SOEE3410: Lecture 14 Sea-ice & the cryosphere. SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics Sea-ice & the cryosphere Ice-production Formation

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


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.


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:

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


(NASA)

Weddell Sea Polynya


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


•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


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)


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


Winter mean temperature change for doubling CO2

From IPCC Assessment, Houghton, et al., 1990

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

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

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).

Sea-ice thickness – from submarine measurements


From Rothrock et al., 1999 http://nsidc.org

Arctic Oceans: freshwater input – air temperature (1936-1999)


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


Salinities through water column: 1950-2001• Rapid & long-term freshening

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.


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

Documents

SOEE3410: Lecture 14 Sea-ice & the cryosphere. SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics Sea-ice & the cryosphere Ice-production Formation