White Paper: Upper ocean layer impacts Arctic sea ice Polyakov et al. April 2015
Page 1 of 2
Upper ocean as a regulator of atmospheric and oceanic heat transports to the sea ice in the Eurasian Basin of the Arctic Ocean
Executive Summary
Summary Processes that redistribute heat in the Arctic Ocean are expected to play an increasing role in changes in sea ice cover as summer ice extent and overall ice volume decrease. Our existing conceptual understanding of ocean heat flux processes and ocean-‐ice coupling is inadequate for quantifying the pathways for heat within the modern Arctic system. We outline a research strategy to reduce uncertainty in fluxes, including atmosphere-‐ice-‐ocean feedbacks, to the level required to predict the transition of sea ice state through projected changes in global climate. Our focus is on the Eurasian Basin (EB), eastern Arctic, which experiences multiple energetic heat flux processes that are distinct from the better-‐sampled western Arctic.
Eurasian Basin regional focus
The principal sources of heat content change in the upper Arctic Ocean are atmospheric surface fluxes, Atlantic Water (AW) and Pacific Water (PW) inflows, and warm freshwater from river inputs in summer. The relative contributions of each differ between the western and eastern Arctic. Many factors including distinct regional stratification and circulation lead to large differences in likely dominant mechanisms for delivering ocean heat to the ice base. The balance of mechanisms in each region will change as global climate affects each of these ocean heat sources.
Measured spatial variability of ocean heat content identifies the EB as the region of largest heat fluxes from the ocean interior. AW heat content declines rapidly as the AW flows as a boundary current along the continental slope around the EB. The large seasonal pulse of warm freshwater input from the large Russian Arctic rivers impacts stability over the broad eastern Arctic shelf seas. Sea ice is predominantly first-‐year, with a long ice-‐free season over much of the region. However, relative to the more intensively sampled western Arctic, there have been few direct measurements of ocean processes in the EB that would explain the inferred ocean heat fluxes. Our research strategy addresses this data sparseness by focusing on measurements within the EB.
Dominant sources of uncertainty in ocean heat flux in the EB
The following processes and unique EB features are primary ocean sources of uncertainty in understanding and modeling the current and future Arctic Ocean and sea-‐ice state.
• Mechanisms for subsurface storage of summer insolation and sensible heat through open water and thin ice to delay subsequent freeze-‐up in fall and winter.
• Size distributions of ice floes and leads as controls on partitioning of heat, freshwater and momentum exchanges at the ocean surface through development of secondary circulations.
• The EB Cold Halocline Layer (CHL) should be an effective barrier to vertical heat flux into the surface mixed layer (SML) from below but is observed to be porous.
• Upward fluxes in the pycnocline above the AW layer in the EB due to double diffusion is predicted to be roughly an order of magnitude larger than in the western Arctic; however, flux estimates rely on lab-‐based parameterizations that do not consider interactions of DD with external sources of shear.
White Paper: Upper ocean layer impacts Arctic sea ice Polyakov et al. April 2015
Page 2 of 2
• Barotropic and baroclinic tides along the path of the AW boundary current around the EB cause energetic mixing at the seabed, sea-‐ice base, and in the pycnocline; however, distribution of mixing is poorly mapped and is sensitive to poorly known benthic and surface boundary layer structure.
• The energy of wind-‐forced inertial waves is sensitive to SML depth and density contrast relative to the deep ocean, and to sea-‐ice state.
• Many of the above processes are sensitive to SML properties and ice dynamics, leading to complex atmosphere-‐ice-‐ocean feedbacks as each medium affects fluxes of heat, freshwater and momentum within the coupled Arctic system.
Recommended research actions
• Detailed process studies using drifting ice camps in the EB for different seasons: Late-‐summer and late-‐winter campaigns targeting the unique mechanisms of oceanic heat exchange through the CHL and surface mixed layer in the eastern EB (Fig. 1).
• Improve ocean monitoring: required at critical locations within the EB where major water mass transports and transformations take place. These sites include the outflow from the St. Anna Trough (where the Barents Sea branch of AW meets the Fram Strait branch), and sites in the central deep basins to monitor changes in processes influencing upward heat fluxes from the spreading AW layer.
• Develop new technologies and interdisciplinary programs: Synergistic combinations of different types of observations and technologies (e.g., microstructure vertical profiles coordinated with spatial surveys using autonomous underwater vehicles, multidisciplinary buoys, and high-‐resolution aircraft and satellite observations) is essential to avoid potential ambiguity in data analyses.
• Coordinate US activities with existing Arctic Observational Network (AON) and international programs: Link the EB observations with another elements of the international AON, thus providing the large-‐scale spatial and long-‐term temporal coverage required for optimized interpretation of new data sets.
• Integrate observations with models: Use existing high-‐resolution pan-‐Arctic models to optimize fieldwork sampling strategies, and prioritize fieldwork sampling towards reducing uncertainty from largest modeled sources of error in explicit and parameterized heat fluxes. Develop new process models aimed at better understanding and more accurate parameterization of sub-‐grid-‐scale heat fluxes in pan-‐Arctic and larger-‐domain ocean and coupled climate models.
Figure 1. A suite of coordinated late summer – early fall (September-‐October) and winter (March-‐April) field campaigns that would contribute to developing a comprehensive, quantitative understanding of heat transports within the upper Eurasian Basin. Circulation of the surface water and intermediate Atlantic Water of the Arctic Ocean is shown by blue and red arrows, respectively.