Edited by: Gilles Reverdin, Centre National de la Recherche Scientifique (CNRS), France
Reviewed by: Inga Monika Koszalka, GEOMAR Helmholtz Centre for Ocean Research Kiel (HZ), Germany; Oliver Zielinski, University of Oldenburg, Germany
*Correspondence: Daniel F. Carlson
This article was submitted to Ocean Observation, a section of the journal Frontiers in Marine Science
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
Icebergs and bergy bits makes up a significant component of the total freshwater flux from the Greenland Ice Sheet to the ocean. Observations of iceberg trajectories are biased toward larger icebergs and, as a result, the drift characteristics of smaller icebergs and bergy bits are poorly understood. In an attempt to fill this critical knowledge gap, we developed the open-source EXpendable Ice TrackEr (EXITE). EXITE is a low-cost, satellite-tracked GPS beacon capable of high-resolution temporal measurements over extended deployment periods (30 days or more). Furthermore, EXITE can transform to a surface drifter when its host iceberg capsizes or fragments. Here we describe basic construction of an EXITE beacon and present results from a deployment in Godthåbsfjord (SW Greenland) in August 2016. Overall, EXITE trajectories show out-fjord surface transport, in agreement with a simple estuarine circulation paradigm. However, eddies and abrupt wind-driven reversals reveal complex surface transport pathways at time scales of hours to days.
Solid ice calved from tidewater glaciers in Greenland makes up a significant component of the total freshwater flux from the ice sheet to the ocean. The freshwater phase and magnitude of the flux vary, both from one fjord to another and also seasonally within individual fjords. Accurate representation of the spatial and temporal variability of solid and liquid freshwater flux from Greenlandic fjords can be an important parameter in numerical ocean models, as well as models of present and future climate (Martin and Adcroft,
Recent studies have focused on large (>100 m) icebergs in Greenland waters as icebergs of this scale are easier to detect using remote sensing (Buus-Hinkler et al.,
Bergy bits typically calve from a larger parent iceberg (Savage et al.,
In an attempt to advance our general understanding of fjord circulation and ice-ocean interactions, and transport of bergy bits (i.e.,
Here we describe the design and construction of the EXITE beacon and present findings from a recent deployment of 7 beacons near a marine-terminating glacier in Godthåbsfjord (southwest Greenland; Figure
The greater Godthåbsfjord region with the EXITE deployment area near the terminus of the fjord indicated by the pink triangle. The inset at the bottom right shows individual EXITE deployment locations. The inset at the upper left shows the location of Godthåbsfjord (outlined in red) in Greenland. Greenland's capital city of Nuuk (NK) is located near the mouth of the fjord (lower left). CTD profile locations are indicated by yellow circles, with the first profile coinciding with the EXITE deployment location. The location of the Asiaq Greenland Survey meteorological station (MET) is shown by the light blue triangle. The outflow from Lake Tasersuaq (LT) is indicated by a red circle and the turbid outflow plume is visible. The three tidal-outlet glaciers, Narssap Serbia (NS), Akugdlerssûp Serbia (AS), and Kangiata Nunâta Sermia (KNS), are also labeled.
The EXITE beacon was designed specifically to observe trajectories of smaller icebergs and bergy bits in Greenlandic fjords in near-real-time via satellite. The design process considered the following requirements. First, the total cost of each beacon must reflect the relatively short expected lifetime on a host iceberg or bergy bit and the high probability of damage to the EXITE beacon when the iceberg capsizes or disintegrates. Minimizing the cost would also increase the number of beacons available for deployment and make it accessible not only to the academic research community but also to citizen-science initiatives. Second, the EXITE beacon must be constructed from off-the-shelf (OTS) components and must not require any specialized tools or expertise to build. Third, the batteries must provide a minimum lifetime of 30 days. Fourth, and finally, a sufficient number of EXITE beacons must be easily transported on and deployed from the small boats commonly available for charter in western Greenland.
