ALPS in the Antarctic
Sarah Purkey and Pierre Dutrieux
Antarctica and its surrounding oceans play a critical role in global climate. The Southern Ocean circulation acts as a door into the deep ocean, driving the upper and lower cells of the meridional overturning circulation (MOC) that controls the exchange of heat, carbon, and nutrients between the surface and the deep ocean. In addition, Antarctica holds the largest reservoir of glacial ice, some of which is retreating rapidly, with large implications for sea level rise. Yet, the Antarctic region remains poorly sampled owing to harsh conditions, inaccessibility during most months of the year, and treacherous evolving icescapes. This has made Antarctica a desirable region to utilize ALPS technology to overcome existing monitoring challenges. These challenges include increased ability to navigate in complex and enclosed cavities under ice shelves, operate around and under rapidly evolving sea ice, and resolve the Southern Ocean’s physical and biogeochemical spatial and temporal scales that are important to climate.
Many autonomous platforms including gliders, floats, autonomous underwater vehicles (AUVs), and animal tags have been adapted for use in this unique environment, but many impediments still lay ahead. Here we discuss some of the advancements and applications of ALPS technologies and how they might be used in the future to continue to advance our scientific understanding of the physical and biogeochemical processes operating in this unique region.
Overarching Scientific Questions
The Southern Ocean is the center of the global MOC, lifting carbon-rich waters from the deep ocean through wind-driven Ekman divergence and converting these deep waters into abyssal and intermediate water through buoyancy exchange with the atmosphere. This process results in a strong meridional gradient in the flux of heat and carbon into and out of the ocean. Despite this region’s climatic importance, monitoring these fluxes and any change in the ocean reservoirs has proven difficult to determine due to limited spatial and temporal data.
Another important climatic process that occurs along the Antarctic coastline is melting of the ice shelves that buttress the flow of ice from land to the Southern Ocean. In steady state, this flow of ice is balanced by precipitation over the ice sheet. However, over the past few decades, satellite observations have demonstrated a persistent and accelerating contribution to global sea level rise from diminishing ice sheets (Shepherd et al., 2012). This ice loss is driven by oceanic melting in West Antarctica, and in particular the Amundsen Sea (Depoorter et al., 2013; Rignot et al., 2013), where ocean heat content is large and efficiently reaches the ice shelves (Jenkins et al., 2010; Jacobs et al., 2011; Dutrieux et al., 2014a) leading to growing concerns about future contributions to sea level rise and the large associated uncertainties (Scambos et al., 2017).
Core (2,000 m), Deep (6,000 m), and Biogeochemical (BGC) Argo floats are key ALPS platforms that have enabled monitoring of the Southern Ocean. The Southern Ocean remains more sparsely covered than the tropical and subtropical oceans. It has not yet reached the Argo goal of 3° x 3° spatial coverage, but better ice-avoidance technology and current scientific interest in the region are paving the way to rapid progress. A number of pilot studies have placed Argo floats under seasonal ice with great success. In addition, Argo, ALAMO, and EM-APEX floats have been placed in polynyas on the Ross Sea, the Sabrina Coast, near the Adélie Depression and the Amundsen Sea, and over Maud Rise. Some of these are locations of past and current deep-water formation while others are areas where the ocean actively melts the ice sheet. In both cases, these instruments are providing the first full-depth, full-year monitoring of these climatically essential regions.
The under-ice Argo floats are able to detect possible surface freezing conditions during their assent and can decide to not surface until conditions are more favorable. One remaining issue with these floats is the large uncertainty in profile position during the winter months given that under-ice profiles cannot get a GPS fix. Techniques for deriving under-ice position include linear interpolation, interpolation informed by numerical models, using bottom bathymetry where floats come aground, and uses of RAFOS acoustics triangulation where available.
Work is currently underway to expand the core Argo array into the deep ocean and add BGC sensors. The Southern Ocean Carbon and Climate Observation and Modeling (SOCCOM) project is currently in the process of deploying 200 Argo floats with BGC sensors (oxygen, pH, phosphate, and optics) throughout the Southern Ocean, including seasonal ice zones (see Gray, 2018, in this report). Preliminary results have already revealed seasonal cycles in carbon fluxes and shown large discrepancies in the annual net Southern Ocean carbon uptake from previous studies (Grey et al., in prep). These data are being incorporated into biogeochemical models to further quantify the Southern Ocean’s role in the carbon cycle (Mazloff et al., 2010). In addition, the first deep Argo floats in the Southern Ocean are planned for deployment in the Australian-Antarctic basin in January 2018, directly downstream from deep-water formation sites along the Adélie coast. If successful, this will allow for continuous and direct monitoring of Antarctic Bottom Water properties and volume near the initiation of the bottom limb of the MOC.
Finally, work is also underway to use float technologies under ice shelves. While chances of instrumental loss remain high in mostly unknown ice cavity geometries, the demonstrated persistence of floats and their low cost compared to moored instruments through ice drilled holes opens interesting possibilities for exploration and monitoring. Underwater acoustic geolocation and software development are being implemented to make such missions possible.
