Reconciling Drivers of Seasonal Terminus Advance and Retreat at 13 Central West Greenland Tidewater Glaciers

The majority of Greenland tidewater glaciers undergo a seasonal cycle in terminus position, characterized by wintertime advance and summertime retreat. Understanding mechanisms that control seasonal cycles can help elucidate how tidewater glaciers regulate dynamic ice loss on longer timescales. However, controls on terminus position are numerous and complex. To address this, we compare time series of satellite-derived terminus positions for tidewater glaciers in central west Greenland with observations of environmental forcings, including runoff at the grounding line, mélange presence, and, where available, ocean temperature in the proglacial fjord. We show that for most glaciers, seasonal terminus positions are more sensitive to glacial runoff than mélange or ocean thermal forcing. The strength of this relationship differs for two end-member glacier types in the region, defined by their terminus geometry and dominant calving style. First, we find a strong relationship between magnitudes of runoff and terminus retreat at tidewater glaciers with shallow grounding lines (<400 m) that calve primarily through small-magnitude serac failures. At these glaciers, subglacial plumes drive submarine melt and locally enhance retreat, causing heterogeneous position change across the terminus and local embayments where seasonal terminus changes are largest. In contrast, deep termini susceptible to buoyant flexure retreat sporadically through full ice thickness calving events less dependent on runoff. While less common, these glaciers deliver larger ice fluxes to the ocean. With predicted surface melt increases and diminished mélange coverage in a warming climate, our results reveal the impact of environmental forcings on diverse tidewater glacier systems in the region.