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The seasonal response to surface melting of the Northeast Greenland Ice Stream outlets, Zachariae and 79N, is investigated using new highly temporally resolved surface velocity maps for 2016 combined with numerical modeling. The seasonal speedup at 79N of 0.15km/yr is suggested to be driven by a decrease in effective basal pressure induced by surface melting, whereas for Zachariae its 0.11km/yr seasonal speedup correlates equally well with the breakup of its large ice melange. We investigate the influence 76km long floating tongue at 79N, finding it provides little resistance and that most of it could be lost without impacting the dynamics of the area. Furthermore, we show that reducing the slipperiness along the tongue-wall interfaces produces a velocity change spatially inconsistent with the observed seasonal speedup. Finally, we find that subglacial sticky spots such as bedrock bumps play a negligible role in the large-scale response to a seasonally enhanced basal slipperiness of the region.

Plain Language Summary The Northeast Greenland Ice Stream may potentially contribute significantly to near-term sea level rise and is one of the lesser studied Greenlandic systems, partly due to its remoteness. We present a new high temporally resolved velocity data set derived from Sentinel 1-A which allows capturing changes on a seasonal timescale, a feature which only the newest generation satellites now permit. We show how surface melting may be linked to the observed seasonal velocity changes, giving important insights into the possible future (range of) behavior and sensitivity of the ice stream outlets to atmospheric changes. In addition, we present a detailed study of possible moderating factors on the seasonal velocity response. In particular, we find that (i) the large ice melange in front of the Zachariae outlet might be dampening the outlet's response, (ii) small-scale subglacial topographical bumps (sticky spots) exert very limited control on the flow, and (iii) the 76km long floating tongue of the 79N outlet is largely a passive feature, suggesting that most of it (approximate to 80%) could be lost without effecting the outlet's contribution to near-term sea level rise. This has broad implications for assessing the future mass loss of ice sheets since it points to the importance of studying every major calving event individually.