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SEAea ICEce INin THEthe ARCTIİCrctic:OBSERVATIİONSbservations, CHANGES AND

SELECTED IMPLICATIONShanges and selected Implications

By Stefan KERNern1

Burcu ÖZSOY ÇİÇEK2,

Abstract

Sea ice is an essential part in the Climate System. Sea ice is further an impediment for navigation and off-shore activities in the Polar Regions. It reacts sensitively to changes in environmental conditions determining its formation and melt. This chapter gives an introduction into sea ice and describes parameters and terms relevant to understand sea ice and ocean-sea ice -atmosphere interaction processes. Given and described are the main stages of development of sea ice. These are illustrated with plates showing photos of sea ice in the respective stages of development. Information is given about the typical internal structure of sea ice. The difference between bare and snow-covered sea ice is explained and illustrated. Processes such as melting and deformation are explained and illustrated. Sea ice cover maps at minimum and maximum sea ice cover in winter and summer in the respective hemisphere are shown to illustrate the typical location. The trend over the sea ice area during the last two decades is shown. Finally, it is stressed that the sea ice cover (or area) is just one parameter which needs to be monitored to fully understand sea ice and the processes related to sea ice in the Climate System.

1 Integrated Climate Data Center (ICDC), University of Hamburg, Hamburg, Germany

2 Yrd. Doç., İstanbul Teknik Üniversitesi, Denizcilik Fakültesi, Deniz Ulaştırma İşletme Mühendisliği

76 Sea Ice in the Arctic: Observations, Changes and selected Implications

Sea Ice in the Arctic: Observations, Changes and selected Implications

Sea ice is saline ice floating on the oceans in the Arctic and Antarctic; this presentation will focus on the Arctic. Sea ice plays an important role in the Earth’s Climate System: Depending on its thickness and snow cover sea ice reflects between 40 and 90% of the incident solar radiation back to space; in comparison: open water reflects 7%. Sea ice effectively hampers ocean-atmosphere heat exchange and hence acts like an insulating blanket during winter. Sea ice formation and sea ice melt cause water mass modifications which can have global impacts.

Sea ice covers vast and remote areas. Its monitoring hence requires satellite remote sensing. Various techniques and satellite sensors exist to derive different physical parameters describing the sea ice cover: its area, its thickness, its motion, and its type. Observation time series obtained during the satellite era are in the meantime long enough to – together with knowledge from indigenous people and conventional observations – conclude that global climate change has reached the Arctic and has caused – partly – dramatic changes of the sea ice cover. These can be seen in the area, thickness and type and partly also in its motion.

These changes have implications in various fields. One socio-economic one is the enhanced potential usage of the Arctic as an area to exploit minerals and to carry out oil- and gas-related off-shore activities. Another socio-economic one is the opening of the northern sea routes across the Arctic for transportation of goods between Europe and, e.g., Japan or China, via ships. Besides these there a climate impacts such as reduced spring but enhanced fall snow cover in Siberia. Some of these are more secure than others – as they have been already observed; many need further observations and studies employing physical numerical computer models.

This presentation is organized as follows. After a short introduction and motivation into topic and terminology we will guide you through key technologies to monitor sea ice from space.

Stefan Kern and Burcu Özsoy Çiçek 77

Subsequently we will present and explain changes in the Arctic sea ice cover which are currently underway. After having summarized these and having also discussed a bit their reliability we will switch to some selected socio-economic and climatic impacts related to observed and projected Arctic sea ice cover changes.

Arktik Deniz Buzu: Gözlemler, Değişimler ve Etkileri

Stefan Kern

Çeviri: Burcu Özsoy-Çiçek

Arktik ve Antarktik deniz buzları okyanusların üzerinde yüzen tuzlu buzlardır; bu sunumda Arktik deniz buzları üzerinde durulacaktır. Deniz buzları dünya'nın iklim sistemde önemli bir rol oynar: deniz buzu kalınlığı ve üzerindeki kar örtüsüne bağlı olarak, güneş ışınlarını % 40 ile % 90 arasında geri atmosfere yansıtır; karşılaştırma yapılacak olursa su güneş ışınlarını % 7 geri yansıtır. Deniz buzu okyanus-atmosfer ısı değişimini etkili bir şekilde engellerken, kış aylarında da deniz yüzeyinde battaniye gibi davranır ve yalıtım sağlar. Deniz buz oluşumu ve erimesi su kütlesi değişikliklerine, dolayısıyla, deniz buzları küresel etkilere neden olur.

