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Science Strategies for the Arctic Ocean

Mary-Louise Timmermans*


Observational evidence suggests the Arctic is undergoing significant climate change; records show increasing atmospheric and ocean temperatures, ocean freshening, rising sea levels, melting permafrost and decline of sea ice. The rapid loss of permanent sea ice suggests emphasis is needed on sustained, uninterrupted Arctic observations and focused analyses to understand and predict Arctic change on seasonal, interannual, and decadal time scales. Some research suggests atmospheric circulation, rising global temperatures and ice-albedo feedbacks will lead to ice-free summers in the Arctic Ocean in as little as 10 years from now, while other studies indicate that strong natural variability of the Arctic system will inhibit further loss of summer sea ice. The next two decades will be of great significance in Arctic research.

The following are specific questions motivated primarily by the direct need to understand and predict the state of Arctic sea ice. Where and how is the heat that is transported to the Arctic Ocean from lower latitudes lost, and what role does the ocean play in the mass balance of sea ice? How might the Arctic Ocean internal wave field change with reductions in sea-ice extent, and what feedback mechanisms might then arise as a result of higher mixing? What types of feedbacks are associated with the observed general freshening and strengthening of the stratification of the upper Arctic Ocean (for example, in terms of ocean heat loss or the structure of ice formed from a fresher ocean)? What mechanisms cause storage

*

Woods Hole Oceanographic Institution



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Science Strategies for the Arctic Ocean Mary-Louise Timmermans* Observational evidence suggests the Arctic is undergoing significant climate change; records show increasing atmospheric and ocean tem- peratures, ocean freshening, rising sea levels, melting permafrost and decline of sea ice. The rapid loss of permanent sea ice suggests emphasis is needed on sustained, uninterrupted Arctic observations and focused analyses to understand and predict Arctic change on seasonal, inter- annual, and decadal time scales. Some research suggests atmospheric circulation, rising global temperatures and ice-albedo feedbacks will lead to ice-free summers in the Arctic Ocean in as little as 10 years from now, while other studies indicate that strong natural variability of the Arctic system will inhibit further loss of summer sea ice. The next two decades will be of great significance in Arctic research. The following are specific questions motivated primarily by the direct need to understand and predict the state of Arctic sea ice. Where and how is the heat that is transported to the Arctic Ocean from lower latitudes lost, and what role does the ocean play in the mass balance of sea ice? How might the Arctic Ocean internal wave field change with reductions in sea-ice extent, and what feedback mechanisms might then arise as a result of higher mixing? What types of feedbacks are associated with the observed general freshening and strengthening of the stratification of the upper Arctic Ocean (for example, in terms of ocean heat loss or the struc- ture of ice formed from a fresher ocean)? What mechanisms cause storage * Woods Hole Oceanographic Institution 10

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105 MARY-LOUISE TIMMERMANS and release of large volumes of fresh water and ice in the Arctic Ocean on seasonal, interannual and decadal time scales?* What intermittent and spatially variable processes in the continental shelf and slope regions (e.g., eddies, cross-shelf exchange driven by buoyancy flux in the seasonal ice zone, winter shore leads and polynyas) are important to ventilation of the deep Arctic Ocean? How can regional Arctic and global models be formulated and used more effectively (for example, by improvements in ice rheology and ridging dynamics, ice-ocean friction parameterizations, better treatment of the basin boundaries, and proper validations of ocean stratification, circulation and seasonality). How might natural variability of climate parameters, such as the Arctic Oscillation index, impact the Arctic Ocean and ice cover on decadal and multi-decadal time scales, and can we identify the interplay between natural and anthropogenic forcing? How will Arctic ecosystems change with reduced ice cover and shorter winters, and to what extent will these changes be irreversible? Considered measurements of the Arctic system are needed to answer these questions. Study of the Arctic Ocean is restricted both by limited opportunities for access and by the lack of appropriate instrumentation. Standard observational practice to sample in August-September (when the sea-ice coverage is at its seasonal minimum and the Arctic is acces- sible by research icebreakers) and April-May (using aircraft when the sea ice is sufficiently strong and there is adequate daylight) does not capture seasonal and shorter time scale variability and provides only limited spa- tial coverage. In recent years, advances in our understanding of the Arctic have been made through the use of autonomous Ice-Based Observatories (IBOs). IBOs combine suites of different sensors mounted in the drifting permanent Arctic ice pack, providing (via satellite) year-round automated measurements of the ocean, ice and atmosphere. Advances in IBO instrument design and capability are required to both improve long-term functionality and to return additional oceano- graphic information. For example, in the coming years velocity sensors will be incorporated on ocean profiling IBOs to provide the capability of measuring deep ocean features such as internal waves and eddies, as well as smaller-scale flows, and thus heat transport, in the ocean beneath the ice (surface ocean velocity measurements are already being made by the Naval Postgraduate School’s [NPS] Autonomous Ocean Flux Buoys). Direct velocity measurements from an extensive array of IBOs will allow us to quantify ocean dynamics and upward heat fluxes over a substantial * It is speculated that heat and fresh water exchange between the Arctic Ocean and the North Atlantic depends upon both the process of Ekman pumping associated with the climatological atmospheric circulation over the Arctic Ocean, and on seasonal sea-ice trans- formations in the Arctic, leading to complex seasonal variability.

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10 OCEANOGRAPHY IN 2025 fraction of the ice-covered Arctic Ocean and over all seasons. A particular focus will also be placed on specializing and adding biogeochemical sen- sors to IBOs, and developing reliable processing techniques, to monitor properties such as dissolved oxygen levels, phytoplankton biomass, and dissolved organic matter concentrations. In addition to developing sensor technologies, modifications to IBOs will be required if existing measure- ment techniques are disrupted by the emerging changes in sea ice; these adaptations would include increased floatation, enhanced buoy design for survival over freeze-up and variable under-ice tether lengths. Additional shelf observatories based on autonomous vehicles and bottom-mounted instruments will be employed to investigate shallow shelf regions where sea ice is seasonal, and where winter ice cover destroys conventional instrumentation. Arctic researchers at Woods Hole Oceanographic Institution (WHOI), the Applied Physics Laboratory at University of Washington (UW-APL) and elsewhere are in the early stages of designing floats, gliders and autonomous vehicles for long-term use under ice to provide broad spatial coverage, and high-resolution measurements of, for example, Arctic Ocean circulation, under-ice rough- ness and seafloor topography along critical sections in both the seasonal ice zone and the central Arctic basin. The instruments will be integrated with basin-scale acoustic navigation and communications systems incor- porated in IBOs to provide navigation data to autonomous platforms by relaying their position via acoustic data link. IBOs will be made capable of acting as communication relays for data passed to them from passing vehicles or to relay commands and data from shore to visiting vehicles. Other advances in Arctic observing capability will include remote cali- bration technologies, greater resolution due to increased data storage density, advanced battery chemistries, and lower power consumption. Federal research funds are required not only for advances in Arctic instrumentation and new field techniques (for example, through collab- orative NSF Science and Technology Centers), but for associated process- oriented studies which emphasize collaboration between engineers, mod- elers, observationalists and theoreticians, as well as interdisciplinary connections between physical oceanographers, chemists and biologists. In the coming years, process and climate studies to interpret extensive new observations of the Arctic Ocean and answer the specific questions outlined above will include analyses of: fresh water and heat accumula- tion and release; dynamics and evolution of water mass fronts; mixing mechanisms; evolution of the surface layers and ice-ocean interactions; seasonal and higher-frequency biological processes; and property fluxes.