False bottom is a form of sea ice that forms at the interface between meltwater and seawater via the process of double-diffusive convection of heat and salt. [2]
False bottom is a form of sea ice that forms at the interface between meltwater and seawater via the process of double-diffusive convection of heat and salt. [2]
False bottoms have been observed under drifting Arctic sea ice, under land-fast ice in Greenland, and at Ward Hunt Ice Shelf. Being located under ice, false bottoms are not easy to investigate, and the current observations are quite variable. For example, the areal coverage of false bottoms was 50% at the drifting station Charlie in 1959, [3] 15% during SHEBA expedition in 1998 [4] and 20% during MOSAiC expedition in 2020. [1] Both physical modelling [5] [6] and in situ observations [7] suggest that false bottoms may decrease sea ice melt up to 8%. The salinity and temperature of under-ice meltwater and false bottoms are controlled by both ice melt and desalination. The salinity of false bottoms was 1.0 during the ARCTIC 91 expedition, [8] 0.4 during SHEBA and 2.3 during MOSAiC. The average thickness of false bottoms was 20 cm during the ARCTIC 91 expedition, 15 cm during SHEBA, and 8 cm during MOSAiC. The presence of false bottoms can increase the rates of sea ice desalination. [8]
During Arctic summer, snow and ice melt results in the accumulation of low-salinity meltwater. Most of this meltwater is transferred to the ocean, while some of it migrates to the surface melt ponds, the sea ice matrix, and under-ice meltwater layers. False bottoms form due to a substantial difference in freezing temperatures of water with different salinities. Their formation in summer was first documented by Fridtjof Nansen in 1897. [9] During MOSAiC expedition, false bottoms occurred in areas of thin and ponded sea ice encircled by thicker sea ice ridges and were formed at the same time when surface melt ponds drained. [10] False bottoms are formed at the upper part of the interface of meltwater and seawater. The ice crystals initially grow downwards towards seawater, and further grow horizontally until a formation of a horizontal ice layer. After the formation of this horizontal layer, false bottoms constantly migrate upwards due to conductive heat flux, supported by the temperature difference between meltwater and seawater, and the rate of such migration is mostly defined by its thickness. [11] The growth and melt of false bottoms are controlled by the physical parameters of the ocean. [12] False bottoms are often observed in areas of thin ice covered by surface melt ponds and encircled by thicker pressure ridges, with ridge draft limiting the depth of under-ice meltwater layers. [7]
The false bottom formation is directly linked to the appearance of under-ice meltwater layers. The appearance of such meltwater layers often happens after surface melt pond drainage during the melt season. The depth of under-ice meltwater layers is usually limited by the draft of thicker and usually deformed ice, surrounding thinner ice with under-ice meltwater. The salinity of under-ice meltwater depends on the sources of meltwater including snow and ice, on the desalination of the ice above under-ice meltwater layers, and on the presence of false bottoms. During the MOSAiC expedition in Fram Strait, the average thickness of meltwater layers was 0.46 m under first-year ice and 0.26 m under second-year ice. The thickness of meltwater layers under multiyear ice during the SHEBA expedition in the Beaufort Sea was 0.35–0.47 m. Observations for fast multiyear ice in the Wandel Sea in North Greenland showed under-ice meltwater layers with 1.1–1.2 m thickness, later transformed into thick platelet ice layer with 0.01 m thick false bottoms under it. [13]
False bottoms may create errors in estimates of sea ice thickness from its draft measurements. They can be investigated manually using ice coring and drilling, [1] or remotely using underwater sonars. [7] Ground-based upward-looking sonar cannot distinguish "normal" or parental sea ice from false bottoms. Similarly, drifting buoys measuring sea-ice temperature (ice mass balance buoys) cannot accurately detect false bottoms but are able to identify thicker under-ice meltwater layers.