Deniz buzları geniş ve uzak alanları kapsamaktadır, dolayısıyla gözlemler uydu/uzaktan algılama tekniği gerektirir. Çeşitli teknikler ve uydu/uzaktan algılama sensörleri deniz buzu fiziksel parametreleri ortaya koymaktadır, örneğin: buz alanları, kalınlığı, hareketi ve türü. Arktik bölgedeki küresel ısınma sonucu oluşan değişimler özellikle deniz buzlarındaki yoğun azalmalar, o bölgede yaşayan yerli halkın geleneksel gözlemleri ve uzaktan algılama çağının başlamasıyla beraber yapılan gözlemler ile ortaya konmuştur. Deniz buzundaki azalmalar ve değişimler: buz alanları, kalınlığı, hareketi ve türünde gözlemlenmiştir.

Bu değişikliklerin çeşitli alanlarda etkileri vardır. Bir sosyo-ekonomik alan minerallerin çıkarılması, petrol ve gaz ile ilgili off-shore faaliyetlerin yürütülmesiyle Arktik bölgeyi potansiyel olarak kullanmaktadır. Başka bir sosyo-ekonomik kesim gemileri aracılığıyla, örneğin Avrupa ve Japonya/Çin arasında ticari malların taşınması için Arktik genelinde açılan kuzey deniz yollarını kullanmaktadır. Bunların yanı sıra iklimsel etkiler olarak Sibirya’da daha az süreli ilkbahar mevsimi yaşanıp, kışları daha fazla miktarda kar yağmaktadır. Bu etkilerden birkaçı zaten halihazırda uzun sürelerdir gözlenmişte olsa, birçok etkiler için sayısal bilgisayar modelleri kullanarak daha fazla gözlem ve çalışmalara ihtiyaç vardır.


Sunumda: konu ve terminoloji ile ilgili kısa bir giriş ve motivasyondan sonra rehberlik etmesi için anahtar teknolojiler ve uzaydan deniz buzu izleme konularına değinilecektir. Daha sonra Arktik deniz buz örtüsü değişiklikleri açıklanacaktır. Bu özetlerin ardından, bu bilgilerin güvenilirliğine değinerek, Arktik deniz buz örtüsü değişimleriyle ilgili olarak bazı seçilmiş sosyo-ekonomik ve iklimsel etkilerden bahsedilecektir.

[1] Introduction NTRODUCTION

Sea ice covers up to 7 percent of the worlds’ oceans. It covers the polar and sub-polar seas (Figure 1). Because of its physical properties (see next section) sea ice plays a major role in the Earths’ climate system. Sea ice reflects a large fraction of the incoming solar radiation back to space. Sea ice formation and melt modify the oceans’ water masses. Snow accumulation on sea ice further contributes to water mass modification during melt. In shelf areas sediments can be incorporatedluded into the sea ice (Plate 3, a) b)). Because most of the sea ice drifts, it transports mass and, via the potentially incorporatedluded sediments, also nutrients. Sea ice can host, e.g. algae (Plate 3 c)) and is a unique habitat for species which play essential roles in the food web of the Polar Regions. Finally, sea ice is an impediment for off-shore activities, transportation and shipping.

Figure 1: Regions with subsect to regular formation of sea ice. Note that for the northern hemisphere (right image) the Bohai Sea, the Caspian Sea, the Gulf of St. Lawrence, and the Black Sea are not included even though sea ice forms there regularly.


[2] Terminology& and Physics

Sea ice is frozen sea water. Its appearance and its physical properties depend on the formation process. It is usually snow covered except during the early stages of formation. Sea ice formation can be divided into four main stages of development and hence types. A complete overview of sea ice types and stages of development is compiled in [WMO, 1970]. In this book chapter only a sub set of these can be given.

Figure 2 defines the terminology used to describe sea ice and relevant features such as its snow cover in form of schematic vertical transects for thin ice (Figure 2 a) and thick ice (Figure 2 b). The figure illustrates the complexity of sea ice which is also exemplified by the various plates with photographs given throughout this book chapter.

Figure 2: Schematic illustration of typical terms and structures relevant to describe sea ice.

