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Antarctic Sea Ice and Polynyas

Antarctic Sea Ice and Polynyas. EPS 131 Itay Halevy ihalevy@fas.harvard.edu. Overview. Antarctic Sea Ice: Definition Classification and formation Extent and variability Observation Climatic significance Polynyas: Definition Formation mechanisms Ecological and climatic significance.

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Antarctic Sea Ice and Polynyas

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  1. Antarctic Sea Ice and Polynyas EPS 131 Itay Halevy ihalevy@fas.harvard.edu

  2. Overview • Antarctic Sea Ice: • Definition • Classification and formation • Extent and variability • Observation • Climatic significance • Polynyas: • Definition • Formation mechanisms • Ecological and climatic significance

  3. Antarctic Sea Ice • Definition - any form of ice at sea that has originated from the freezing of sea water. • Forms a “girdle” around Antarctica every austral winter with immense maximal extent. • Impacts ocean-atmosphere interaction (heat, momentum, gas, salinity).

  4. Classification and Formation • Fast ice - attached to: • Land • Ice wall • Grounded icebergs • Pack ice - moves with winds and currents in groups of “floes”.

  5. Classification and Formation • Classification by age (thickness): • New ice (<10 cm thick): grease frazil pancake nilas

  6. Classification and Formation • Classification by age (thickness): • Young ice (10-30 cm thick): grey grey-white

  7. Classification and Formation • Classification by age (thickness): • First-year ice (>30 cm thick): - Old ice (> 1 yr).

  8. Rafting and Ridging rafting ridging

  9. Extent and Variability • Minimal extent: 4 million km2 (feb) • Maximal extent: 19 million km2 (sep)

  10. Dynamics • The “ice drift”: • Southerly winds drive the ice northwards: • water exposed to the atmosphere. • rapid formation of new ice. • Northerly winds cause convergence: • The newly-formed ice thickens. • The overall drift is divergent, tending to disperse ice to the north. • The “ice drift” is central to the characteristics of Antarctic ice. • Tracking of floes reveals coupling to wind speed and direction and less so to currents.

  11. Observation • Satellite images • Aerial photographs • Drifting buoys • Ship-based observations • Moored instruments • In situ measurements

  12. Climatic Significance • Changes the ocean-atmosphere interaction: • Atmospheric composition: • Physical barrier to gas exchange • Radiative balance: • Insulating layer - up to a factor of 2 • Highly reflective - increases albedo • Ocean circulation and water mass formation: • Decreases momentum transferred by wind • Changes the salinity of the surrounding water

  13. Polynyas • Areas of open, ice-free water surrounded by developed sea ice. • Sometimes partially covered in new ice. • Variable size (few km2 to 105 km2). • Duration can be from one season to several years. • Focal point of biological activity.

  14. Recurring Polynyas • Constant Location

  15. Formation Mechanisms Sensible Heat Polynyas • Warm (2°C) water upwells due to wind driven transport of water at the surface or due to bottom topography. • The ice thins and finally melts. • The polynya doesn’t refreeze due to constant supply of warm water. • Sensible heat polynyas are taken to be evidence for ocean circulation.

  16. Formation Mechanisms Latent Heat Polynyas • Katabatic winds drive newly formed ice away from land or from fast ice. • New ice forms in the exposed water and is also driven away by the winds. • The latent heat released by freezing is lost to the atmosphere. • Water is salinified by brine rejected from frozen sea water.

  17. Polynya Formation Mechanisms

  18. Ecology of Polynyas • Seasonal polynyas - few weeks to 6 months. • Sensible heat polynyas: • A fresh water lens forms at the surface. • The lens is quickly warmed by the sun. • Stratification keeps microflora in the photic zone. • Microalgal blooms produce ample organic matter which draws diverse fauna. • Latent heat polynyas: • Katabatic winds • Salinification • Mixing sends photosynthesizers into the dark until the summer sun causes temperature-driven stratification.

  19. Climatic Significance of Polynyas - Carbon Uptake • CO2 sink: • Summer photosynthesis. • Winter freezing.

  20. Climatic Significance of Polynyas - Circulation

  21. References • www.britanica.com • www2.fsg.ulaval.ca/giroq/now/what.htm • http://earthobservatory.nasa.gov/Study/Polynyas • http://nsidc.org/seaice/characteristics/polynyas.html • www.atmosp.physics.utoronto.ca/ANTARCTIC/ • www.antcrc.utas.edu.au/aspect/seaice.html • iup.physik.uni-bremen.de:8084/ amsr/amsre.html

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