![]() The winter speed of Tunabreen is only 0.2 m per day, rising to a peak of ∼1 m per day in October. ![]() Kronebreen has a winter speed of 1.5–2 m per day, with summer peaks of 3–4 m per day associated with positive air temperatures and periods of high rainfall. 2) show significantly different behaviours between the three glaciers during our observation period. These include local weather data (air temperature and precipitation), sea-surface temperature (SST), fjord ice presence and water temperatures at depth in the glacier fjords (henceforth ‘sub-surface temperatures' Methods section). To investigate the controls on measured frontal ablation rates we explore a variety of environmental variables that have previously been associated with calving behaviour. In the cases of Kronebreen and Aavatsmarkbreen, we incorporate images from more than one satellite track, giving a greater frequency of observations during some time periods. The images, in ‘Stripmap' mode (∼2 m ground range pixel size), were acquired every orbital cycle for 19 months during 2013 and early 2014, providing an image pair every 11 days barring a few acquisition failures. We use equation (1) to calculate the frontal ablation rate between pairs of TerraSAR-X images from surface velocities derived by feature tracking, and ice-front position change derived by manual digitization and geographical information systems (GIS) (Methods section). Our findings imply that submarine melt undercut and collapse is the dominant calving mechanism at these glaciers, and that frontal ablation is therefore controlled primarily by ocean temperatures. We find that rates of frontal ablation are not dependent on glacier dynamics, nor reduced by the onset of glacier surface freeze-up, but closely follow the temperature of the fjord waters at depth. Analysis of these data allows us to quantify evolution of frontal ablation rates in unprecedented detail through a complete seasonal cycle, and to identify the key environmental controls on calving behaviour. We derive time series of frontal velocities, terminus positions and frontal ablation rates for three Svalbard tidewater glaciers using 11-day repeat, 2 m resolution, TerraSAR-X images spanning more than a year. We address this problem by combining high temporal and spatial resolution satellite radar data with oceanographic and meteorological time series to investigate controls on frontal ablation rates. This scarcity of direct measurements of frontal ablation rates and their relationship to conditions both above and below the waterline places severe limits on our ability to predict the response of tidewater glaciers to oceanographic forcing 10. Some attempt has been made to directly measure frontal ablation rates at tidewater glaciers 8, 9 but temporal and spatial resolution is often poor, and winter months are inadequately sampled. Almost all estimates of the latter are based on calculations of the frontal heat budget using oceanographic data, for example, refs 1, 6, 7, and because these estimates rely on extrapolations from spatially and temporally limited CTD and current measurements, they tend to be subject to large uncertainties. However, there remains a distinct lack of direct measurements of either calving or subaqueous melt rates. Several recent studies have concluded that submarine melting can dominate mass loss at tidewater termini, particularly where glaciers are impacted by incursions of warm water from the continental shelf or beyond 2, 4, 5. In addition to its direct contribution, submarine melting can amplify calving by undercutting and destabilizing the glacier front 3. Loss of mass at the termini of tidewater glaciers, or frontal ablation, occurs by a combination of calving and submarine melting 1, 2. Our findings illustrate the potential for deriving simple models of tidewater glacier response to oceanographic forcing. We conclude that calving proceeds by melt undercutting and ice-front collapse, a process that may dominate frontal ablation where submarine melt can outpace ice flow. We find that frontal ablation is not dependent on ice dynamics, nor reduced by glacier surface freeze-up, but varies strongly with sub-surface water temperature. We combine ocean data, ice-front position and terminus velocity to investigate controls on frontal ablation. Here we derive frontal ablation rates for three dynamically contrasting glaciers in Svalbard from an unusually dense series of satellite images. Measurements are difficult because mass lost is replaced by ice flow at variable rates, and frontal ablation incorporates sub-aerial calving, and submarine melt and calving. Rates of ice mass loss at the calving margins of tidewater glaciers (frontal ablation rates) are a key uncertainty in sea level rise projections. ![]()
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