Antarctic Sea Ice and Antarctic Sea Ice and PolynyasPolynyas

EPS 131EPS 131Itay HalevyItay Halevyihalevy@fas.harvard.eduihalevy@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

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).

Classification and Formation

• Fast ice - attached to:– Land– Ice wall– Grounded icebergs

• Pack ice - moves with winds and currents in groups of “floes”.

Classification and Formation• Classification by age (thickness):

– New ice (<10 cm thick):

frazil

grease

nilaspancake

Classification and Formation• Classification by age (thickness):

– Young ice (10-30 cm thick):

grey

grey-white

Classification and Formation• Classification by age (thickness):

– First-year ice (>30 cm thick):

- Old ice (> 1 yr).

Rafting and Ridging

rafting

ridging

• Minimal extent: 4 million km2 (feb)

• Maximal extent: 19 million km2 (sep)

Extent and Variability

• 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.

Dynamics

Observation

• Satellite images

• Aerial photographs

• Drifting buoys

• Ship-based observations

• Moored instruments

• In situ measurements

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

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.

Recurring Polynyas• Constant Location

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.

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.

Formation Mechanisms

Polynya Formation Mechanisms

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.

Climatic Significance of Polynyas - Carbon Uptake

• CO2 sink:

– Summer photosynthesis.– Winter freezing.

Climatic Significance of Polynyas - Circulation

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