Seaice the cryosphere SOEE 3410 Lecture 14 Seaice

Sea-ice & the cryosphere SOEE 3410: Lecture 14

Sea-ice & the cryosphere • Ice-production • Formation of “polynyas” • Heat exchanges • Feedback systems • Extent and seasonality of sea-ice • Climate implications SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

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. SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

Antarctic Bottom Water (AABW) - Weddell Sea (deep convection site) SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

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 SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

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: 1. Latent (or coastal) polynyas: 2. 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 SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

Weddell Sea Polynya (NASA) SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

Weddell Sea Polynya Low ice concentration close to coast – coastal Polynya SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

Two mechanisms for sustaining polynyas: Sensible Heat Latent Heat loss Qnet ~ -200 Wm-2 Qnet ~ -300 Wm-2 Brine Formation Offshore wind Water -1. 9 C Convection Cells Cold Saline Waters Cold Dense water 2000 - 4000 m AABW Antarctic Continental Shelf

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 SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

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 SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

Feedbacks: clouds (over Arctic Seas) Schweiger, A. J. (2004) Changes in seasonal cloud cover the Arctic seas from satellite and surface observations, Geophysical Research Letters, Vol. 31, L 2207, doi: 10. 1029/2004 GL 020067, 2004. SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

Feedbacks: increasing GHGs on temperature Winter mean temperature change for doubling CO 2 From IPCC Assessment, Houghton, et al. , 1990 SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

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 SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

Extent & seasonal variation of ice • North pole: - semi-enclosed basin => relatively little seasonal variation • South pole: - ice cover shifts ~20 o latitude - almost all sea ice melts in summer SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

Annual and seasonal sea-ice extent in N hemisphere: 1901 -1999 (Annual values from Vinnikov et al. , 1999 b; seasonal values updated from Chapman and Walsh, 1993). SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

Sea-ice thickness – from submarine measurements From Rothrock et al. , 1999 http: //nsidc. org SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

Arctic Oceans: freshwater input – air temperature (1936 -1999) Peterson et al. , Science Vol. 298, 2002 1 Sv = 106 m 3 s-1 Hydrologic sensitivity is the main control variable that determines the future response of the THC! SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

Labrador Sea: freshening at all depths Salinities through water column: 1950 -2001 • Rapid & long-term freshening SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

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. SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

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 SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics