Drivers of Seasonal Land‐Ice‐Flow Variability in the Antarctic Peninsula.

Land‐ice flow in Antarctica has experienced multi‐annual acceleration in response to increased rates of ice thinning, ice‐shelf collapse and grounding‐line retreat. Superimposed upon this trend, recent observations have revealed that land‐ice flow in the Antarctic Peninsula exhibits seasonal velocity variability with distinct summertime speed‐ups. The mechanism, or mechanisms, responsible for driving this seasonality are unconstrained at present, yet detailed, process‐based understanding of such forcing will be important for accurately estimating Antarctica's future contributions to sea level. Here, we perform time‐series analysis on an array of remotely sensed, modeled and reanalysis data sets to examine the influence of potential drivers of ice‐flow seasonality in the Antarctic Peninsula. We show that both meltwater presence and ocean temperature act as statistically significant precursors to summertime ice‐flow acceleration, although each elicits an ice‐velocity response after a distinct lag, with the former prompting a more immediate response. Furthermore, we find that the timing and magnitude of these local drivers are influenced by large‐scale climate phenomena, namely the Amundsen Sea Low and the El Niño Southern Oscillation, with the latter initiating an anomalous wintertime ice‐flow acceleration event in 2016. This hitherto unidentified link between seasonal ice flow and large‐scale climatic forcing may have important implications for ice discharge at and beyond the Antarctic Peninsula in the future, depending upon how the magnitude, frequency and duration of such climate phenomena evolve in a warming world. Plain Language Summary: Over the past three decades, the speed of Antarctica's seaward‐flowing glaciers has accelerated, resulting in increased ice mass‐loss to the ocean and corresponding global sea‐level rise. Recent work has found that glaciers in the Antarctic Peninsula also experience changes in their flow speed throughout the year, with faster flow (of up to 15% relative to annual‐averaged rates) occurring during the summertime. The physical processes responsible for controlling this seasonal behavior, however, are unknown. Here, we examine a range of satellite, atmosphere, and ocean records to determine the main environmental drivers of this seasonal response. Our results show that both surface and oceanic processes are important in driving this behavior, but that the influence of surface processes is more immediate than the relatively time‐delayed, or "lagged," effects of the ocean. We also find that both surface and oceanic processes are themselves subject to the influence of larger‐scale climate phenomena, including the El Niño Southern Oscillation. Overall, our results provide new insights into the environmental controls on the recently observed seasonal pattern of ice flow in Antarctica. Key Points: Surface meltwater extent and sea‐ice‐modulated oceanic forcing act as statistically significant precursors to summertime ice‐flow acceleration in the Antarctic PeninsulaSurface meltwater presence and enhanced ocean temperatures presage summertime acceleration with different temporal lead timesA relationship exists between El Niño Southern Oscillation and Antarctic ice‐flow seasonality, with the unprecedented magnitude El Niño event of 2016 driving an anomalous wintertime speed‐up through enhanced local surface and oceanic forcing [ABSTRACT FROM AUTHOR]