Sastrugi, or zastrugi, are features formed by erosion of snow by wind. They are found in polar regions, and in snowy, wind-swept areas of temperate regions, such as frozen lakes or mountain ridges. Sastrugi are distinguished by upwind-facing points, resembling anvils, which move downwind as the surface erodes. These points usually lie along ridges parallel to the prevailing wind; they are steep on the windward side and sloping to the leeward side. Smaller irregularities of this type are known as ripples or wind ridges.
RV Polarstern is a German research icebreaker of the Alfred Wegener Institute for Polar and Marine Research (AWI) in Bremerhaven, Germany. Polarstern was built by Howaldtswerke-Deutsche Werft in Kiel and Nobiskrug in Rendsburg, was commissioned in 1982, and is mainly used for research in the Arctic and Antarctica. The ship has a length of 118 metres and is a double-hulled icebreaker. She is operational at temperatures as low as −50 °C (−58 °F). Polarstern can break through ice 1.5 m thick at a speed of 5 knots. Thicker ice of up to 3 m (9.8 ft) can be broken by ramming.
Sea ice arises as seawater freezes. Because ice is less dense than water, it floats on the ocean's surface. Sea ice covers about 7% of the Earth's surface and about 12% of the world's oceans. Much of the world's sea ice is enclosed within the polar ice packs in the Earth's polar regions: the Arctic ice pack of the Arctic Ocean and the Antarctic ice pack of the Southern Ocean. Polar packs undergo a significant yearly cycling in surface extent, a natural process upon which depends the Arctic ecology, including the ocean's ecosystems. Due to the action of winds, currents and temperature fluctuations, sea ice is very dynamic, leading to a wide variety of ice types and features. Sea ice may be contrasted with icebergs, which are chunks of ice shelves or glaciers that calve into the ocean. Depending on location, sea ice expanses may also incorporate icebergs.
An ice shelf is a large floating platform of ice that forms where a glacier or ice sheet flows down to a coastline and onto the ocean surface. Ice shelves are only found in Antarctica, Greenland, Northern Canada, and the Russian Arctic. The boundary between the floating ice shelf and the anchor ice that feeds it is the grounding line. The thickness of ice shelves can range from about 100 m (330 ft) to 1,000 m (3,300 ft).
The East Greenland Current (EGC) is a cold, low-salinity current that extends from Fram Strait (~80N) to Cape Farewell (~60N). The current is located off the eastern coast of Greenland along the Greenland continental margin. The current cuts through the Nordic Seas and through the Denmark Strait. The current is of major importance because it directly connects the Arctic to the Northern Atlantic, it is a major contributor to sea ice export out of the Arctic, and it is a major freshwater sink for the Arctic.
Ice algae are any of the various types of algal communities found in annual and multi-year sea or terrestrial ice. On sea ice in the polar oceans, ice algae communities play an important role in primary production. The timing of blooms of the algae is especially important for supporting higher trophic levels at times of the year when light is low and ice cover still exists. Sea ice algal communities are mostly concentrated in the bottom layer of the ice, but can also occur in brine channels within the ice, in melt ponds, and on the surface.
Melt ponds are pools of open water that form on sea ice in the warmer months of spring and summer. The ponds are also found on glacial ice and ice shelves. Ponds of melted water can also develop under the ice, which may lead to the formation of thin underwater ice layers called false bottoms.
Anchor ice is defined by the World Meteorological Organization as "submerged ice attached or anchored to the bottom, irrespective of the nature of its formation". It may also be called bottom-fast ice. Anchor ice is most commonly observed in fast-flowing rivers during periods of extreme cold, at the mouths of rivers flowing into very cold seawater, in the shallow sub or intertidal during or after storms when the air temperature is below the freezing point of the water, and the subtidal in the Antarctic along ice shelves or near floating glacier tongues, and in shallow lakes.
A false bottom is an internal partition in a container, typically at the bottom of a drawer, suitcase or similar item, enabling the concealment of objects from a cursory examination of the item's contents. A false bottom may be present if the internal dimensions of a container do not match its external dimensions after taking into account the thicknesses of constituent materials and structures used to build the object.
