Sea ice: processes, observaons and models Cryosphere and Climate Change CIE 4602 2015 – 2016 February 10th OUTLINE 1) Sea ice lifecycle: formaon, growth and melt 2) Observaons: – Microwave and op/cal signatures – Arc/c sea ice – Antarc/c sea ice 3) Models: dynamics and thermodynamics 4) Sea ice model performance References: • IPCC FiQh Assessment report (AR5) • hSps://nsidc.org/cryosphere/seaice • hSp://earthobservatory.nasa.gov/ • Canadian Ice Service (www.ec.gc.ca/glaces-ice) • JPL Polar Oceanography Group • hSp://www.arc/c.noaa.gov • Sea Ice Physics and Remote Sensing (Shokr & Sinha, Wiley, 2015) Introduc/on to Sea Ice • A major component of the polar ecosystem: – Habitat for plants and animals at all trophic levels (plankton, algae, fish, birds, seals, penguins, bears and whales) • Also a component of the climate system: – changes the ocean surface albedo (incoming SW) – insulates the ocean from heat loss (outgoing LW) – barrier to gas (H2O, CO2) and momentum exchanges – alters ocean density à thermohaline circulaon àearly indicator of climate change and amplifier of climate perturbaons Ice-albedo feedback: mel/ng ice has a lower albedo (absorbs more sunlight, thus melts more) Excursus – Thermohaline circulaon • Also MOC for Meridional [Morrison, Frolicher & Sarmiento, Phys. Today, 2015] Overturning Circulaon (turns every ~1000 years) or Great Ocean Conveyor Belt. • Slow density-driven ocean circulaon (as opposed to the fast wind-driven circulaon that dominates the ocean upper few hundred meters). • Downwelling: salty cold water from new sea ice formaon (e.g. Weddell Sea, Barents Sea) gets this circulaon started. • Upwelling: thought to occur mostly along density surfaces in the Southern Ocean by à Important because of heat uptake (of excess energy in the climate system), carbon sink and nutrient supply (from wind-driven Ekman transport. deep water enriched by the biological pump). Figure I.12, [WMO,1970]). Since its formation from crystal suspensions as young ice, the process of growth into first-year and multi-year ice is one of steady desalination, thickness increase,1 - Sea ice lifecycle and surface erosion. The thickness of the overlying snow layer on top of the sea ice increases in general as the sea ice grows. In this section, we will review the • Sea ice may be roughly divided into five different age classes (new, young, development stages of sea ice and the basic terminology required for its classification. thin first year, first year and mul/year ice) used as a proxy for ice thickness. small pressure ridge lead large pressure ridge rafting Sea Snow level Salt rejec/on desalinaon New Young Thin First Year Ice Multiyear Ice Ice Ice First Year Thickness Since its formaon from crystal suspensions as new and young ice, the <10 cm <30 cm <70 cm <200 cm ~400 cm process of growth into first year and mul/year ice is one of steady Salinity 25 ‰ 15 ‰ 4-15 ‰ 4-5 ‰ 2 ‰ thickness increase (by thermodynamic growth and deformaon), surface erosion and desalinaon, with an increasing layer of snow depth. Snow ~10 cm ~10 cm ~30 cm – – Figure I.12 – Sea ice terminology ([WMO, 1970]) Formation Sea ice formation into new ice [see Figure I.13(a)] begins at the sea surface with a suspension of ice crystals known as frazil [Tucker et al., 1992]. When ice forms in calm seas, the frazil rises to form an unconsolidated layer of crystals known as grease ice, which stabilizes the sea surface and suppresses the formation of capillary waves in the presence of wind. Continued freezing results in a smooth, thin, elastic ice known as dark nilas. Consolidation progresses by water crystallization, with a resulting increase in salinity of the remaining liquid. Some of the saline brine is forced out of the ice mass to the sea beneath and to the surface. The remainder of brine is trapped within the ice in 21 Figure I.12, [WMO,1970]). Since its formation from crystal suspensions as young ice, the process of growth into first-year and multi-year ice is one of steady desalination, thickness increase,1 - Sea ice lifecycle and surface erosion. The thickness of the overlying snow layer on top of the sea ice increases in general as the sea ice grows. In this section, we will review the development stages of sea ice and the basic terminology required for its classification. small pressure ridge lead large pressure ridge rafting Sea Snow level New Young Thin First Year Ice Multiyear Ice Ice Ice First Year Thickness <10 cm <30 cm <70 cm <200 cm ~400 cm Salinity 25 ‰ 15 ‰ 4-15 ‰ 4-5 ‰ 2 ‰ Snow ~10 cm ~10 cm ~30 cm – – Figure I.12 – Sea ice terminology ([WMO, 1970]) Formation Sea ice formation into new ice [see Figure I.13(a)] begins at the sea surface with a suspension of ice crystals known as frazil [Tucker et al., 1992]. When ice forms in calm seas, the frazil rises to form an unconsolidated layer of crystals known as grease ice, which stabilizes the sea surface and suppresses the formation