1[3] Stages of development

The first stage of development is new ice. This type mainly comprises frazil ice and grease ice. Frazil ice is a mixture of sea water and freely floating ice crystals. It is the first stage of sea ice formation. Grease ice can be regarded as a dense mat of frazil ice floating at the sea surface. While frazil ice is hard to recognize


visually grease ice can be seen as a greyish layer at the water surface dampening the small-scale surface waves generated by the wind. Bands of frazil and grease ice are shown in Plate 1 a). For ice types similar in appearance such as shuga and slush we refer to [WMO, 1970]. The second stage of development is young ice. Ice types belonging to this stage are dark nilas, light nilas, grey ice, grey-white ice, and white ice. Also pancake ice belongs to this stage of development. Pancake ice forms under turbulent conditions, i.e. under the action of wind waves and swell, whereas the other young ice types require quiescent conditions without swell and no or only light winds. Pancake ice forms from grease ice as more or less round ice floes. Their diameters are between a few ten centimeters to a few meters. Typical thicknesses are between a few centimeters to up to about half a meter. Pancake ice usually carries a ring of re-frozen slush or grease ice on its surface. This ring of ice originates from ice scraped off from neighboring pancake ice floes under the action of the floe movement in the wave field. Pancake ice canoften carryies a substantial amount of some snow on its top [e.g. Ozsoy-Cicek et al., 2011]. In open seas like the Southern Ocean, in particular the Weddell Sea, pancake ice is the dominant ice type during ice formation. One even speaks of the pancake ice cycle [Lange et al., 1991]. Also in the northern hemisphere pancake ice occurs – predominantly in the peripheral seas like the Greenland Sea (see Figure 1)

Table 1: Overview about ice types and their thickness range (for undeformed, level ice)

Ice type Thickness rangeDark nilas 2 - 5 cmLight nilas 5 - 10 cmGrey ice 10 - 15 cmGrey-white ice 15 - 20 cmWhite ice 20 - 30 cmThin first-year ice 30 - 70 cmMedium thick first-year

ice70 - 120 cm

Thick first-year ice 120 - 200cmMultiyear ice 200 - 500 cmPancake ice 5 - 30 cm (60 cm)


Plate 1: Illustration of different stages of development and ice types. New ice: frazil and grease ice (a); young ice: dark nilas (b); young ice: light nilas / grey ice (c); young ice in transition to thick ice: white ice (d); first-year ice: small (~ 20 m diameter) floes (e); first-year ice: large (~ 500 m) broken floes (f); multiyear ice (g); close-up of a ridge on multiyear ice (h). Images a), b), e), g) and h) are taken from aboard a ship of from the ice itself. Images c), d), and f) are taken from aboard a helicopter. Images a) and b) were taken


during September 2007, all other images during March/April 2003. All images are copyright Stefan Kern.

The other young ice types (see above) form when grease ice accumulating at the sea surface forms an ice sheet. First this ice sheet is still elastic and swell can penetrate it without destroying it. Once the sheet becomes thicker it loses its elasticity and grows vertically. Its growth rate is determined by the atmosphere-ocean temperature difference. Thickness ranges for the above-mentioned young ice types are compiled in Table 1. Examples for dark nilas, light nilas/grey ice, and white ice are given in Plate 1 b), c), and d), respectively. Light nilas is also shown in Plate 3 d) and g). Plate 2 shows the transition from light nilas/grey ice (a) to grey-white ice (b). Depending on its thickness these young ice types can carry an increasing amount of snow. The temperature gradient within young ice is usually large. This paired with the possibility to expel brine onto the ice surface during the formation, building up a skim of brine (see [Notz and Worster, 2009]), favors the growth of so-called frost-flowers – the most beautiful form of sea ice found in Polar Regions (Plate 4).


Plate 2: Illustration of different deformation forms and melting. Rafted light nilas / grey ice (a); transition between rafting and ridging on grey-white ice (b); ridging and rafting on white ice (c); fresh ridging of first-year ice in the background versus an old weathered ridge on multiyear ice in the center (d); melt ponds on sea ice (e); distribution of melt ponds and ridges (f). Images c) to e) are taken from aboard a ship. Images a), b), and f) are taken from a helicopter. Image f) is taken from an altitude of about 700 m; the dark spot in the center of the image is the German Research Vessel Polarstern with an approximate length of 120 m. Images a) to d) were taken in March/April 2003, images e) and f) in August 2007. All images are copyright Stefan Kern.

New and young ice are the typical stages of development encountered during freeze-up, i.e. in late fall / early winter. At the equator-ward border of the sea ice cover, which is often also called marginal ice zone, these stages of development are encountered


during the entire freezing period. Other places where these stages of development occur during the entire freezing season are openings in the sea ice covered created by winds or ocean currents. Such openings are called leads (e.g. plate 3 e) if they are elongated features or called polynyas (e.g. plate 3 g) if they have an irregular shape; again see WMO [1970] for a more specific definition. Very often these openings are sites of intense ocean-atmosphere heat exchange (Plate 3 d) and hence of substantial intense ice producformation. Knowing location of leads and polynyas and quantifying the associated ice production is one key for the understanding of changes in the sea ice mass balance in Polar Regions.

