| Summary: | The loss of the floating ice tongue on Jakobshavn Isbræ, Greenland, in the early 2000s has been concurrent with a pattern of thinning, retreat and acceleration leading to enhanced contribution to global sea level. The loss of the floating ice tongue on Jakobshavn Isbræ, Greenland, in the early 2000s has been concurrent with a pattern of thinning, retreat and acceleration leading to enhanced contribution to global sea level. These changes on decadal timescales have been well documented. Here we identify how the glacier responds to forcings on shorter timescales, such as from variations in surface melt, the drainage of supraglacial lakes and seasonal fluctuations in terminus position. Ice motion and surface melt were monitored intermittently from 2006 to 2008. Dual-frequency GPS were deployed 20–50 km upstream of the terminus along the glacier center line. Gaps in surface melt measurements were filled using a temperature-index model of ablation driven by surface air temperatures recorded during the same time period. Our results corroborate the premise that the primary factors controlling speeds on Jakobshavn Isbræ are terminus position and geometry. We also observe that surface speeds demonstrate a complex relationship with meltwater input: on diurnal timescales, velocities closely match changes in water input; however, on seasonal timescales a longer, more intense melt season was observed to effectively reduce the overall ice flow of the glacier for the whole year. Support for this project was provided by NASA’s Cryospheric Sciences Program (NNG06GB49G). Logistical support was provided by CH2M Hill Polar Field Services and instrument support was provided by the University Navstar Consortium (UNAVCO). We thank J. Brown, M. Lu ̈thi and R.J. Motyka for help in the field, and S. Herreid for his GIS wizardry. Helpful discussions with A. Aschwanden greatly improved the clarity of the figures. We thank two anonymous reviewers for comments which greatly improved the clarity of the manuscript. Yes
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