of surface ponding based on air temperature, MOA, and a weighted combination of MOA and a grounding line proximity index (GLPI). We tested three RACMO resolutions (27, 11, and 2 km) and three surface meltwater products. The meltwater product that is best aligned with MOA identifies ponding locations using aggregate meltwater depths at 27 km resolution. For this product, the calibrated MOA and GLPI threshold predicts present-day ponding with an F1 score over twice as high as the theoretical threshold of MOA ≥ 0.7 (F1 = 0.587 vs. 0.261). Under emissions pathway SSP1-2.6, the empirical threshold predicts 2.3 times more lake coverage by 2100 than the theoretical threshold, underscoring the importance of calibrating climatic ponding thresholds. If MOA-based thresholds are to be used in future ponding projections, we recommend they be applied at relatively coarse spatial scales, calibrated against present-day, depth-based meltwater products, and combined with grounding line proximal processes.