The third stage of development is first-year ice. Different forms of first-year ice are discriminated by their thickness ranges: thin first-year ice, medium thick first-year ice, and thick first-year ice (see Table 1). First-year ice usually carries snow. It is also often called seasonal ice and forms the majority of the sea ice in the Southern Ocean; it also forms an increasing percentage of the Arctic sea ice cover. Plate 1 e) and f) give examples of freely drifting first-year ice. There is another form of first-year ice which is attached to the coast (or to glaciers and ice sheets) and which is not drifting under the action of the combined forces of wind and ocean currents. It is called fast ice or land fast ice. An example of this is giveshown in Plate 2 f). This type of ice is usually completely level and is free of any deformation. The only surface roughness is due to snow dunes (see Plate 2 f) which form under the action of the surface wind. Fast ice is typically the thickest form of purely thermodynamically grown first-year ice. Investigation of its thickness at the end of winter over time allows conclusions about changes in the forcing conditions – namely the near surface air temperature and the length of the freezing season [e.g. Gerland et al., 2008].

The fourth stage of development is multiyear ice. In many places the seasonal ice formed during winter melts off completely during the following summer. This is true for most of the Southern Ocean, the Arctic peripheral seas, e.g. the Hudson Bay or the Bering Sea (see Figure 1), and parts of the Arctic Ocean. However, depending on location, oceanic and atmospheric forcing, sea ice may survive the summer melt. Sea ice which survives at least one summer melt season is called multiyear ice. It is usually thicker than first-year ice because it starts to grow from some residual thickness. Its typical thickness ranges between ~3 m and ~6 m and it forms the bulk of the sea ice volume of the Arctic Ocean. It also usually carries a thicker snow cover because the snow has more time to accumulate. Snow falling in fall contributes to the snow cover on multiyear ice but cannot contribute to the snow cover on


first-year ice which first needs to grow up to a certain thickness to carry snow. In sub-section “Melting” we will come back to the main difference between first-year ice and multiyear ice. As multiyear ice is thick it requires substantial forces to deform it. Usually it is rather bended than ridged and usually thinner ice types forced against multiyear ice start to deform and ridge rather than the multiyear ice itself starts to deform. Ridges seen on multiyear ice are hence old, weathered ridges which usually do not have the blocky structure as is typical for ridges in first-year ice (see Plate 2 d). In the Southern Ocean multiyear is more often interspersed with thick first-year because it is more mobile than in the Arctic Ocean; this mixture of multiyear and first-year ice is called perennial ice.

Plate 3: Illustration of some special forms of sea ice and processes. Sediment laden “brown” ice (a and b); algae at sea ice underside (c); grey-white ice with openings illustrating large ocean-atmosphere heat flux (d); a freshly


opened (about 4 hours) ~200 m wide lead (e); landfast sea ice south of Svalbard (f); a polynya south of Svalbard (g). Images a) to c) and e) were taken from aboard a ship; the other images were taken from a helicopter. All images were taken in March/April 2003. All images are copyright Stefan Kern. 2[4] Formation and sea ice structure

The different environments in which sea ice forms: turbulent, with wind waves and swell on the one hand and quiescent with little sea surface movement on the other hand, cause a different internal structure, or in other words, a different form, size distribution and orientation of the ice crystals. One can discriminate between so-called granular ice and so-called columnar ice (or growth). Granular ice comprises rounded ice crystals of a more or less circular shape. This growth type is typical for turbulent environments. It is the main crystal type found in pancake ice. Columnar ice comprises elongated crystals which grow vertically downward into the water column. This growth type is typical for quiescent environments. It is typical for grey / grey-white / white ice and is usually also found in and underneath first-year ice and multiyear ice. Another ice type which is particularly important for Antarctic sea ice but not further considered here is so-called platelet ice.

During sea ice formation air is included in form of small bubbles. Their concentration determines together with the concentration of entrapped salt the sea ice density. The concentration of air bubbles in sea ice is large enough to cause a smaller density than sea water which explains why sea ice floats aton the sea surface. The concentration and shape distribution of the air bubbles in the sea ice depends on the formation process and on the age of the sea ice. Air bubbles (or voids) included during winter freeze-up are usually smaller than the air voids remaining in sea ice after summer melt (see section “Melting”).

