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 PSU in the west
C and 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: to -1.4 C
S: – 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. SOEE3410 : 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 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,
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. SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics
Weddell Sea Polynya
(NASA)
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

8. SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics
Weddell Sea Polynya
Low ice concentration close 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
Brine Formation
Water -1.9 C
Convection Cells
Cold Saline Waters
Cold Dense water
AABW m
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: /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.,
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

19. 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

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

21. 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

22. SOEE3410 : 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
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics