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Sea-ice & the cryosphere

Sea-ice & the cryosphere. SOEE3410: Lecture 14. Sea-ice & the cryosphere. Ice-production Formation of “polynyas” Heat exchanges Feedback systems Extent and seasonality of sea-ice Climate implications. SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics.

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Sea-ice & the cryosphere

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  1. Sea-ice & the cryosphere SOEE3410: Lecture 14

  2. Sea-ice & the cryosphere • Ice-production • Formation of “polynyas” • Heat exchanges • Feedback systems • Extent and seasonality of sea-ice • Climate implications SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  3. N Atlantic: Brine expulsion & deep convection The temperature, salinity ranges of NADW: 0-2 C and 34.88 - 34.93 PSU in the west 1.8 - 3 C and 34.98 - 35.03 PSU in the east. SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  4. Antarctic Bottom Water (AABW) - Weddell Sea (deep convection site) SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  5. Antarctic Bottom Water T: -0.25 to -1.4 C S: 34.63 – 34.65 PSU. • Cold air near Antarctica cools the ocean to the point that sea ice begins to form • Ice forms, salt is released to the water beneath the new ice. • Both the low temperatures and higher salinity make the surface water sufficiently dense enough to sink 4 km to the bottom of the ocean SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  6. Polynyas • Extensive open-water (ice-free) areas bordered by winter ice cover, • generally in well-defined areas • Size: 100 m - 1000 km • Two mechanisms can contribute to keeping polynyas open: • Latent (or coastal) polynyas: Sea ice grows in open-water and is continually removed by winds and currents (often off shoreline) - latent heat released to the ocean during ice formation perpetuates the process – described as “sea-ice factories” – balances loss to atmosphere. • 2. Sensible heat (or open-ocean) polynyas: • Upwelling warm waters, vertical heat diffusion, or convection may provide enough oceanic heat flux to maintain ice-free region • Reading: “Polynyas and leads:…”, Smith et al., 1990, JGR, Vol. 95, 9461-9479 • Examples of major Polynya in the Antarctic: • Ronne Ice Shelf Polynya, Ross Sea Polynya,Terra Nova, Bay Polynya SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  7. Weddell Sea Polynya (NASA) SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  8. Weddell Sea Polynya Low ice concentrationclose to coast – coastal Polynya SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  9. Two mechanisms for sustaining polynyas: Sensible Heat Qnet ~ -200 Wm-2 Latent Heat loss Qnet ~ -300 Wm-2 Offshore wind Water -1.9 C Brine Formation Convection Cells Cold Saline Waters Cold Dense water AABW 2000 - 4000m Antarctic Continental Shelf

  10. Heat exchange: ocean - sea-ice - atmosphere • Ice acts as blanket over the ocean • i.e. sea ice prevents the ocean heating lower atmosphere • Ice and the sustained snow cover prevent the turbulent exchange of heat and momentum at the ocean/atmosphere interface • A cooler atmosphere is supported by high albedo • (reflectivity of sea ice) • Ice (high albedo) reduces absorption of short-wave radiation • Qlw is similar for both ice and water • Sea ice is highly dynamic • i.e. constantly susceptible to the effects of the wind, precipitation, ocean tides SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  11. Feedbacks: ocean - sea-ice - atmosphere • Albedo-Temperature • An atmospheric warming (cooling) implies: • A decrease (increase) in the sea-ice area • Smaller (larger) regional surface albedo • Sea ice modification of evaporation rates • A decrease in the sea-ice compactness through melting: • Higher water-vapour concentration in the lower atmosphere • Stimulates the absorption of long wave radiation – warming air • Further ice ablation • An increase in the sea-ice compactness through freezing: • Lower water-vapour concentration in the lower atmosphere • Inhibition of long wave radiation absorption • Supporting further ice accretion SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  12. Feedbacks: clouds (over Arctic Seas) Schweiger, A.J. (2004) Changes in seasonal cloud cover over the Arctic seas from satellite and surface observations, Geophysical Research Letters, Vol. 31, L2207, doi:10.1029/2004GL020067, 2004. SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  13. Feedbacks: increasing GHGs on temperature Winter mean temperature change for doubling CO2 From IPCC Assessment, Houghton, et al., 1990 SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  14. Polar surface temperature trends: 1981-2000 • North pole: • - +ve T trend on Canadian side • - -ve T rend on Russian side • South pole: • - +ve T trend over sea-ice cover • - -ve T trend over parts of continent From Sea Ice, Thomas and Dieckmann, 2003 SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  15. Extent & seasonal variation of ice • North pole: • - semi-enclosed basin => relatively little seasonal variation • South pole: • - ice cover shifts ~20o latitude • - almost all sea ice melts in summer SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  16. Annual and seasonal sea-ice extent in N hemisphere: 1901-1999 (Annual values from Vinnikov et al., 1999b; seasonal values updated from Chapman and Walsh, 1993). SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  17. Sea-ice thickness – from submarine measurements From Rothrock et al., 1999 http://nsidc.org SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  18. Arctic Oceans: freshwater input – air temperature (1936-1999) Peterson et al., Science Vol. 298, 2002 1 Sv = 106 m3 s-1 Hydrologic sensitivity is the main control variable that determines the future response of the THC! SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  19. Labrador Sea: freshening at all depths • Salinities through water column: 1950-2001 • Rapid & long-term freshening SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  20. Climate implications: thermohaline circulation collapse Model outputs: change in annual temp, 30 years after collapse of thermohaline circulation Figure courtesy of Michael Vellinga, Hadley Centre. SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  21. Summary: sea-ice • Formation of sea-ice at poles • Changes in heat exchanges due to sea-ice • Production of polynyas – importance in heat exchange • Feedback loops associated with sea-ice • Geographical / seasonal / climatological variations in sea-ice • Evidence of freshening of Nordic Oceans • Implications for thermohaline circulation SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

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