In addition to air bubbles sea ice contains so-called brine pockets. These include sea water with an elevated salt concentration, the so-called brine. The salt solved in the sea water cannot be incorporated into the ice crystal matrix during ice formation. Sea ice can be regarded as a mixture of fresh water ice crystals, air, and brine. The brine is distributed throughout the sea ice in form of brine pockets. Their concentration, i.e. the number of brine pockets per volume, and shape distribution as well as the salt concentration of the brine depends on the formation process of the sea ice, the age of the sea ice, and the vertical temperature distribution in the sea ice. Usually the amount of brine trapped in the sea ice is expressed as the sea ice salinity. For remote sensing applications and / or numerical modeling of sea ice properties the brine pocket size distribution and the brine volume fraction, i.e. the volume of brine solution per unit volume of sea ice, are used. One discriminates between the bulk salinity, which is the average


salinity of the entire sea ice column, and the vertical salinity profile. Because the brine concentration and brine volume are a function of the temperature and because the temperature varies with depth also the sea ice salinity varies with depth.

Brine has a higher salt concentration than sea water. Therefore it is denser and slowly drains downwards through the sea ice. One speaks of gravity drainage (see, e.g. [Notz and Worster, 2009]). This drainage is temperature dependent. While cold sea ice is quite impermeable and brine pockets are small and contain a highly up-concentrated salt solution, warm sea ice, i.e. > -5°C bulk temperature, has larger brine pockets which eventually connect to form brine channels through which brine drains downwards. This is illustrated schematically in Figure 3 by means of the larger number and larger vertical extent of brine channels for the warmer sea ice (left, bulk ice temperature Tice: -5°C) compared to the colder sea ice (right, bulk ice temperature Tice: -8°C).

The temperature distribution in sea ice can be approximated by a linear profile between the freezing point of sea water at the ice underside and the surface temperature (Figure 3, right). Snow on top of sea ice insulates it from the cold atmosphere. The temperature distribution in the snow-sea ice system can be approximated by a stepwise linear profile with a comparably small vertical temperature gradient in the sea ice itself followed by a comparably large vertical temperature gradient in the snow on top (Figure 3, left).

Figure 3: Schematic illustration of some differences in sea ice properties between snow covered (left) and snow free (right) sea ice. Yellow arrows depict schematically the shortwave (solar) radiation. Red arrows depict schematically the net surface heat exchange. The profiles given right of the sea ice illustrate in a simplified way the different temperature gradients in


these two sea ice types under winter conditions. Tice is the bulk ice temperature, Ts is the surface temperature. Tair is an arbitrarily chosen near surface air temperature and Twater is the freezing point temperature of sea water at the typically encountered sea water salinity. Dashes and twig-like features in the sea ice denote brine pockets and channels.

3[5] Snow

Snow plays a few key roles in context with sea ice. On the one hand it insulates sea ice from warm air and direct solar radiation. Hence snow protects sea ice from melt. On the other hand it insulates sea ice also from cold air. This decreases the vertical temperature gradient in sea ice and slows down thermodynamic ice growth at the bottom (compare the temperature gradients in the sea ice in Figure 3). Hence snow retards sea ice growth. In other words, sea ice (the ocean underneath) loses more heat into the atmosphere for bare (snow free) sea ice compared to snow-covered sea ice (see red arrows in Figure 3).

Snow has a higher reflectivity than bare sea ice. Therefore it reflects more solar radiation back to space. This is illustrated by the yellow arrows above the sea ice in Figure 3. Snow is also less permeable for solar radiation. Therefore the amount of solar radiation which is transmitted through sea ice is smaller under a snow cover. This is illustrated by the yellow arrows below the sea ice in Figure 3. This has consequences for the progress of summer melt (see section “Melting”) but also for species living beunderneath the sea ice.

4[6] Deformation

Sea ice is a rigid body which deforms and fails under the action of external forces. The rigidity of sea ice is a function of its temperature and internal structure which is in turn a function of its growth history. It is beyond the scope of this chapter to explain different modes of failure and deformation of sea ice. We concentrate on the main aspects and differentiate between un-deformed sea ice (see Plate 3 f) and deformed sea ice (see Plate 1 g), h).

Sea ice deformation can occur in three forms. The first is bending, meaning that sea ice floes bend under the action of external forces without recognizable deformation features. This form of deformation can cause sea ice floes or parts of it to be elevated above or suppressed below their vertical position relative to the sea surface given by the buoyancy forces. Such bending can occur for instance close to coasts or ridges (see below).


Figure 4: Schematic illustration of rafting (a, b) and ridging (c, d).

