Core theme 3: Ocean
The Arctic Ocean is affected by large-scale climate variability and change. Oceanic heat imported from the North Atlantic has the potential to affect the ice cover in the Eurasian Arctic, while the Arctic Ocean itself plays a major role in collecting and delivering freshwater to the North Atlantic.
The 10 % of the global river runoff which it receives is exported into the North Atlantic where it may impact the global oceanic thermohaline circulation (THC). Our aim is to improve the understanding of the large-scale circulation in the Arctic Ocean, its interaction with changing sea ice conditions, its 2-way exchanges with the North Atlantic, and the balance of local and remote forcing which causes this system to change---- eg the recently discovered connections between the North Atlantic wind stress curl and volume transport in the Norwegian Atlantic Current, or the hypothesized link between successive warming events in the Norwegian Sea, Fram Strait and the Nansen Basin. Since these connections offer the possibility of prediction, they are important to the integration of observations and modelling in core themes 3 and 4.
In the Arctic Ocean, a relatively thin layer of fresh surface water and its associated cold halocline, whose origins lie partly on the shelf, exert a powerful effect in insulating the ice from the underlying warm and salty water mass of Atlantic origin, the only source of heat for the Arctic. During the 1990s, the distribution of this fresh insulating layer within the Arctic was observed to change, while at the same time, the temperature and extent of the warm Atlantic-derived layer at depth were observed to increase, probably due to a warming and strengthening of the Atlantic water inflow from the Nordic Seas. The mutual boundary between the Atlantic and Pacific water types was pushed east into the Canada Basin to an extent not previously observed. Both these distributional changes have potentially crucial consequences for the Arctic sea-ice cover.
The sources of the fresh water for the Arctic Ocean ----ie the discharge of the major circumarctic rivers and the relatively fresh Pacific surface water which enters through Bering Strait, -----are both subject to change as are their circulation and residence times within the Arctic Ocean. For example, the more anticyclonic Arctic Ocean circulation during the low-index phase of the North Atlantic Oscillation (NAO) is thought to have led to an accumulation of fresh water in the intensified Beaufort gyre before being released to the North Atlantic, with the opposite sense of change during the NAO-positive phases. In the cyclonic/NAO-positive phase of the late 1990s the upper ocean was both warmer and substantially less saline than in previous years.
High-density waters form on Arctic shelves through brine rejection during freezing and drain down-slope where they encounter the circum-Arctic boundary current. Together with the water that they entrain from this current, they contribute about 30% to the dense water which overflows from the Nordic seas to 'drive' the abyssal limb of the Atlantic Meridional Overturning Circulation (AMOC). The shelves also form the conduit for particulate matter, nutrients, dissolved inorganic and organic carbon, dissolved metals and a range of other contaminants. Large methane pools within the sediments of the circum-Arctic shelf are a potential major contributor to atmospheric greenhouse gases. Changing the ice cover, heat and fresh water storage and circulation on the shelves will alter the amount, density and contaminant loading of the sinking shelf water in an unpredictable way.
The influence of Arctic change is also known or suspected to be carried south in the ocean circulation to exert significant and rapid changes on climate, particularly through the impact of the Arctic freshwater outflux on the AMOC, and through changes in the transport and character of the dense water overflows that ‘drive’ its abyssal limb.
For these reasons, the DAMOCLES ocean programme is variously concerned with documenting and understanding changes in the factors controlling oceanic exchanges with the North Atlantic through the major gateways, with the modification of inflowing watermasses on the shelves and their communication with central Arctic via the continental slopes, and with the redistribution of oceanic heat as well as fresh water within the Arctic Ocean proper.
Objectives
- To document and understand
- The variation of the exchanges with the North Atlantic by observing fluxes through the major gateways connecting the Arctic and Atlantic Oceans,
- the redistribution of oceanic heat as well as fresh water within the Arctic Ocean proper,
- the modification of shelf water, its conversion to dense water, subsequent convection and the communication with central Arctic via the continental slopes.
- To develop and implement the improved observational system with capabilities to monitor the input of mass and heat to the Arctic Ocean.
- To identify the mix of local and remote forcing that controls the input of heat to the Arctic Ocean.
- To evaluate the capabilities to predict variability of these fluxes.
- To develop and implement new cost-effective observing techniques in the Fram Strait which will complement and enhance the capabilities of the existing conventional array of moorings.
- To observe and understand processes important for the interaction between the shelves and the deep basins, including the transformation of water masses and the variability in the strength and properties of the boundary current.
- Detect and estimate volume and physical properties of brine enriched bottom water flowing out of the Storfjorden.
- Determine the time and spatial variability of the velocity, temperature and salinity fields in the boundary current.
- Estimate the cooling of Atlantic layer by cold shelf plumes.
- Assess the regional loss of mass, heat and salinity from the boundary current to the Arctic interior. Determine the relative roles of eddies and intrusions.
- Determine the spreading pattern of river water plumes.
- To assess the heat flux from the Atlantic layer in the central Arctic to the ice and atmosphere.
- To observe and understand the variability of the circulation and properties of fresh surface mixed layer in relation to large scale atmospheric patterns.
- To observe the propagation of the Atlantic-derived sublayer in the central Arctic.
- To identify the "switchgear" that determines whether Arctic freshwater will pass directly to the Atlantic "conveyor" or recirculate in the Nordic Seas.
- To assess the scale and extent of changes associated with ocean freshwater fluxes, and their impact on the AMOC.
Description of work
Historically, poor access has meant that the Arctic Ocean is poorly documented, but the time is now ripe for concerted multi-platform, multi-sensor studies that rely on the smart, automated and autonomous systems that are now available. We will make use of ice-tethered buoys with sensors for atmospheric, ice and ocean data, floats, gliders, tomography, and long-term moorings with ice-strengthened components. Other innovative techniques currently under development will address underwater communication between systems, while integrated information on pathways will come from the use of natural and anthropogenic tracers. Regional models will be used to link and interpret the observations.
