sea-ice & the cryosphere
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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. - PowerPoint PPT PresentationTRANSCRIPT
SOEE3410: Lecture 14
Sea-ice & the cryosphere
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics
Sea-ice & the cryosphere
• Ice-production
• Formation of “polynyas”
• Heat exchanges
• Feedback systems
• Extent and seasonality of sea-ice
• Climate implications
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
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics
Antarctic Bottom Water (AABW) - Weddell Sea (deep convection site)
Antarctic Bottom Water
• 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
T: -0.25 to -1.4 C
S: 34.63 – 34.65 PSU.
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics
Polynyas
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics
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:
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
Weddell Sea Polynya
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics
(NASA)
Weddell Sea Polynya
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics
Low ice concentrationclose to coast – coastalPolynya
Sensible HeatQnet ~ -200 Wm-2
2000 - 4000m
Latent Heat lossQnet ~ -300 Wm-2
Brine Formation
Cold Saline Waters
Cold Dense water
Offshorewind
AABW
Water -1.9 C
Convection Cells
Antarctic Continental Shelf
Two mechanisms for sustaining polynyas:
Heat exchange: ocean - sea-ice - atmosphere
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics
•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
Feedbacks: ocean - sea-ice - atmosphere
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics
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
Feedbacks: clouds (over Arctic Seas)
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics
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.
Feedbacks: increasing GHGs on temperature
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics
Winter mean temperature change for doubling CO2
From IPCC Assessment, Houghton, et al., 1990
Polar surface temperature trends: 1981-2000
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics
• 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
Extent & seasonal variation of ice
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics
• North pole:
- semi-enclosed basin =>
relatively little seasonal
variation
• South pole:
- ice cover shifts ~20o
latitude
- almost all sea ice melts
in
summer
Annual and seasonal sea-ice extent in N hemisphere: 1901-1999
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics
(Annual values from Vinnikov et al., 1999b; seasonal values updated from Chapman and Walsh, 1993).
Sea-ice thickness – from submarine measurements
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics
From Rothrock et al., 1999 http://nsidc.org
Arctic Oceans: freshwater input – air temperature (1936-1999)
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics
Hydrologic sensitivity is the main control variable that determines the future response of the THC!
Peterson et al., Science Vol. 298, 2002
1 Sv = 106 m3 s-1
Labrador Sea: freshening at all depths
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics
Salinities through water column: 1950-2001• Rapid & long-term freshening
Climate implications: thermohaline circulation collapse
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics
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
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