the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 May 2024
Ice speed of a Greenlandic tidewater glacier modulated by tide, melt, and rain
Abstract. Ice discharge from the Greenland ice sheet is controlled by tidewater glacier flow speed, which shows significant variations in different timescales. Short-term speed variations are key to understanding the physical processes controlling glacial motion, but studies are sparse for Greenlandic tidewater glaciers, particularly near the calving front. Here, we present high-frequency ice speed measurements performed at 0.5–4 km from the front of Bowdoin Glacier, a tidewater glacier in northwestern Greenland. Three GPS (global positioning system) receivers were operated for several weeks in July of 2013–2017 and 2019. Horizontal ice speed varied over timescales of hours to days, including short-term speed-up events as well as diurnal and semidiurnal variations. Frequency analysis revealed that semidiurnal signals decay upglacier, whereas diurnal signals are consistently observed over the area of study. Speed-up events were associated with heavy rain, and longer-term variations were correlated with air temperature. Uplift of the glacier surface was observed during fast-flowing periods, suggesting basal separation due to elevated water pressure. These observations confirm the strong and immediate impact of melt/rainwater on subglacial water pressure and sliding speed. Tidally modulated ice speed peaks coincided with or slightly before low tide, which demonstrates the key role viscoelastic ice dynamics play in response to changing hydrostatic pressure acting on the glacier front. Our study results reveal details of short-term flow variations near the front of a Greenlandic tidewater glacier and provide insights into calving glacier dynamics. During melt season, ice speed is controlled by atmospheric conditions through meltwater production and rain events as commonly observed in alpine glaciers, but additional complexity arises from tidal influence near the calving front.
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RC1: 'Comment on egusphere-2024-1476', Anonymous Referee #1, 18 Jun 2024
This paper uses detailed GPS-based ice velocity records from near the terminus of Bowdoin Glacier, NW Greenland, to explore controls on the short term variability in ice motion along the lowermost 4 km of the glacier. This methodology is quite similar to that employed in previous studies of the dynamics of alpine glaciers and outlet glaciers from the Greenland Ice Sheet, and correspondingly some of the findings are quite familiar from earlier work. However, studies on the short term dynamics of tidewater glaciers remain comparatively uncommon, and this study thus adds value in demonstrating how far our understanding of glacier dynamics applies (and doesn’t apply) in this context. The GPS data on which the paper is based is of high quality and the analysis and interpretation seem largely sound and well supported. I have no major concerns but just a few minor comments:
L31. Can you be more quantitative on the relative contribution of these processes?
L33. I think it’s overstating things to say that this is ‘key’ (implying it’s the single most important factor) – it is one many topics that are important in understanding the current and future mass loss of the ice sheet.
L249. More so than simply the time taken for water to get to the bed, I think an important consideration here is what controls the timing of the diurnal peak in water pressure. Throughout the middle part of the day when melting is greatest, meltwater input to the glacier will exceed meltwater discharge from the glacier, causing water storage and pressure to increase (the opposite occurs during the remainder of the day). Thus the period of maximum pressure will not likely coincide with maximum melt rates, but rather will occur slightly later (towards the end of the higher melt period of the day), as you observe (see for example Cowton et al 2016).
L255-257. Could snow cover also be a factor here? I assume there was little in the study area at Bowdoin Glacier, but if the study area at Helheim extended 37 km from the terminus, then perhaps remaining snowpack could have played a role in slowing the runoff of meltwater.
L282. Suggest ‘The amount and…’
L283. Suggest ‘…subglacial meltwater input on the basal water pressure…’
L283. It’s not totally clear whether Bartholomaus et al (2008) is being cited as an example of where glacier motion is or isn’t a simple function of meltwater production.
L283-285. I think you can see indications of this even within the short time windows in which data is available each year, with comparable temperature spikes appearing to generate a smaller velocity response later in each measurement period – this is particularly apparent in 2013 and 2019. Also, this sentence would benefit from one or more references – there are plenty to choose from on this topic.
L292. Suggest ‘…glacier acceleration has been documented…’
L295-6. It would perhaps be fair to say that incidences of rain induced acceleration may be increasing in Greenland as the climate warms, but if the authors feel it is truly of ‘critical importance’ then it would be good to see some justification for this with respect to its contribution to ice discharge.
L311-313. It also looks like temperature is falling at this time, so it may be that the addition of rainfall is partially compensated by a reduction in meltwater.
L316. Rather that saying the subglacial drainage system ‘had already developed’ (which implies the system can reach a specific ‘developed’ state beyond which it no longer changes), it might be more correct to say something like ‘had already evolved to a more efficient state’.
L316-317. As for L311-313, it looks like temperature is dropping at this point too.
L399. It’s also been previously observed (e.g. Cowton et al 2016) that there is a stronger correlation between horizontal velocity and the rate of surface uplift (i.e. the vertical velocity) – have you checked to see whether there is any correlation in this instance?
L453-4. I’ve flagged this here as there is a specific reference to mountain glaciers in this sentence, but the comment applies more generally. It’s great that earlier work on the hydrology and dynamics of mountain glaciers is cited – this forms the foundation of the topic and it’s important that its contribution is recognized. There is however also a substantial body of literature on the hydrology and dynamics of land terminating glaciers in Greenland which builds on this work from mountain glaciers and develops it in a Greenlandic context. This is largely overlooked in the current manuscript, but it would seem appropriate to give this a little more reference, as it is the logical stepping stone between earlier work on alpine glaciers and the current study (and addresses many of the same themes as this manuscript). For a review, see Davison et al (2019).
References
Cowton, T., Sole, A., Nienow, P., Slater, D., Wilton, D., & Hanna, E. (2016). Controls on the transport of oceanic heat to Kangerdlugssuaq Glacier, east Greenland. Journal of Glaciology, 62(236), 1167-1180.
Davison, B. J., Sole, A. J., Livingstone, S. J., Cowton, T. R., & Nienow, P. W. (2019). The influence of hydrology on the dynamics of land-terminating sectors of the Greenland ice sheet. Frontiers in Earth Science, 7, 10.
Citation: https://doi.org/10.5194/egusphere-2024-1476-RC1 - AC2: 'Reply on RC1', Shin Sugiyama, 26 Sep 2024
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CC1: 'Comment on egusphere-2024-1476', Ralf Greve, 02 Jul 2024
I think the authors should discuss their findings against the results of the modelling study by Seddik et al. (2019). Quoting the abstract:
"Reduction of the basal drag by 10-40% produces speed-ups that agree approximately with the observed range of speed-ups that result from warm weather and precipitation events. In agreement with the observations, tidal forcing and surface speed near the calving front are found to be in anti-phase (high tide corresponds to low speed, and vice versa). However, the amplitude of the semi-diurnal variability is underpredicted by a factor ~3, which is likely related to either inaccuracies in the surface and bedrock topographies or mechanical weakening due to crevassing."
In particular, it would be interesting whether there is any new insight in possible reasons for the underprediction of the amplitude.
Reference:
Seddik, H., R. Greve, D. Sakakibara, S. Tsutaki, M. Minowa and S. Sugiyama. 2019. Response of the flow dynamics of Bowdoin Glacier, northwestern Greenland, to basal lubrication and tidal forcing. Journal of Glaciology 65 (250), 225-238, doi: 10.1017/jog.2018.106.
Citation: https://doi.org/10.5194/egusphere-2024-1476-CC1 - AC3: 'Reply on CC1', Shin Sugiyama, 26 Sep 2024
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RC2: 'Comment on egusphere-2024-1476', Anonymous Referee #2, 09 Sep 2024
- AC1: 'Reply on RC2', Shin Sugiyama, 26 Sep 2024
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