To satisfy the above-mentioned requirements, the EXITE beacon is composed of a Spot Trace GPS powered by a 10 Ah 6V lantern battery and housed in a 50 cm section of 10 cm diameter PVC drain pipe (Figure
The EXITE beacons were designed to be buoyant, given the short lifetime expectancies on the iceberg as mentioned above. After becoming dislodged from the iceberg or bergy bit, the EXITE beacon would transition from an iceberg tracker to a surface drifter to provide information about meltwater pathways. We planned to identify this transition using observed changes in velocity and/or acceleration, as will be described in detail below. An additional 1.2 kg of ballast was added in the form of scrap metal to the bottom of the PVC pipe. With the battery installed on top of the ballast the EXITE beacon floated nearly vertically with approximately 10 cm of pipe exposed to the air to provide the GPS with a clear view to the sky.
For the cylindrical housing and tag line, the slip speed is estimated following Suara et al. (
where |
Godthåbsfjord (GF) is a long (180 km), narrow (5 km), and deep (400–800 m) sill fjord in southwestern Greenland near Nuuk (Figure
7 EXITE beacons were thrown by hand into depressions and/or grooves on bergy bits near KNS on 8 August 2016 (Figures
Temperature profiles (Figure
Time series of
Estimates of iceberg mass and surface area at the time of tagging are necessary to understand velocities and accelerations computed from EXITE trajectories. Most methods to estimate iceberg keel depth, volume, and mass require
Following Ralph et al. (
where ρ and
where
Savage et al. (
where κ = 0.45. Equation (4) can then be used to estimate mass by multiplying by ice density.
An iceberg of mass
where
where
and
In Equations (7) and (8) ρ,
Icebergs and bergy bits calved from the Greenland Ice Sheet are typically smaller and irregularly shaped when compared to their large tabular Antarctic counterparts (Bigg,
where
Ice mass and, by extension, volume and cross-sectional area vary over time, both gradually due to melting (Bigg et al.,
The observed temperature profile (Figure
Savage et al. (
where
In Equation (11) κ is the inverse of the aspect ratio and is set to 0.45 (Savage et al.,
In Equation (12),
Wave erosion has been suggested as the dominant process responsible for iceberg mass loss (Huppert,
Observed positions in degrees of latitude and longitude were converted to universal transverse Mercator (UTM) easting (
Length along the waterline, average height above the waterline, and above-water surface area were estimated from photographs taken during deployment. A 50.6 megapixel Canon 5DSR Mk. III full-frame DSLR camera equipped with a 24 mm lens was used to photograph bergy bits. Lens distortion was corrected using the Camera Calibration Toolbox for Matlab (
Using the maximum observed EXITE velocities and wind speeds we can estimate the forces due to air and water friction using Equations (7) and (8) using the dimensional estimates for each iceberg (Table
Shape and estimated dimensions of icebergs tagged with EXITE beacons.
ICE01 | B | 31 | 2 | 33 | 21 | 62 | 1.29 × 104 | 1.18 × 104 | 43 | 12 |
ICE02 | W | 40 | 1.5 | 40 | 28 | 105 | 2.9 × 104 | 2.6 × 104 | 17 | 10 |
ICE03 | B+D | 33 | 2 | 34 | 23 | 91 | 1.5 × 104 | 1.4 × 104 | 43 | 0.67 |
ICE06 | W+D | 26 | 2.5 | 29 | 18 | 79 | 7.58 × 103 | 6.9 × 103 | 25 | NA |
ICE07 | W | 33 | 3.2 | 35 | 23 | 53.2 | 1.64 × 104 | 1.49 × 104 | 46 | 12 |
ICE08 | W | 24 | 3 | 28 | 17 | 72 | 6.1 × 103 | 5.6 × 103 | 31 | 16 |
ICE10 | B+D | 15 | 2 | 18 | 9 | 40 | 1.08 × 103 | 9.8 × 102 | 33 | NA |
The EXITE beacons exceeded expectations, with 5 of the 7 meeting or exceeding the 30 day battery life requirement (Table
The trackers reveal complex bergy bit and surface water transport with evidence of the influence of topographic eddies, collisions with fjord walls, freshwater outflows, and a large flow reversal (Figures
A trajectory of an EXITE beacon initially deployed at the large pink triangle. Locations are color-coded according to time, in days, since deployment. Every 4th measurement is shown for clarity. The location where the critical acceleration was exceeded is indicated by the large pink square.
