. Semi-periodic Dynamic Thickening of a Tidewater Glacier in Køge Bugt, Greenland

Abstract
Numerous Greenland tidewater glaciers experience seasonal fluctuations in speed (e.g. Joughin et al, 2008; Howat et al, 2010; Sole et al, 2011). Warm summer temperatures coupled with the draining of supraglacial meltwater to the bed enhances flow that can trigger dynamic processes along submarine termini. While the majority of tidewater glaciers along Greenland's southeast coast exhibit melt-induced seasonal oscillations in speed (Moon et al, 2014), one glacier in Koge Bugt behaves as a notable outlier; it demonstrates multi-year semi-periodic departures in ice flow. Identified here as Koge Central, it is an understudied glacier that produces the third highest ice flux in Greenland (Enderlin et al, 2014). Here we combine NASA's MEaSUREs and GoLIVE products, ArcticDEM elevations, and Landsat panchromatic scenes to create a long time-series of speeds, surface elevations, and terminus positions for tidewater glaciers in Koge Bugt. We show that on multiple occasions Koge Central experienced large (+30%) increases in speed, sustained high flow rates for between 2 and 4 years, then rapid and equally high (-30%) reductions in speed. The decelerations were accompanied by large terminus advance (>1 km) and a significant increase (> 45 m) in surface elevation that occurred over several months. The cyclic behavior occurred three times over the last 15 years. During each, the advance and reduction in speed began in late summer and culminated in winter, suggesting a meltwater trigger for deceleration events; however, the lack of annual variability and prolonged periods of accelerated flow suggest an atypical response to meltwater input. Contrasting the anomalous behavior of Koge Central are two unnamed tidewater glaciers located 20 km north (referred to here as Koge North) and 18 km south (Koge South) that show a characteristic response to melt-induced velocity variations. We compare observations for all three tidewater glaciers with a particular focus on Koge Central and investigate surface melt and sea surface temperature records for potential source perturbations that initiated the observed changes in ice dynamics.