Gliders have also been use to resolve the physical environment across boundaries and on the continental shelf around Antarctica. Some gliders have been deployed along the West Antarctic Peninsula to supplement annual ship-based hydrographic work to quantify pathways of relatively warm Southern Ocean deep water onto the shelf (Mckee et al., in prep) or the processes involved (Thompson et al., 2014). In addition, gliders equipped with microstructure sensors along the southern end of Drake Passage measure mixing and water mass transformation (Ruan et al., in review). Finally, sparse glider sections have mapped ocean properties near ice shelves (Miles et al., 2016).
All missions to date were conducted in summer and mostly in open water. However, some ventured, voluntarily or not, under the ice for small amounts of time, so they did not involve specific technological developments to persistently obtain observations under ice during winter. Projects are now underway to try to make progress in these areas using a combination of underwater acoustic geolocation and software development to manage complex geometries and drifts.
Tagged marine mammals armed with temperature-salinity sensors capable of profiling under ice, with dive depths up to 2,000 m and wide rooming ranges from the coast to open water covering most of the Southern Ocean, are also currently being used to radically increase the number of CTD profiles south of 40°S. These additional CTD data greatly improve Southern Ocean assimilation models by providing under-ice data (see Roquet and Boehme, 2018, in this report). They also provide crucial winter observations in areas that are mostly devoid of them (Årthun et al., 2012; Heywood et al., 2016; Williams et al., 2016). One major issue with these data is sensor accuracy. Current and ongoing work to improve the precision and accuracy of the sensors has shown promising results, and animal platforms will likely be a major source of quality Southern Ocean data in the future.
Autonomous Underwater Vehicles (AUV)
Owing to their relatively short endurance, AUVs are not yet suited for studying systems over more than a week at a time. But their large payloads and ability to reach otherwise inaccessible areas makes them platforms of choice to explore cavities under ice shelves and therefore radically expand on the visions previously obtained from point observations. Following a preliminary loss of Autosub2 under the Fimbul ice shelf (Nicholls et al., 2008), Autosub3 successfully mapped over 500 km of ocean properties (Jenkins et al., 2010), seabed (Graham et al., 2013), and ice shelf base (Dutrieux et al., 2014b) geometries under Pine Island ice shelf in West Antarctica in the austral summer of 2009 and 2014. These first, detailed observations of two unexplored cavities will undoubtedly inspire many others using similar technologies, and many groups are preparing to do just that.
Other platforms with similar technologies and payloads have also been deployed near and under Antarctic ice shelves and sea ice. The majority remain tethered for insurance purposes or to test future deployment and retrievals through ice shelf drilled holes. These more local explorations are limited to a few kilometers from where they are deployed, but offer very interesting perspectives to explore detailed boundary layer processes as well as the local biogeochemistry.
Challenges for the Next Decade
While the past decade has seen amazing advances in the use of floats, gliders, AUVs, and tagged animals, challenges remain to fully utilize ALPS technology to monitor Antarctica and the Southern Ocean. Some of the key issues to address in the coming decade include:
- Improved coverage of Core (>2,000 m) and Deep (>6,000 m) Argo throughout the Antarctic oceans, including under seasonal ice for full monitoring of the Southern Ocean
- Improved estimates of the positions of under-ice Argo floats
- Continuous monitoring of Circumpolar Deep Water circulation near and under ice to improve our understanding of ocean driven basal melting
- Monitoring Antarctic Bottom Water formation regions and understanding the processes controlling production rates
- Improved accuracy of marine mammals data to reach Argo standard of quality
The deployment of ALPS technology in Antarctic settings remains expensive and fraught with danger for the instruments. Yet experience has been gained, and recent explorations have demonstrated that the scientific benefits largely outweigh potential losses. Thus, we are sure to see a continuation in the positive trajectory of the use of ALPS technology in and around Antarctica in the coming decade.
Årthun, M., K.W. Nicholls, and L. Boehme. 2012. Wintertime water mass modification near an Antarctic ice front. Journal of Physical Oceanography 43(2):359–365, https://doi.org/10.1175/JPO-D-12-0186.1.
Depoorter, M.A., J.L. Bamber, J.A. Griggs, J.T.M. Lenaerts, S.R.M. Ligtenberg, M.R. van den Broeke, and G. Moholdt. 2013. Calving fluxes and basal melt rates of Antarctic ice shelves. Nature 502:89–92, https://doi.org/10.1038/nature12567.
Dutrieux, P., J. De Rydt, A. Jenkins, P.R. Holland, H.K. Ha, S.H. Lee, E.J. Steig, Q. Ding, E. P. Abrahamsen, and M. Schröder. 2014a. Strong sensitivity of Pine Island ice-shelf melting to climatic variability. Science 3(7):468–472, https://doi.org/10.1126/science.1244341.
Dutrieux, P., C. Stewart, A. Jenkins, K.W. Nicholls, H.F.J. Corr, E. Rignot, and K. Steffen. 2014b. Basal terraces on melting ice shelves. Geophysical Research Letters 41(15):5,506–5,513, https://doi.org/10.1002/2014GL060618.