Meltwater is water released by the melting of snow or ice, including glacial ice, tabular icebergs and ice shelves over oceans. Meltwater is often found during early spring when snow packs and frozen rivers melt with rising temperatures, and in the ablation zone of glaciers where the rate of snow cover is reducing. Meltwater can be produced during volcanic eruptions, in a similar way in which the more dangerous lahars form.
A pressure ridge, when consisting of ice, is a linear pile-up of sea ice fragments formed in pack ice by accumulation in the convergence between floes.
Rotten ice is a loose term for ice that is melting or structurally disintegrating due to being honeycombed by liquid water, air, or contaminants trapped between the initial growth of ice crystals. It may appear transparent or splotchy grey, and it is generally found after spring or summer thaws, presenting a danger to those traveling or spending time in outdoor recreation. The increase of rotten ice vs. solid ice in the Arctic affects ocean-atmosphere heat transfer and year-to-year ice formation, as well as the lives of the Inuit, sea mammals such as walrus and polar bear, and the microorganisms that live inside the ice.
Measurement of sea ice is important for safety of navigation and for monitoring the environment, particularly the climate. Sea ice extent interacts with large climate patterns such as the North Atlantic oscillation and Atlantic Multidecadal Oscillation, to name just two, and influences climate in the rest of the globe.
Sea ice is a complex composite composed primarily of pure ice in various states of crystallization, but including air bubbles and pockets of brine. Understanding its growth processes is important for climate modellers and remote sensing specialists, since the composition and microstructural properties of the ice affect how it reflects or absorbs sunlight.
The Surface Heat Budget of the Arctic Ocean (SHEBA) study was a National Science Foundation-funded research project designed to quantify the heat transfer processes that occur between the ocean and the atmosphere over the course of a year in the Arctic Ocean, where the sun is above the horizon from spring through summer and below the horizon the rest of the time. The study was designed to provide data for use in global climate models, which scientists use to study global climate change.
Brine rejection is a process that occurs when salty water freezes. The salts do not fit in the crystal structure of water ice, so the salt is expelled.
Antarctic sea ice is the sea ice of the Southern Ocean. It extends from the far north in the winter and retreats to almost the coastline every summer. Sea ice is frozen seawater that is usually less than a few meters thick. This is the opposite of ice shelves, which are formed by glaciers, they float in the sea, and are up to a kilometre thick. There are two subdivisions of sea ice: fast ice, which are attached to land; and ice floes, which are not.
Sea Ice Microbial Communities (SIMCO) refer to groups of microorganisms living within and at the interfaces of sea ice at the poles. The ice matrix they inhabit has strong vertical gradients of salinity, light, temperature and nutrients. Sea ice chemistry is most influenced by the salinity of the brine which affects the pH and the concentration of dissolved nutrients and gases. The brine formed during the melting sea ice creates pores and channels in the sea ice in which these microbes can live. As a result of these gradients and dynamic conditions, a higher abundance of microbes are found in the lower layer of the ice, although some are found in the middle and upper layers. Despite this extreme variability in environmental conditions, the taxonomical community composition tends to remain consistent throughout the year, until the ice melts.
The Multidisciplinary drifting Observatory for the Study of Arctic Climate expedition was a one-year-long expedition into the Central Arctic. For the first time a modern research icebreaker was able to operate in the direct vicinity of the North Pole year round, including the nearly half year long polar night during winter. In terms of the logistical challenges involved, the total number of participants, the number of participating countries, and the available budget, MOSAiC represents the largest Arctic expedition in history.
An ice mass balance buoy (IMB) allows scientists studying sea ice to measure its temperature and the evolution of its interfaces remotely. The autonomous mass balance buoys usually consist of a data controller module and a temperature string. Some ice mass balance buoys also include acoustic sounders above and below ice measuring the positions of the snow-ice and ice-water interfaces.