Ocean processes of climatic importance in the Arctic

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February 9, 2010, 3:14 am
May 7, 2012, 5:27 pm
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This is Section 9.2.3 of the Arctic Climate Impact Assessment
Lead Author: Harald Loeng; Contributing Authors: Keith Brander, Eddy Carmack, Stanislav Denisenko, Ken Drinkwater, Bogi Hansen, Kit Kovacs, Pat Livingston, Fiona McLaughlin, Egil Sakshaug; Consulting Authors: Richard Bellerby, Howard Browman,Tore Furevik, Jacqueline M. Grebmeier, Eystein Jansen, Steingrimur Jónsson, Lis Lindal Jørgensen, Svend-Aage Malmberg, Svein Østerhus, Geir Ottersen, Koji Shimada

The marine Arctic plays an important role in the global climate system (Box 9.1). A number of physical processes will be affected by the changes (Climate change) anticipated in global climate during the 21st century, but this assessment focuses on those that are expected to have strong impacts on the climate or biology of the Arctic. These include the effects of wind on the transport and mixing of water, and the circulation systems generated by freshwater (Freshwater discharge in the Arctic) input and thermohaline ventilation (Fig. 9.4). A key issue is the extent to which each of these processes contributes to driving the inflow of Atlantic water to the Arctic (Arctic Ocean). Models[1] have shown that the heat transported by this inflow in some areas elevates the sea surface temperature to a greater extent than the temperature increase projected for the 21st century (see Chapter 4 (Ocean processes of climatic importance in the Arctic)). A weakening of the inflow could therefore significantly reduce warming in these areas and might even induce regional cooling, especially in parts of the Nordic Seas. Thus, special attention is paid to the processes that affect the inflow, especially the thermohaline circulation (THC).

Freshwater and entrainment (9.2.3.1)

Fig. 9.4. Two types of processes create unique current systems and conditions in the marine Arctic. The input of freshwater, its outflow to the Atlantic, and the en-route entrainment of ambient water create an estuarine type of circulation within the marine Arctic. In addition to this horizontal circulation system, thermohaline ventilation creates a vertical circulation system. Both patterns of circulation are sensitive to climate change

Freshwater is delivered to the marine Arctic by atmospheric transport through precipitation (Precipitation and evapotranspiration in the Arctic) and by ocean currents, and to the coastal regions through river (River and lake ice in the Arctic) inflows[2]. Further net distillation of freshwater may occur within the region during the melt/freeze cycle of sea ice, provided that the ice and rejected brine formed by freezing in winter can be separated and exported before they are reunited by melting and mixing the following summer[3].

The freshwater has decisive influences on stratification and water column stability as well as on ice formation. Without the freshwater input, there would be less freezing, less ice cover, and less brine rejection[4]. This is also illustrated by the difference between the temperature-stratified low latitude oceanic regime and the salinity-stratified high latitude oceanic regime[5].

Box 9.1. Role of the marine Arctic in the global climate system

300px-ACIA Box 9.1a.png.jpeg

The marine Arctic is an interconnected component of the global climate system whose primary role is to balance heat gain at low latitudes and heat loss at high latitudes. At low latitudes about half the excess heat is sent poleward as warm (and salty) water in ocean currents (sensible heat, QS) and the other half is sent poleward as water vapor in the atmosphere (latent heat, QL). At low latitudes the subtropical gyres in the ocean collect excess heat and salt, the western boundary currents carry them poleward, and the Atlantic inflow brings them into the marine Arctic. Heat carried by the atmosphere is released at high latitudes by condensation, thus supplying freshwater to the ocean through precipitation and runoff. Freshwater is stored in the surface and halocline layers of the marine Arctic.To prevent the build-up of salt (by evaporation) at low latitudes, freshwater is exported from the high latitudes, thus completing the hydrological cycle by reuniting the atmospheric water content and the salty ocean water. At high latitudes the return flows include export by ice and transport in low-salinity boundary currents, intermediate water (which forms and sinks along the subpolar fronts), and deep water (which sinks on shelves and in gyres). Export of these low-salinity waters southward couples the Arctic to the world thermohaline circulation (THC) through intermediate and deep-water formation.The role of intermediate water in governing THC is unclear.

300px-ACIA Box 9.1b.png.jpeg

The marine Arctic plays an active role in the global climate system with strong feedbacks, both positive and negative. For example: albedo feedback, thermohaline feedback, and greenhouse gas feedback.

Albedo feedback – Ice and snow reflect most of the solar radiation back into space. With initial warming and sea-ice melting, more heat enters the ocean, thus melting more sea ice and increasing warming.

Thermohaline feedback – If the export of freshwater from the Arctic Ocean should increase, then stratification of the North Atlantic would probably increase, and this could slow the THC.A decrease in the THC would then draw less Atlantic water into high latitudes, leading to a slowdown in the global overturning cell and subsequent localized cooling. (This scenario does not take into account the formation of intermediate water.)

Greenhouse gas feedback – Vast amounts of methane and carbon dioxide are currently trapped in the permafrost and hydrate layers of the arctic margins[6]. With warming, arctic coastal lakes will act as a thermal drill to tap this greenhouse gas source and further exacerbate warming.

