On (in)correctness of estimating heat flux across a single section Wieslaw Maslowski Naval Postgraduate School AOMIP Workshop, Woods Hole, MA, October, 2009 AOMIP Workshop, Woods Hole, MA, October, 2009

Observed Arctic sea ice extent (a,b) and modeled sea ice thickness (c,d) during September 1979 (a,c) and 2002 (b,c) SSM/I – 2D MODEL - 3D 09/7909/7909/0209/02 Significant decrease in observed sea ice extent (17-20%; top) and in modeled ice thickness (up to m or ~35%; bottom) in the 2000s. Note that largest changes are downstream of Pacific / Atlantic water inflow into the Arctic Ocean. (Maslowski et al., 2007)

Observed MY Ice Fraction

Forcing of Arctic sea ice melt “Atmospheric circulation trends are weak over the record as a whole, suggesting that the long-term retreat of Arctic sea ice since 1979 in all seasons is due to factors other than wind-driven atmospheric thermal advection.” - Deser and Teng, J. Clim Oceanic Forcing can locally play critical role in melting sea ice via: –horizontal advection of warm Pacific / Atlantic water into/under the sea ice cover –Locally induced (upwelling, topographically controlled flow, eddies) upward heat flux into the mixed layer

1/12 o Model domain and bathymetry Gateways/Margins of Pacific Water and Atlantic Water Inflow into the Arctic Ocean Main uncertainties of importance to global climate 1. Northward heat transport from the N. Atlantic/Pacific to Arctic Ocean * 2. Arctic sea ice thickness and volume * 3. Freshwater export from the Arctic to North Atlantic

Ocean Sci. Discuss., 6, 1007–1029, Problems with estimating oceanic heat transport – conceptual remarks for the case of Fram Strait in the Arctic Ocean, U. Schauer and A. Beszczynska-Moller Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany Published: 18 May 2009 Correspondence to: U. Schauer While the concept of oceanic heat transport – or rather heat transport divergence – is known since long, it is sometimes applied inaccurately. Often temperature transports are computed across sections with unbalanced volume flow which then depend entirely on the choice of the temperature scale. The consequences of such arbitrariness are demonstrated with a simple calculation exercise for the passages to the Arctic Ocean. To circumvent the arising difficulties for the Fram Strait as an example we propose a stream tube concept to define a net zero volume flow section which can, with coarse assumptions, be used to determine oceanic heat transport by the Atlantic water flow. Weaknesses of this approach and consequences for observational strategies are discussed. Is it (not) OK to calculate a heat flux across a single section?

Wikipiedia: heat flux definition Heat flux or thermal flux, sometimes also referred to as heat flux density or heat flow rate intensity is a flow of energy per unit of area per unit of time. In SI units, it is measured in [W·m -2 ]. It has both a direction and a magnitude so it is a vectorial quantity. To define the heat flux at a certain point in space, one takes the limiting case where the size of the surface becomes infinitesimally small.energySIWmvectorialflux

‘However, computation of heat transport by evaluating observations or even model results are sometimes far from straightforward. This is partly due to the complexity of ocean currents and the resulting necessity to determine both velocity and temperature along the boundary at a sufficiently high spatial resolution.’ (Schauer and Beszczynska-Moller, 2009) In any model velocity and temperature is determined with 100% detail of spatial resolution, relative to that model grid.

Mean Oceanic Heat Convergence: m; T ref = T freezing Modeling Challenges: Inflow of Pacific / Atlantic Water into the Arctic Ocean Pacific Water entering via Pacific Water entering via narrow (~60mi) Bering Strait narrow (~60mi) Bering Strait outflow through Fram outflow through Fram Strait vs. Atlantic Water Strait vs. Atlantic Water inflow (FSBW) inflow (FSBW) Atlantic (BSBW) and Pacific Atlantic (BSBW) and Pacific Water each losses majority Water each losses majority of heat to the atmosphere of heat to the atmosphere before entering Arctic Basin before entering Arctic Basin Arctic ocean-ice-atm feedbacks – not represented realistically in climate models Modeling Challenges: Inflow of Pacific / Atlantic Water into the Arctic Ocean Pacific Water entering via Pacific Water entering via narrow (~60mi) Bering Strait narrow (~60mi) Bering Strait outflow through Fram outflow through Fram Strait vs. Atlantic Water Strait vs. Atlantic Water inflow (FSBW) inflow (FSBW) Atlantic (BSBW) and Pacific Atlantic (BSBW) and Pacific Water each losses majority Water each losses majority of heat to the atmosphere of heat to the atmosphere before entering Arctic Basin before entering Arctic Basin Arctic ocean-ice-atm feedbacks – not represented realistically in climate models Heat Loss High resolution is one of the top requirements for advanced modeling of High resolution is one of the top requirements for advanced modeling of Arctic climate FSBW BSBW Heat Loss (Maslowski and Clement Kinney, 2009; Maslowski et al., 2008, Clement et al., 2005)

Modeled Upper Ocean Heat Content and Ice Thickness Anomalies ( Mean Annual Cycle Removed) Heat content accumulated in the sub-surface ocean since mid-1990s explains at least 60% of total sea ice thickness change

EW Modeled Oceanic heat flux exiting the Chukchi Shelf Heat Flux via Alaska Coastal Current accounts for ~67% of the Total Heat Flux across Chukchi Shelf Line Sept Sept. 2002

Emergence of open-ocean Polynya in the Arctic Ocean Vertical section of temperature along 150W (Yellow line in the sea ice concentration map (08/27), CCGS Louis S. St-Laurent JWACS2006) – courtesy of K. Shimada, JAMSTEC/TUMST Corr. coef. (MJJAS) R = or ~65% of variance Increased northward heat flux off the Chukchi Shelf coincides with the sea ice retreat in the late 1990s and 2000s SNACS 08/2005 off Barrow Courtesy S. Okkonen UAF

1/48 o (2.36-km) pan-Arctic model ‘produces’ many more mesoscale eddies in the western Arctic SSH and 0-25m velocity Eddies with diameter of ~30 km resolved but …. 1/48 o is ~50x more expensive than 1/12 o model configuration! Eddy-driven oceanic heat advection 1/12 o (9-km) pan-Arctic configuration above freezing temperature and m velocity in the western Arctic

Heat Flux/Content and Sea Ice thickness – Box 3 Box 3

Heat Flux/Content and Sea Ice thickness – Box 4 Box 4

boxMo. meanMo. Mean anom Correlations between lateral heat flux and ice thickness Correlations between heat content and ice thickness boxMo. meanMo. Mean anom

Are the following conclusions justified? 1.Oceanic heat advection / storage can contribute significant forcing (>60%) to local sea ice melt during the last decade 2.The rate of melt of sea ice volume possibly much greater than that of sea ice extent 3.Oceanic heat accumulating in the western Arctic is potentially a critical initial factor in reducing ice concentration / thickness before & during the melt season 4.Ice-edge & shelf/slope upwelling, eddies and other mesoscale circulation features in the Canada Basin provide a mechanism for horizontal heat distribution throughout the basin and up into the mixed layer Conclusions