The second form: rafting, and the third form: ridging, are those which cause recognizable changes in the sea ice structure. These two forms are illustrated in Figure 4. Rafting typically occurs with young ice. During rafting the majority of the sea ice floe remains intact. Sea ice floes move on top or are submerged below each other without creating vertically extensive structures (Figure 4 a, b). Rafting can occur, however, over large regions and several times. Multiple rafting events can build up a sea ice cover which is much thicker than the sea ice encountered typically in a certain region and time. Examples for rafting are illustrated in Plate 1 b) and c), and Plate 2 a) and b). Plate 2 b) and also Plate 2 c) show the transition from rafting to ridging where ice floes are broken into pieces which pile up on top of the sea ice, forming a ridge, and below the sea ice (see Figure 4 c, d); the structure created below the sea ice is called keel. Typically the keel extends deeper into the water than the ridge extends into the air. Examples for ridges are given in Plate 1 d), Plate 1 f) to h), and Plate 2 d). The fact that sea ice is thicker where ridged can also be observed during summer where the level, mostly un-deformed and hence thinner parts of the sea ice carry melt ponds and are separated by ridges (see Plate 2 f). This plate also depicts nicely one example for the spatial distribution of ridges.

5[7] Melting

Sea ice can melt in various ways. The transition between sea ice growth and melt is determined by the net surface radiation balance (short- and long-wave), the ocean-atmosphere temperature gradient and the oceanic heat flux at the bottom of the sea ice. Sea ice melts from below and the sides (lateral melt) once the water temperature


exceeds the freezing point at the respective water salinity; in most ocean waters the freezing point is -1.8°C. In the Baltic Sea and in other seas with substantial input of fresh water, e.g. from rivers like on the Siberian Shelf, the freezing point could be higher. Sea ice melts from top once air temperatures are above the freezing point and, more importantly, as a function of a positive net surface radiation balance.

In the Arctic, first the snow on top of the sea ice melts. Meltwater forms puddles on top of the sea ice developing into so-called melt ponds. These appear darker than the surrounding sea ice (see Plate 2 e), f)) and enhance absorption of solar radiation which in turn enhances melt of the sea ice. Melt ponds can cover large parts of the sea ice (Plate 2 f). The sea ice itself reflects less solar radiation once the snow is gone (see Figure 3). This also enhances sea ice melt. In addition more solar radiation can be transmitted into and through the sea ice (see Figure 3) which both also enhances sea ice melt. A number of positive feedback mechanisms are working together here during summer. Eventually a melt pond can melt through the entire ice column.

This melting process triggers two substantial changes in the sea ice structure. The melt water is percolating the sea ice and flushes out the brine pockets with fresh water. Hence the bulk salinity of multiyear ice: 2-3 psu, is smaller than the bulk salinity of first-year ice: 4-6 psu. Brine pockets above the water line are flushed out completely. These become air voids which stay once freezing conditions return. Hence the concentration of air bubbles or voids in multiyear ice is larger than in first-year ice – particularly in the topmost few (ten) centimeters. Consequently, multiyear ice has a smaller overall (bulk) density than first-year ice. Both, the different salinity and different density helps distinguishing between first-year and multiyear ice when drilling an ice core.

Melting of Antarctic sea ice is different. The snow cover on Antarctic sea ice is often thicker than on Arctic sea ice. Hence it takes longer to melt. It has been shown that during summer the snow on Antarctic sea ice typically changes its morphology, i.e. its grain size and its density. It also becomes thinner but it rarely melts off completely. Accordingly, the amount of solar radiation available to melt the ice in the Antarctic is smaller than in the Arctic. More solar radiation is reflected back to space because of the higher reflectivity of snow compared to bare ice (see Figure 3) and because melt ponds, which substantially accelerate melt of Arctic sea ice, are absent in the Antarctic. Why does most of the Antarctic seasonal sea ice melt then every summer? This can be explained by two things. Antarctic seasonal sea ice is usually thinner than most of the Arctic seasonal ice. Vertical oceanic heat fluxes are usually larger in the Antarctic, causing a stronger bottom melt than in the


Arctic. Because of these two factors Antarctic seasonal sea ice often melts out before the snow could melt completely. There are a few locations around Antarctica, mainly the Weddell Sea and parts of the Bellingshausen-Amundsen Sea (see Figure 6), where sea ice also survives the summer melt and becomesing multiyear ice. However, because the melt process differs much from the one in the Arctic Antarctic sea ice does not necessarily undergo the same changes in salinity and density as described above.

