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

Overview Antarctic Sea Ice: Polynyas: Definition Classification and formation Extent and variability Observation Climatic significance Polynyas: 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). Different from some forms of ice found at sea that originated from precipitation and compaction and then cleaved into the sea. The girdle, as we’ll see, has variable extent and interacts with the ocean currents and with the winds. The ice is a physical barrier to gas exchange and can trap CO2 produced by summer biological productivity. It also acts to insulate the warm (>-1.9 deg C) ocean from the very cold atmosphere, as much as halving the heat transferred. The high albedo of sea-ice causes less incident radiation to be absorbed by the ocean - positive cooling feedback. Transfer of momentum from the atmosphere to the ocean through winds is greatly reduced by the existence of sea ice. Brine rejection, caused by exclusion of salts as water freezes, salinifies underlying water. Melting in turn freshens the surrounding water. Notice the potential effect on density by the ice’s effect on temperature and salinity.

Classification and Formation Fast ice - attached to: Land Ice wall Grounded icebergs Pack ice - moves with winds and currents in groups of “floes”. There is an abundance of terms for the description of sea ice in its different stages of development. Ice forms by freezing of sea water. Thickness increases mainly through “rafting”, “ridging” when floes converge and collide but also by congelation (growth on the underside). Classification by age and the formation process: Small individual ice crystals (frazil) form where T<1.8 deg C. Gives the ocean an oily look. Further freezing causes the water to coagulate and become more viscous. This is termed grease ice. Frazil and grease ice freeze to form nilas under calm conditions. Under rougher conditions the nilas “rafts”, increasing the thickness of the ice. Sometimes pancakes form as a result of breaking of nilas. Raised rims result from collisions. Thicker young ice “rafts” and “ridges” due to pressure caused by the wind, the currents and the tides. Fast ice is ice that is attached to land, an ice wall, grounded icebergs etc. Pack ice is not attached to anything stationary and moves with the winds and currents in groups of “floes” - contiguous pieces of ice.

Classification and Formation Classification by age (thickness): New ice (<10 cm thick): grease frazil There is an abundance of terms for the description of sea ice in its different stages of development. Ice forms by freezing of sea water. Thickness increases mainly through “rafting”, “ridging” when floes converge and collide but also by congelation (growth on the underside). Classification by age and the formation process: Small individual ice crystals (frazil) form where T<1.8 deg C. Gives the ocean an oily look. Further freezing causes the water to coagulate and become more viscous. This is termed grease ice. Frazil and grease ice freeze to form nilas under calm conditions. Under rougher conditions the nilas “rafts”, increasing the thickness of the ice. Sometimes pancakes form as a result of breaking of nilas. Raised rims result from collisions. Thicker young ice “rafts” and “ridges” due to pressure caused by the wind, the currents and the tides. Fast ice is ice that is attached to land, an ice wall, grounded icebergs etc. Pack ice is not attached to anything stationary and moves with the winds and currents in groups of “floes” - contiguous pieces of ice. pancake nilas

Classification and Formation Classification by age (thickness): Young ice (10-30 cm thick): grey There is an abundance of terms for the description of sea ice in its different stages of development. Ice forms by freezing of sea water. Thickness increases mainly through “rafting”, “ridging” when floes converge and collide but also by congelation (growth on the underside). Classification by age and the formation process: Small individual ice crystals (frazil) form where T<1.8 deg C. Gives the ocean an oily look. Further freezing causes the water to coagulate and become more viscous. This is termed grease ice. Frazil and grease ice freeze to form nilas under calm conditions. Under rougher conditions the nilas “rafts”, increasing the thickness of the ice. Sometimes pancakes form as a result of breaking of nilas. Raised rims result from collisions. Thicker young ice “rafts” and “ridges” due to pressure caused by the wind, the currents and the tides. Fast ice is ice that is attached to land, an ice wall, grounded icebergs etc. Pack ice is not attached to anything stationary and moves with the winds and currents in groups of “floes” - contiguous pieces of ice. grey-white

