2. The Ice/Ocean Interface During Summer: Implications for Ice-Albedo Feedback Miles McPhee McPhee Research Company SEARCH OSM 28 Oct 2003 Underice melt ponds and false bottoms Storage and sequestration of heat in the upper ocean

3. False bottoms/underice meltponds may significantly impact IAF by: 1)Shielding thin ice from oceanic heat flux and bottom melting, and 2)Decreasing the aggregate ocean-to-ice heat flux by acting as a source of heat at the ice/water interface rather than the usual latent heat sink. The first direct estimates of the interface heat and salt exchange coefficients (from WARPS 2003) indicate  T ~ 12.5 x 10 -3 with  T /  S ~ 50

4. Heat storage in the summer mixed layer has an appreciable direct impact on IAF by absorbing solar radiation (that would otherwise go to melting) during the time of maximum solar angle. A less direct effect on IAF is sequestering heat below the midsummer meltwater cap. Evidence suggests that in the Canada Basin, the ice/mixed layer system is a net source of heat for the upper pycnocline.

5. Underice melt ponds and false bottoms Citation: Notz, D., M. G. McPhee, M. G. Worster, G. A. Maykut, K. H. Schluenzen, and H. Eicken, Impact of underwater-ice evolution on Arctic summer sea ice, J. Geophys. Res., 108(C7), 3223, doi:10.1029/2001JC001173, 2003.

6. During the 1975 AIDJEX Project in the Beaufort Gyre, Arne Hanson maintained an array of depth gauges at the main station Big Bear. Here are examples showing a decrease in ice thickness for thick ice, but an increase at several gauges in initially thin ice.

7. Estimated friction velocity for different values of bottom surface roughness, z 0 = 0.6 and 6 cm respectively Changes in ice bottom elevation relative to a reference level on day 190, at the “false bottom” sites. Note that false bottoms appear to form at all sites during the relative calm starting about day 205, and start migrating upward on or near day 210, when the wind picks up

8. Thick ice (BB-4 – BB-6) ablated 30-40 cm by the end of melt season. “False bottom” gauges showed very little overall ablation during the summer. The box indicates a 10-day period beginning in late July, when false bottoms apparently formed at several sites.

9. Thick ice gauges False Bottom Gauges

11. Assuming a linear temperature gradient in the thin false bottom: If the upper layer is fresh, temperature 0 o C:

12. This modifies the heat equation slightly from one in which the conductive heat flux in the ice is specified directly, but nevertheless leads to a quadratic for S 0

19. false bottom “true” bottom upward heat flux down “water table”

21. Winter ARctic Polynya Study – Mar-Apr, 2003 Special thanks to Anders Sirevaag, Ilker Fer and Ursula Schauer

22. Measure these quantities with a turbulence cluster 1 m below the ice. Estimate from the temperature gradient in the ice

29. Summer Heat Storage in the Upper Ocean

35. 1976

40. A modest anticyclonic surface motion field can induce the downwelling velocity needed to drive the trapped summer heat downward. During AIDJEX this was about 20 MJ m -2 (equivalent to about 8 cm ice ablation).

41. Conclusions Heat storage in the summer mixed layer has an appreciable direct impact on IAF by absorbing solar radiation (that would otherwise go to melting) during the time of maximum solar angle. A less direct effect on IAF is sequestering heat below the midsummer meltwater cap. Evidence suggests that in the Canada Basin, the ice/mixed layer system is a net source of heat for the upper pycnocline

42. The first turbulence measurements providing direct estimates of the interface heat and salt exchange coefficients indicate  T ~ 12.5 x 10 -3 with  T /  S ~ 50 False bottoms/underice meltponds may significantly impact IAF by: 1)Shielding thin ice from oceanic heat flux and bottom melting, and 2)Decreasing the aggregate ocean-to-ice heat flux by acting as a source of heat at the ice/water interface rather than the usual latent heat sink.