Graham, A.G.C., P. Dutrieux, D.G. Vaughan, F.O. Nitsche, R. Gyllencreutz, S.L. Greenwood, R.D. Larter, and A. Jenkins. 2013. Seabed corrugations beneath an Antarctic ice shelf revealed by autonomous underwater vehicle survey: Origin and implications for the history of Pine Island Glacier. Journal of Geophysical Research 118:1,356–1,366, https://doi.org/10.1002/jgrf.20087.
Gray, A.R. 2018. Observing the global ocean with the Argo array. Pp. 21–24 in ALPS II – Autonomous Lagrangian Platforms and Sensors. A Report of the ALPS II Workshop. D. Rudnick, D. Costa, K. Johnson, C. Lee, and M.-L. Timmermanns, eds, February 21–24, La Jolla, CA.
Heywood, K., L.C. Biddle, L. Boehme, P. Dutrieux, M. Fedak, A. Jenkins, R.W. Jones, J. Kaiser, H. Mallett, A.C. Naveira Garabato, and others. 2016. Between the devil and the deep blue sea: The role of the Amundsen Sea continental shelf in exchanges between ocean and ice shelves. Oceanography 29(4):118–129, https://doi.org/10.5670/oceanog.2016.104.
Jacobs, S.S., A. Jenkins, C.F. Giulivi, and P. Dutrieux. 2011. Stronger ocean circulation and increased melting under Pine Island Glacier ice shelf. Nature Geoscience 4(8):519–523, https://doi.org/10.1038/ngeo1188.
Jenkins, A., P. Dutrieux, S.S. Jacobs, S.D. McPhail, J.R. Perrett, A.T. Webb, and D. White. 2010. Observations beneath Pine Island Glacier in West Antarctica and implications for its retreat. Nature Geoscience 3(7):468–472, https://doi.org/10.1038/ngeo890.
Mazloff, M., P. Heimbach, and C. Wunsch. 2010. An eddy-permitting Southern Ocean state estimate. Journal of Physical Oceanography 40:880–899, https://doi.org/10.1175/2009JPO4236.1.
Miles, T., S.H. Lee, A. Wåhlin, H.K. Ha, T.W. Kim, K.M. Assmann, and O. Schofield. 2016. Glider observations of the Dotson Ice Shelf outflow. Deep Sea Research Part II 123:16–29, https://doi.org/10.1016/j.dsr2.2015.08.008.
Nicholls, K.W., E.P. Abrahamsen, K.J. Heywood, K. Stansfield, and S. Østerhus. 2008. High-latitude oceanography using the Autosub autonomous underwater vehicle. Limnology and Oceanography 53:2,309–2,320, https://doi.org/10.4319/lo.2008.53.5_part_2.2309.
Rignot, E., S. Jacobs, J. Mouginot, and B. Scheuchl. 2013. Ice-shelf melting around Antarctica. Science 341(6143):266–270, https://doi.org/10.1126/science.1235798.
Roquet, F., and L. Boehme. 2018. On the use of animal-borne instruments to monitor the ocean. Pp. 12–15 in ALPS II – Autonomous Lagrangian Platforms and Sensors. A Report of the ALPS II Workshop. D. Rudnick, D. Costa, K. Johnson, C. Lee, and M.-L. Timmermanns, eds, February 21–24, La Jolla, CA.
Ruan, X., A.F. Thompson, M.M. Flexas, and J. Sprintall. In review. Topographic closure of the overturning circulation in the Southern Ocean.
Scambos, T.A., R.E. Bell, R.B. Alley, S. Anandakrishnan, D.H. Bromwich, K. Brunt, K. Christianson, T. Creyts, S.B. Das, R. DeConto, and others. 2017. How much, how fast?: A science review and outlook for research on the instability of Antarctica’s Thwaites Glacier in the 21st century. Global and Planeteary Change 153:16–34, https://doi.org/10.1016/j.gloplacha.2017.04.008.
Shepherd, A., E.R. Ivins, A. Geruo, V.R. Barletta, M.J. Bentley, S. Bettadpur, and K.H. Briggs. 2012. A reconciled estimate of ice-sheet mass balance. Science 338(6111):1,183–1,189, https://doi.org/10.1126/science.1228102.
Thompson, A.F., K.J. Heywood, S. Schmidtko, and A.L. Stewart. 2014. Eddy transport as a key component of the Antarctic overturning circulation. Nature Geoscience 7:879–884, https://doi.org/10.1038/ngeo2289.
Williams, G.D., L. Herraiz-Borreguero, F. Roquet, T. Tamura, K.I. Ohshima, Y. Fukamachi, A.D. Fraser, L. Gao, H. Chen, C.R. McMahon, and others. 2016. The suppression of Antarctic bottom water formation by melting ice shelves in Prydz Bay. Nature Communications 7(6):12577, https://doi.org/10.1038/ncomms12577.
Sarah Purkey, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA, USA, firstname.lastname@example.org
Pierre Dutrieux, Lamont–Doherty Earth Observatory of Columbia University, Palisades, NY, USA, email@example.com