6[8] Areas of sea ice occurrence

Arctic

The areas in which sea ice can form in the Arctic Ocean and its peripheral seas are given in Figure 1. Figure 5 illustrates the average maximum (a) and average minimum (b) sea ice cover in the Arctic together with the temporal evolution of the sea ice area (c).

Figure 5: 20-year (1992-2011) average sea ice concentration (= percentage of a known area covered with sea ice) for the Arctic; a) March, b) September. Image c) illustrates the seasonal development of the monthly mean Arctic sea ice area for 1992 to 2014.


Shown in Figure 5 (and Figure 6) is the sea ice concentration. This is the fraction of sea ice covering a known area. For the maps shown in Figure 5 (and Figure 6) observations from a polar-orbiting satellite are used to compute the sea ice concentration at daily temporal resolution and at a grid resolution of 12.5 km x 12.5 km. Daily temporal resolution means that such a map is computed every day. The grid resolution stems from the fact that the data are interpolated onto a geographic projection; in this case a polar-stereographic projection is used and each grid cell has a size of 12.5 km by 12.5 km. Because the satellite observations used in this case are obtained in the microwave frequency range of the electromagnetic spectrum they are independent of daylight and clouds (see Chapter: Remote Sensing).

The sea ice concentration data shown in Figure 5 and Figure 6 are computed with the ARTIST Sea Ice (ASI) sea ice concentration algorithm [Kaleschke et al., 2001]. They are provided as an IFREMER – ICDC co-production and are available in netCDF-Format at http://icdc.zmaw.dehttp://icdc.zmaw.de. For the maps shown first the monthly average sea ice concentration is computed. Subsequently the monthly sea ice concentration maps of the months used, i.e. March and September, are averaged over the years 1992-2011.

In the Arctic, during time of the maximum sea ice cover (Figure 5 a), the sea ice concentration is close or equal to 100% or 100% in the entire Arctic Ocean and over parts of the peripheral seas. During time of the minimum sea ice cover (Figure 5 b), only the central Arctic Ocean reveals a sea ice concentrations close to 100% or 100%. Sea ice concentration decreases towards the coasts bordering the Arctic Ocean and the peripheral seas are ice free. The sea ice cover seen in Figure 5 b) could be considered as the average coverage with multiyear ice because in September freezing starts overconditions start to kick in.

The time series of the monthly mean sea ice area shown in Figure 5 c) is based on the same data set – except that it is longer and ends in 2014. The sea ice area is the sum of the area covered by all grid cells which contain at least 15% sea ice. Sea ice area is expressed in square kilometers. The maximum Arctic sea ice area is usually found in March and takes a value between 12 and 14 million square kilometers. The minimum Arctic sea ice area is found in September and takes values around 5 million square kilometers. Figure 5 c) illustrates that the Arctic sea ice area is decreasing – during winter (maxima) but more so during summer and fall (minima) as is known from various literature sources (e.g. [Cavalieri and Parkinson, 2013]). Figure 5 c) reveals that the minimum sea ice

Stefan Kern, 06/15/15,

Bisher dachte ich dass beide Artikel im gleichen Buch sein würden. Das müßte man dann hier korrigieren.


area has decreased from about 6 million square kilometers in the 1990ties to about 4 million square kilometers in the recent 5 years.

AntarcticThe areas in which sea ice can form in the Southern Ocean are

given in Figure 1. Figure 6 illustrates the average maximum (a) and average minimum (b) sea ice cover in the Antarctic together with the temporal evolution of the sea ice area (c). The maps and time series shown in Figure 6 are based on the same data and the same algorithm as is used to generate Figure 5. Figure 6 a) and b) exemplify the average (years 1992-2011) sea ice concentration for September and February, respectively. The map of the September sea ice concentration (Figure 6 a) appears less “white” than the corresponding map close to maximum sea ice cover in the Arctic. This is because the Antarctic sea ice cover is a bit more open than the Arctic one. This is caused by the fact that the sea ice is more free to move in the Antarctic; no land masses are bordering the sea ice to the north. The sea ice cover in September is vast compared to the one in February (Figure 6 b) when the minimum is reached. Most sea ice survives summer melt in the Weddell Sea, in the Amundsen Sea and in the Ross Sea. One could take this as the area where the Antarctic multiyear ice (or perennial ice) occurs on average.

Figure 6: 20-year (1992-2011) average sea ice concentration (= percentage of a known area covered with sea ice) for the Antarctic; a) September, b)


February. Image c) illustrates the seasonal development of the monthly mean Antarctic sea ice area for 1992 to 2014.