A trajectory of an EXITE beacon initially deployed at the large pink triangle. Locations are color-coded according to time, in days, since deployment. Every 4th measurement is shown for clarity. The location where the critical acceleration was exceeded is indicated by the large pink square.
A trajectory of an EXITE beacon initially deployed at the large pink triangle. Locations are color-coded according to time, in days, since deployment. Every 4th measurement is shown for clarity. The location where the critical acceleration was exceeded is indicated by the large pink square.
A trajectory of an EXITE beacon initially deployed at the large pink triangle. Locations are color-coded according to time, in days, since deployment. Every 4th measurement is shown for clarity.
A trajectory of an EXITE beacon initially deployed at the large pink triangle. Locations are color-coded according to time, in days, since deployment. Every 4th measurement is shown for clarity. The location where the critical acceleration was exceeded is indicated by the large pink square.
A trajectory of an EXITE beacon initially deployed at the large pink triangle. Locations are color-coded according to time, in days, since deployment. Every 4th measurement is shown for clarity. The location where the critical acceleration was exceeded is indicated by the large pink square.
A trajectory of an EXITE beacon initially deployed at the large pink triangle. Locations are color-coded according to time, in days, since deployment. Every 4th measurement is shown for clarity.
Zonal (blue) and meridional (red) velocity components for each EXITE beacon.
Magnitude of acceleration for each EXITE beacon during August–September 2016.
In general, most EXITE beacons exceeded their respective critical accelerations during collisions with fjord walls. The estimated transition point in each EXITE trajectory is indicated by a pink square, when applicable, in Figures
Two beacons were transported to within a few km of the calving front of NS (Figures
Contrast-enhanced, color image of the inner Godthåbsfjord region acquired 10 September 2016. The positions of the 3 active EXITE beacons at the time of acquisition are shown by pink triangles.
Additional evidence of the complexity of surface transport in GF comes from a tracker that traveled upstream into the LT outflow (Figure
The EXITE beacons successfully tracked bergy bits and melt water in their first deployment in GF at a fraction of the cost of commercially available drifters and ice trackers. In addition to tracking general out-fjord transport of solid and liquid freshwater, the EXITE trajectories reveal complexity in the shallow circulation in the form of small-scale eddies and wind-driven reversals. One such reversal transported an EXITE tracker to within 8 km of its original deployment location. The differences in dispersion between initially proximal bergy bits highlight the importance of small-scale variability in ocean currents and the chaotic, unpredictable nature of Lagrangian transport. The importance of and interplay between winds and freshwater plumes is also apparent when the EXITE trajectories are compared to high-resolution satellite imagery.
The observed velocities agree well with previous studies of icebergs in both open water (Smith,
Empirical relationships for basal and convective melt also need to be verified for icebergs and bergy bits in a fjord setting. The combined lateral and vertical temperature stratification observed in Godthåbsfjord in August 2016 likely resulted in both depth and spatially dependent melt rates (Figures
While a wide range of size classes of ice can be tracked by radar and time lapse imagery (Turnbull et al.,
The application of historical scaling relationships highlights the need for additional measurements of ice over a wider range of size scales. Understanding of bergy bit drift and deterioration will require comprehensive measurements of both ice geometry and ambient fjord conditions. Ideally, such a comprehensive study would include tagging of drifting icebergs and bergy bits in a region instrumented with acoustic Doppler current profilers (ADCP), CTDs, and weather stations. Time lapse cameras and/or radar would also provide insight into the motion of all the ice in the field of view.
While the EXITE beacons provided the first observations of bergy bit transport in GF, the biggest drawback of the present design is the uncertainty in timing of transition from an ice tracker to a surface drifter. The critical acceleration metric devised to estimate the time of transition relies on dated scaling relationships that were largely developed for larger pieces of ice and in an open ocean setting. The initial mass estimates are, therefore, subject to error, the magnitude of which is difficult estimate given the multiple sources of uncertainty. The critical acceleration metric also uses an estimate of ice mass at the time of tagging and does not take changes in mass due to melting and fragmentation into account. In most cases, the critical acceleration is exceeded when an EXITE beacon decelerates rapidly during a collision with a fjord wall. Given that all of the bergy bits satisfied the criterion for instability (Equation 9) when initially tagged, it comes as no surprise that the bergy bit could capsize during a collision with a solid boundary. Improvements to the existing design are discussed in Section 4.1.
