Funds are provided in support of a study to understand exchanges across a major gateway linking the Arctic with the subpolar North Atlantic " Davis Strait (Canadian Arctic Archipelago). The proposed study purports to quantify, with robust error estimates, Davis Strait watermass variability, volume, liquid freshwater, heat and ice fluxes at weekly to inter-annual timescales. These measurements will be used to advance understanding of the impacts of these exchanges on large scale characteristics of the Atlantic and Arctic Oceans.
The proposed observation system, currently under development in Davis Strait, employs complementary techniques to resolve critical aspects of the exchange. The system includes: (1) A sparse array of subsurface moorings, each instrumented with an upward looking sonar, an Acoustic Doppler Current Profiler (ADCP), conductivity-temperature (CT) sensors, and (deep) conventional current meters, will provide time series of upper ocean currents, ice velocity, and ice thickness, (2) Shelf sites instrumented with ADCPs, CT sensors and an innovative, low-cost package for collecting CT time series in the ice-threatened near-surface region, and (3) Acoustically navigated Seagliders to provide year-round, repeated, high-resolution hydrographic sections across the straits. The Davis Strait network will be undertaken in collaboration with the Bedford Institution of Oceanography.
Freshwater and heat exchange between the Arctic and North Atlantic Oceans also provide critical mechanisms through which Arctic variability and global climate interact. Arctic freshwater discharges through Davis and Fram Straits into deep water formation regions west (Labrador Sea) and east (Greenland/Irminger Seas) of Greenland. Fresh inflow contributes a buoyant surface layer that acts as a barrier inhibiting convective overturning and deepwater formation. Global climate models predict the Atlantic Meridional Overturning Circulation (MOC) to be highly sensitive to variability in northern fresh- water flux, suggesting a delicate competition between freshwater supply and heat loss to the cold, high-latitude atmosphere. Changes in Arctic freshwater outflow can have profound impacts on fisheries and on carbon uptake and storage in this highly productive region. This International Polar Year (IPY) project extended the measurements of transport passing along the western side of Greenland, the primary pathway for liquid (as opposed to ice) freshwater exiting the Arctic into the subpolar seas. The Davis Strait observing system began operating in 2004, which also marked the first year that all of the critical Arctic Gateways had been monitored simultaneously. Long records are needed to make robust assessments of mean state and seasonal cycles, to distinguish between secular and cyclic change and to advance our understanding of the processes that govern observed variations in Arctic-Subarctic exchange. Simultaneous monitoring of all the Arctic gateways, implemented through international collaboration, facilitates investigation of Arctic mass, heat and freshwater budgets, which provide critical information for understanding Arctic change and its role in global climate. This program provided 3 additional years (2007 – 2010) of high-resolution measurements of ocean velocity, temperature, ice drift and thickness, and biogeochemical properties across Davis Strait (the critical choke point for Arctic waters flowing west of Greenland), southern Baffin Bay and the northern Labrador Sea. All data have been made freely available through the Cooperative Arctic Data and Information Service (CADIS), the official data facility for the US Arctic Observing Network. These data have been used in a wide range of published investigations (see project reference list) and are being employed in numerical studies for initialization and evaluation. Results include a robust quantification of volume, heat and freshwater fluxes that initially show little change from previous years (2004 – 2006), but suggest increased freshwater flux leaving the Arctic in 2009/2010. The measurements characterize the seasonal cycle and provide the first time series that include the critical region near the ice-ocean interface, allowing identification of where in the water column most of the freshwater transport occurs. Biogeochemical measurements suggest increasing acidification of waters exiting the Arctic. These data, along with the derived flux estimates and increase understanding of processes, have been employed in pan-Arctic syntheses and numerical efforts to advance understanding of the Arctic Ocean circulation and its role in climate, with the long-term goal of developing predictive capability. This project has also resulted in significant advances in ocean observing technology, as the desire for expanded measurements motivated considerable instrument development. Foremost is the development of long-endurance autonomous underwater gliders capable of working in ice-covered waters. These are small (1.5 meter length, 50 kg mass) buoyancy-driven submersibles that carry a variety of sensors and can be commanded via satellite telephone (Iridium) to navigate between waypoints as they profile from the sea surface to 1000 m depth. This project developed gliders that utilize long-range (100 km) acoustic beacons to navigate for extended periods (weeks to months) under the ice, while using advanced autonomy to make operational decisions that would normally require a human in the loop (e.g. Is there ice cover overheard? should I try to surface now?). In the ice-covered Arctic, our endurance record is currently ~6 months, with over 50 days spend operating under full ice-cover. The ICECAT mooring technology represents another end of our instrument development efforts. This small, inexpensive platform was designed to collect data near the ice-ocean interface. The instrument’s sensing elements sit in the highly important, but ice-threatened, boundary layer, attached via a weak link to a data logger safely situated deep, near the sea floor. Data is passed from sensor to logger via inductive modem. The near-surface sensing element was intentionally designed to be inexpensive, and thus, given that the data is safely archived in a safe (deep) location, potentially expendable. ICECATS have been highly successful and provided critical measurements near the ice-ocean interface. Both of these technologies have been freely shared with other research teams. The under-ice glider technologies have been used in Fram Strait, on the eastern side of Greenland and in the Ross Sea (Antarctica). The ICECATS have been used to collect measurements in the Bering Strait and Bering Sea, and are being proposed for an upcoming array that will measure the Atlantic Meridional Overturning Circulation.