Classification and Formation Classification by age (thickness): First-year ice (>30 cm thick): There is an abundance of terms for the description of sea ice in its different stages of development. Ice forms by freezing of sea water. Thickness increases mainly through “rafting”, “ridging” when floes converge and collide but also by congelation (growth on the underside). Classification by age and the formation process: Small individual ice crystals (frazil) form where T<1.8 deg C. Gives the ocean an oily look. Further freezing causes the water to coagulate and become more viscous. This is termed grease ice. Frazil and grease ice freeze to form nilas under calm conditions. Under rougher conditions the nilas “rafts”, increasing the thickness of the ice. Sometimes pancakes form as a result of breaking of nilas. Raised rims result from collisions. Thicker young ice “rafts” and “ridges” due to pressure caused by the wind, the currents and the tides. Fast ice is ice that is attached to land, an ice wall, grounded icebergs etc. Pack ice is not attached to anything stationary and moves with the winds and currents in groups of “floes” - contiguous pieces of ice. - Old ice (> 1 yr).

Rafting and Ridging rafting ridging

Extent and Variability Minimal extent: 4 million km2 (feb) Maximal extent: 19 million km2 (sep) Maximal extent: More than twice the size of the entire US. The “ice drift” causes antarctic sea ice to be formed of 47% frazil as opposed to about 5% in arctic ice.

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. Maximal extent: More than twice the size of the entire US. The “ice drift” causes antarctic sea ice to be formed of 47% frazil as opposed to about 5% in arctic ice.

Observation Satellite images Aerial photographs Drifting buoys Ship-based observations Moored instruments In situ measurements Satellite images, aerial photos and buoys are used to track the movement, concentration, extent and climatology of pack ice. In situ cores and measurements allow analysis of the composition and morphology of the ice.

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 The ice is a physical barrier to gas exchange and can trap CO2 produced by summer biological productivity. It also acts to insulate the warm (>-1.9 deg C) ocean from the very cold atmosphere, as much as halving the heat transferred. The high albedo of sea-ice causes less incident radiation to be absorbed by the ocean - positive cooling feedback. Transfer of momentum from the atmosphere to the ocean through winds is greatly reduced by the existence of sea ice. Brine rejection, caused by exclusion of salts as water freezes, salinifies underlying water. Melting in turn freshens the surrounding water. Notice the potential effect on density by the ice’s effect on temperature and salinity.

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. The Wendell Sea polynya was measured to be 350,000 sq km in area. Duration can be from one season to several years in otherwise perennially glaciated waters. A well lit environment enables photosynthesis which supplies the bottom of the food chain. Consequently many fish, birds and marine mammals are drawn to polynyas.

Recurring Polynyas Constant Location The Wendell Sea polynya was measured to be 350,000 sq km in area. Duration can be from one season to several years in otherwise perennially glaciated waters. A well lit environment enables photosynthesis which supplies the bottom of the food chain. Consequently many fish, birds and marine mammals are drawn to polynyas.

Sensible Heat Polynyas 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. Wind drives surface currents and water from the deep upwells to conserve mass. Topography diverts the flow of deep water and may also cause upwelling. After the ice melts the water is exposed to the very cold atmosphere. It doesn’t freeze because warm water keeps upwelling from below. As a result, a lot of heat is lost to the atmosphere. The evidence for ocean circulation is that in the height of winter, when there is little insolation, heat finds its way beneath the ice and melts it.

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. Latent heat is not really the reason for the existence of ice-free water in these polynyas - the winds are. Cold water is exposed to the even colder atmosphere resulting in fast and voluminous production of sea ice.

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. The result of the freshening and warming is a layer of water that stays in the well-lit, warm top of the water column. With the addition of the upwelled nutrients, this environment is ideal for microfloral blooms. Consequently, animals that feed on these photosynthesizers are drawn to polynyas. Whales also use polynyas to come up for air. In latent heat polynyas there is no upwelling of warm water or fresh water from melying. Rather the water remaining after ice formation is saltier and very cold and tends to sink. Also the winds that drive the ice away from shore induce mixing. Consequently organisms at the surface are quickly mixed into light-limited depths and productivity at LHPs is lower than at SHPs. Consequently, productivity is 6 months long in SHPs, 4 months long in LHPs and only 2 months long in the rest of the seasonally glaciated ocean.

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