The initial mass used to estimate the critical acceleration also changes in time due to the relatively high (estimated) melt rate. A melt rate of 1.2 m day−1 corresponds to a decrease in ice mass of approximately 700 kg day−1. The highest melt rates will result from wave erosion and, in the absence of waves, the warm (4–5°C) surface (0–10 m) layer will lead to similar melt rates. Both processes result in the undercutting observed below the waterlines of several of the bergy bits tagged in this study (Figure
The recovery of 2 EXITE beacons provided first-hand verification of their drifting status. A dedicated attempt to recover 2 EXITE beacons on 24 August was unsuccessful due to the large amount of growlers and smaller ice fragments in the vicinity of the positions reported by the beacons. The Sentinel-2 image from the same day (not shown) verifies these conditions. The trapping of EXITE beacons in a high concentration of small pieces of ice may have limited their ability to drift freely and, as a result, their observed accelerations likely reflect the collective response, or added mass, of the ice and surface water. In other words, the net effect of many small pieces of ice concentrated in a small area may be to resemble the drift of a larger, single piece of ice. The coherent patches of small ice fragments visible in the Sentinel-2 imagery may result from geostrophic adjustment, especially toward the mouth of the fjord where the density gradients between the localized melt water and ambient water are larger.
A wind-related metric was also devised in an attempt to detect the transition from an ice tracker to a surface drifter. As surface currents typically respond nearly instantaneously to wind forcing (Ursella et al.,
Despite the complex problem of identifying the transition from iceberg tracker to surface drifter we decided that it was a worthwhile exercise to attempt. It is our hope that others will adopt the general design presented here and, should attempts to detect the transition from iceberg tracker to surface drifter fail, the EXITE beacon can be constructed such that it will sink once dislodged from the ice. Negative buoyancy can be achieved simply by increasing the ballast or decreasing the internal volume of the PVC housing, or by doing both. An instrumented tracker, currently under development, is described briefly below. When completed, it provide a more reliable estimate of the time of transition to a surface drifter using conductivity and temperature measurements, as well as observations of these variables along its trajectory.
While the EXITE prototype described here exceeded all expectations, the next generation of EXITE trackers has been modified slightly to extend battery life and improve reliability. Two versions are in development. The first version simply improves on the original design and incorporates a higher capacity, industrial-strength battery (6V, 27 Ah) that should result in significantly longer lifetimes. The battery and GPS are still housed in a section of PVC but the internal frame is now constructed out of laser-cut plywood. One-way vents have been installed in the cap to allow any gas buildup in the housing to escape. An optional external collar can be attached to the top of the tracker to provide the GPS antenna with a better view of the sky while on the ice. The second version in development takes advantage of open-source developments in the unmanned aerial systems (UAS) and remotely operated vehicles (ROV) sectors. This version incorporates more internal and external sensors to provide higher temporal resolution measurements of position, rotation, and acceleration, as well as temperature and conductivity. At first glance, EXITE is similar in construction to other low-cost drifters (Johnson et al.,
Understanding ice sheet-ocean interactions will require application of observations, models, and theory to the fjords where these two systems interact. While remote sensing techniques provide estimates of melt rates of ice in fjords (Enderlin et al.,
DC, WB, LM, SR, and JA contributed to experiment planning, data collection, and interpretation of results. DC designed and constructed the trackers, analyzed the data, and prepared the bulk of the manuscript.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Erling Pedersen assisted in the construction of the trackers and Egon Randa Frandsen, Emmelia Wiley, and Carl Isaksen assisted with logistics. Masayo Ogi assisted with deployment of the EXITE beacons and collection of other field data. Diego Mejia at Globalstar provided SPOT Trace units with custom firmware. We gratefully acknowledge the contributions of Arctic Research Centre (ARC), Aarhus University and by the Canada Excellence Research Chair (CERC). This work is a contribution to the Arctic Science Partnership (ASP)
expendable ice tracker
global positioning system
Lake Tasersuaq. Location in Godthåbsfjord (Figure
Godthåbsfjord.