In the Antarctic the maximum sea ice area usually occurs in September / October and takes a value of around 16 million square kilometers (Figure 6 c). The minimum sea ice area occurs in February and takes a value of 2 to 3 million square kilometers (Figure 6 c). In contrast to the Arctic, the Antarctic sea ice area is increasing – the minimum as well as the maximum area. This is illustrated in Figure 6 c) and can also be read in the literature (e.g. [Parkinson and Cavalieri, 2013]).

[9] Concluding remarks

This chapter gives a brief overview about the nature of sea ice, its stages of development, its types, some of its physical properties and where it can be found in the Polar Regions. Much more can be said about sea ice – like how beautiful it is, how dangerous it is, how complex it is, how difficult it is to include it properly into computer models, how much fun and satisfaction it can give a scientist studying it – but we would like to stay with two things here.

Plate 4: Frost flowers growing on dark nilas. Photography taken in April 2003 in the Arctic, Copyright Stefan Kern.

First, sea ice has shown to be vulnerable to the amplified global warming which the Arctic has been experiencing. It seems to be an


excellent indicator of the changes going on. At the same time, however, sea ice is a riddle, because while it shrinks in the Arctic it – at least at the time of writing this book chapter – tends to increase in the Antarctic. Many studies have dealt with this (apparent) discrepancy between both hemispheres and many more studies will follow. This leads to point two.

In this book chapter we only very briefly looked into sea ice parameters. We picked sea ice concentration and area becauseas these two parameters describe where one can find sea ice in the most easy way. However, in order to describe the sea ice cover of the Polar Regions completely one needs much more information about these vast sea ice areas. This starts with the sea ice thickness which is required to obtain an estimate of the sea ice volume which is required to answer the question whether the sea ice mass is indeed shrinking or increasing. This continues over the sea ice motion which is required to understand where sea ice goes once it has been formed and to quantify to role of deformation processes in shaping the sea ice mass distribution. And this ends with a full suite of additional parameters such as snow depth on sea ice, ice type, ice age, ice surface temperature, etc. which could all enhance our understanding about processes related to sea ice in the Polar Regions.

Many of these parameters have been and could be derived by means of remote sensing – which is perhaps one of the reason why we have a chapter “Remote Sensing” in this book as well.

REFERENCES eferencesASI Algorithm SSMI-SSMIS sea ice concentration data, originally computed

at and provided by IFREMER, Brest, France, were obtained as 5-day median-filtered and gap-filled product for [01-01-1992 to 31-12-2014] from the Integrated Climate Data Center (ICDC, http://icdc.zmaw.de/), University of Hamburg, Hamburg, Germany, accessed Feb. 25 2015.

Cavalieri, D. J., and C. L. Parkinson, Arctic sea ice variability and trends: 1979-2010. The Cryosphere, 6, 881-889, doi:10.5194/tc-6-881-2012, 2012.

Gerland, S., et al., Decrease of sea ice thickness at Hopen, Barents Sea, during 1966-2007, Geophysical Research Letters, 35, L06501, doi: 10.1029/2007GL032716, 2008.

Kaleschke, L., G. Heygster, C. Lüpkes, A. Bochert, J. Hartmann, J. Haarpaintner,and T. Vihma, SSMI sea ice remote sensing for mesoscale ocean-atmosphere interaction analysis. Canadian Journal of Remote Sensing, 27, 526-537, 2001.

Lange, M. A., and H. Eicken, Textural characteristics of sea ice and major mechanisms of ice growth in the Weddell Sea.Annals of Glaciology, 15, 210-215, 1991.

Notz, D., and M. G. Worster, Desalination processes of sea ice revisited. Journal of Geophysical Research – Oceans, 114, C05006, doi: 10.1029/2008JC004885, 2009.

Ozsoy-Cicek, B., et al.,Intercomparison of Antarctic sea ice types from visual ship. RADARSAT-1 SAR, Envisat ASAR, QuikSCAT, and AMSR-E satellite observations in the Bellingshausen Sea. Deep Sea Research – II, 58, 1092-1111, 2011.

Stefan Kern, 06/15/15,

Siehe Kommentar SK1


Parkinson, C. L., and D. J. Cavalieri, Antarctic sea ice variability and trends: 1979-2010. The Cryosphere, 6, 871-880, doi:10.5194/tc-6-871-2012, 2012.

World Meteorological Organization (WMO) Sea ice nomenclature, WMO/OMM/BMO – No. 259, Suppl. 5, 1970, revised March 2010.

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