Antarctic Sea Ice and Polynyas

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Antarctic Sea Ice and Polynyas
EPS 131 Itay Halevy ihalevy_at_fas.harvard.edu
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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

• 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).

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Classification and Formation

• Fast ice - attached to
• Land
• Ice wall
• Grounded icebergs
• Pack ice - moves with winds and currents in
  groups of floes.

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Classification and Formation

• Classification by age (thickness)
• New ice (lt10 cm thick)

grease
frazil
pancake
nilas
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Classification and Formation

• Classification by age (thickness)
• Young ice (10-30 cm thick)

grey
grey-white
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Classification and Formation

• Classification by age (thickness)
• First-year ice (gt30 cm thick)

- Old ice (gt 1 yr).
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Rafting and Ridging
rafting
ridging
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Extent and Variability

• Minimal extent 4 million km2 (feb)
• Maximal extent 19 million km2 (sep)

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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.

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Observation

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

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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

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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.

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Recurring Polynyas

• Constant Location

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Sensible Heat Polynyas
Formation Mechanisms

• Warm (2C) 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 doesnt refreeze due to constant
  supply of warm water.
• Sensible heat polynyas are taken to be evidence
  for ocean circulation.

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Latent Heat Polynyas
Formation Mechanisms

• 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.

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Polynya Formation Mechanisms
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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.

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Climatic Significance of Polynyas - Carbon Uptake

• CO2 sink
• Summer photosynthesis.
• Winter freezing.

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Climatic Significance of Polynyas - Circulation
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References

• www.britanica.com
• www2.fsg.ulaval.ca/giroq/now/what.htm
• http//earthobservatory.nasa.gov/Study/Polynyas
• http//nsidc.org/seaice/characteristics/polynyas.h
  tml
• www.atmosp.physics.utoronto.ca/ANTARCTIC/
• www.antcrc.utas.edu.au/aspect/seaice.html
• iup.physik.uni-bremen.de8084/ amsr/amsre.html

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