of capillary waves in the presence of wind. Continued freezing results in a smooth, thin, elastic ice known as dark nilas. Consolidation progresses by water crystallization, with a resulting increase in salinity of the remaining liquid. Some of the saline brine is forced out of the ice mass to the sea beneath and to the surface. The remainder of brine is trapped within the ice in 21 Sea ice formaon • Formaon of new ice begins at the sea surface with a random suspension of ice crystals known as frazil (aer salt rejec*on). • When ice forms in calm seas, the frazil con/nues to form an unconsolidated layer of crystals known as grease ice, which stabilizes the sea surface and suppresses the formaon of capillary waves in the presence of wind. Salt rejec/on eliminates about 80% of the ini/al seawater salt contents. Ice produc/on and salinity enhancements go hand-in-hand. Sea ice development • Con/nued freezing results in a smooth elas/c thin ice known as dark nilas, becoming brighter as it thickens. • Currents or winds oQen push the nilas around so that they slide over each other, a process known as raing. • Under calm condi/ons, sea ice growth progresses by steady crystallizaon and brine drainage (air filling), some of which will remain trapped in ver/cally elongated brine pockets. Con/nued growth takes place at the boSom of the slab (basal freezing as congelaon ice). Mixture of brine and air pockets give old ice its (electromagne/cally) bright appearance Sea ice development • Wave ac/on (par/cularly in the SH) causes the ice to lump and form small rounded floes called pancakes. Super- • On thin ice, snow deposi/on may weigh down the ice enough to cause flooding imposed ice mainly controlled by the snow cover. Bare sea disintegrates more quickly than snow- covered sea ice, and this pattern of differential melting will result in the appearance of melt ponds, hummocks, drainage channels and weathered ridges [see Figure I.13(e)] [Tucker et al., 1992]. Some first-year ice survives the summer melt, becoming thick multiyear ice, with a top layer transformed into a porous, low salinity cover, and a surface relief that becomes increasingly modulated by snow deposition and wind erosion [see Figure I.13(c)]. Sea ice development • Growth into first-year ice is a process of con/nued brine drainage (desalinaon with formaon of air pockets), with increases in ice thickness and snow load. Meanwhile, the ice surface will undergo con/nual deformaon under forcing from wind, waves and ocean currents, forming leads, pressure ridges and rubble fields. (a) New Ice Leads (b) First-year ice (c) Multi-year ice Rubble fields (d) Ridge and Lead e) Melt Divergence causes lower Convergence sea ice concentraons Figure I.13 – Development stages of sea ice produces thicker ice (Photos courtesyand enhanced sea ice of the Canadian Ice Service and the JPL Polar Oceanography Group) by deformaon. producon. à Dynamical effects on sea ice state Dielectric properties of sea ice The electrical properties of sea ice are relevant, along with structural features such as surface roughness and the distribution of inhomogeneities, in understanding how it interacts with the incident electromagnetic radiation [Onstott, 1979]. At frequencies higher than about 1 GHz, the propagation of electromagnetic energy into sea ice is best 23 mainly controlled by the snow cover. Bare sea disintegrates more quickly than snow- covered sea ice, and this pattern of differential melting will result in the appearance of melt ponds, hummocks, drainageSea ice development channels and weathered ridges [see Figure I.13(e)] [Tucker et • al.,As the water and air temperatures rise each summer, the sea ice starts to melt. 1992]. Some first-year ice survives the summer melt, becoming thick • Bare sea ice disintegrates faster than snow- multiyear ice, with a top layer transformed into a porous, low salinity cover, and a surface covered sea ice (by differen/al mel/ng) resul/ng in the appearance of melt ponds, drainage relief that becomeschannels, weathered ridges and increasingly modulatedhummock fields by snow deposition. and wind erosion [see … snow and sea ice have different albedos (solar input) and Figure I.13(c)].thermal conduc/vi/es (flux of sensible heat from the atmosphere) • Ice that survives the summer melt season is called (perennial) mul/-year ice: characterized by a porous, low salinity ice with a smooth relief modulated by melt and snow deposi/on. (a) New Ice (b) First-year ice (c) Multi-year ice (d) Ridge and Lead e) Melt Figure I.13 – Development stages of sea ice (Photos courtesy of the Canadian Ice Service and the JPL Polar Oceanography Group) Dielectric properties of sea ice The electrical properties of sea ice are relevant, along with structural features such as surface roughness and the distribution of inhomogeneities, in understanding how it interacts with the incident electromagnetic radiation [Onstott, 1979].