Pressure ridge (ice)

Although ice pressure ridges vary greatly in shape (which also evolves in time), this diagram (not to scale) shows how a drifting ridge is often idealized.[7][4]

Field example of a pressure ridge. Only the sail is shown in this photograph. The keel is more difficult to document.

Pressure ridge at North Pole, expedition of University of Giessen, April 17, 1990

A pressure ridge in the Antarctic ice near Scott Base, with lenticular clouds in the sky.

The blocks making up pressure ridges are mostly from the thinner ice floe involved in the interaction, but it can also include pieces from the other floe if it is not too thick.[1] In the summer, the ridge can undergo a significant amount of weathering, which turns it into a smooth hill. During this process, the ice loses its salinity (as a result of brine drainage and meltwater flushing). This is known as an aged ridge.[8] A fully consolidated ridge is one whose base has undergone complete freezing.[8] The term consolidated layer is used to designate freezing up of the rubble just below the water line.[2] The existence of a consolidated layer depends on air temperature — in this layer, the water between individual blocks is frozen, with a resulting reduction in porosity and an increase in mechanical strength. A keel's depth of an ice ridge is much higher than its sail's height - typically about 3-5 times. The keel is also 2-3 times wider than the sail.[9] Ridges are usually melting faster than level ice, both at the surface[10] and at the bottom.[11] Sea-ice ridges also play an important role in confining meltwater within under-ice meltwater layers, which may lead to the formation of false bottoms.[12]

One of the largest pressure ridges on record had a sail extending 12 m above the water surface, and a keel depth of 45 m.[1] The total thickness for a multiyear ridge was reported to be 40 m.[13] On average, total thickness ranges between 5 m and 30 m,[4] with a mean sail height that remains below 2 m.[2] The average keel depth of Arctic ridges is 4.5 m. The sail height is usually proportional to the square root of the ridge block thickness. Ice ridges in Fram Strait usually have a trapezoidal shape with a bottom horizontal section covering around 17% of the total ridge width and with a mean draft of 7 m,[14] while ice ridges in the Chukchi and Beaufort Seas have a concave close to triangular shape.[15]

The average consolidated layer thickness of Arctic ridges is 1.6 m. Usually, ridges are consolidating faster than level ice because of their initial macroporosity. Ridge rubble porosity (or water-filled void fraction of ridge unconsolidated part) is in the wide range of 10-40%. During winter, ice ridges are consolidating up to two times faster than level ice, with the ratio of level ice and consolidated layer thickness proportional to the square root of ridge rubble porosity.[16] This results in 1.6-1.8 ratio of consolidated layer and level ice thickness by the end of winter season.[17] Meanwhile, snow is usually about three times thicker above ridges than above level ice.[18] Sometimes ridges can be found fully consolidated with the total thickness up to 8 m.[19] Ridges may also contain from 6% to 11% of snow mass fraction, which can be potentially linked to the mechanisms of ridge consolidation.[20] Fram Strait ridge observations suggest, that the largest part of ridge consolidation happens during the spring season when during warm air intrusions or dynamic events snow can enter ridge keels via open leads and increase the speed of ridge consolidation.[21] These observations are supported by high snow mass fraction in refrozen leads, observed during the spring season.[22] The ridge consolidation potentially reduces light levels and the habitable space available for organisms, which may have negative ecological impacts as ridges have been identified as ecological hotspots.

The physical characterization of pressure ridges can be done using the following methods:[2]

• Mechanical drilling of the ice with non-coring or coring augers (when the ice core is retrieved for analysis).[23]
• Surveying, whereby a level, theodolite or a differential GPS system is used to determine sail geometry.
• Thermal drilling – drilling involving melting of the ice.[24]
• Observation of the ice canopy by scuba divers.
• Upward looking sonars and multibeam sonars fixed on seabed or moounted on a remotely operated underwater vehicle.[11]
• A series of thermistors (ice mass balance buoy), to monitor temperature changes.[25]
• Electromagnetic induction, from the ice surface or from an aircraft.[26]

From an offshore engineering and naval perspective, there are three reasons why pressure ridges are a subject of investigation.[4] Firstly, because the highest loads applied on offshore structures operating in cold oceans by drift ice are associated with these features.[27] Secondly, when pressure ridges drift into shallower areas, their keel may come into contact with the seabed, thereby representing a risk for subsea pipelines (see Seabed gouging by ice) and other seabed installations. Thirdly, they have a significant impact on navigation. In the Arctic, ridged ice makes up about 40% of the overall mass of sea ice.[9][3] First-year ridges with large macroporosity are important for the ice-associated sympagic communities and identified as potential ecological hotspots and proposed to serve as refugia of ice-associated organisms.[28]

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