In the [[Arctic (Arctic Ocean)] Ocean], freshwater is stored within the various layers above and within the halocline, the latter serving as an extremely complex and poorly understood reservoir. This is especially true for the Beaufort Gyre, which represents the largest and most variable reservoir of freshwater storage in the marine Arctic. The ultimate sink for freshwater is its export southward into the North Atlantic to replace the freshwater evaporating from low latitude oceans and to close the global freshwater budget. This southward transport occurs partly through the THC since the overflow from the Nordic Seas into the Atlantic is less saline than the inflowing Atlantic water. The role of the freshwater is illustrated in Fig. 9.5. The figure shows the processes responsible for the development of the horizontal and vertical circulation systems unique to the marine Arctic.

Fig. 9.7. The Arctic Mediterranean has four current branches that import water into the upper layers; three from the Atlantic (the Iceland, Faroe, and Shetland branches), and one from the Pacific. The outflow occurs partly at depth through the overflows and partly as surface (or upper-layer) outflow through the Canadian Archipelago and the East Greenland Current. The numbers indicate volume flux in Sverdrups (106 m3/s) rounded to half-integer values and are based on observations, with the exception of the surface outflow, which is adjusted to balance[7].

Most of the freshwater in the Arctic Ocean returns southward in the surface outflows of the East Greenland Current and through the Canadian Archipelago. These flows carry low-salinity water as well as sea ice (Sea ice in the Arctic). They include most of the water that enters the Arctic Ocean through the Bering Strait and water of Atlantic origin entrained into the surface flow. Since the estimated total volume flux of the surface outflows greatly exceeds the combined fluxes of the Bering Strait inflow and the freshwater input, most of the surface outflows must derive from entrained Atlantic water. This process therefore induces an inflow of Atlantic water to the Arctic, which by analogy to the flows in estuaries is usually termed "estuarine circulation". This estuarine-type circulation is sensitive to climate change.

Mixed-layer depth (9.2.3.2)

The vertical extent of the surface mixed layer is critical to the primary production and depends on the vertical density stratification and the energy input, especially from the wind. Density stratification is affected by heat and freshwater (Freshwater discharge in the Arctic) fluxes from the atmosphere (Atmosphere layers) or by advection from surrounding ocean areas. Some areas, for example the [[Arctic (Arctic Ocean)] Ocean], are salt-stratified whereas other areas, such as the Nordic Seas and the Bering Sea, are temperature-stratified. In a classic study, Morison and Smith[8] found that seasonal variations in mixed-layer depth are largely controlled by buoyancy (i.e., heat and salt) fluxes.

Winds blowing over the sea surface transfer energy to the surface mixed layer. In ice-free areas, increased winds would tend to deepen the surface mixed layer, depending upon the strength of the vertical density stratification. In the presence of sea ice, however, the efficiency of energy transfer from wind to water is a complex function of sea-ice roughness and internal ice stress which, in turn, is a function of sea-ice concentration and compactness[9]. Because warming will decrease sea-ice (Sea ice in the Arctic) concentrations (and so decrease internal ice stress) and increase the duration of "summer" conditions (i.e., earlier breakup and later freeze-up), the efficiency of wind mixing in summer is likely to increase. This is especially true for late summer in the Arctic Ocean when energy input from storms is greatest. However, owing to the poorly understood role of air–ice–ocean coupling and the present level of salt-stratification, this increased exposure will not necessarily lead to significant increases in mixed-layer depth. Furthermore, the role that lateral advection plays in establishing the underlying halocline structure of the Arctic Ocean must also be considered.

Wind-driven transport and upwelling (9.2.3.3)

A number of studies have shown the effect of wind stress on the circulation of particular regions within the marine Arctic[10]. Winds have also been shown to have a strong influence on exchanges between regions[11]. If winds were to change significantly, wind-driven currents and exchanges would also change. These wind-induced changes in turn would redistribute the water masses associated with the different currents, thereby affecting the location and strength of the fronts separating the water masses[12].

Retraction of the multi-year ice cover seaward of the shelf break in the [[Arctic (Arctic Ocean)] Ocean] may lead to wind-induced upwelling at the shelf break, which is currently not happening. This process might substantially increase the rate of exchange between the shelf and deep basin waters, the rate of nutrient upwelling onto the shelves, and the rate of carbon export to the deep basin[13].

Thermohaline circulation (9.2.3.4)

Thermohaline circulation is initiated when cooling and freezing of sea water increase the density of surface waters to such an extent that they sink and are exchanged with waters at greater depth. This occurs in the Labrador Sea, in the Nordic Seas, and on the arctic shelves. Together, these regions generate the main source water for the North Atlantic Deep Water; the main ingredient of the global ocean "Great Conveyor Belt"[14]. All these arctic areas are therefore important for the global THC. More importantly from the perspective of this assessment is the potential impact of a changing THC on flow and conditions within the marine Arctic. Some areas are more sensitive than others, because the oceanic heat transport induced by the THC varies regionally. The most sensitive areas are those that currently receive most of the heat input from inflowing warm Atlantic water, i.e., the eastern parts of the Nordic Seas and the Arctic Ocean[15], namely the Arctic Mediterranean.

The THC in the Arctic Mediterranean is often depicted as more or less identical to open-ocean convection in the Greenland Sea. This is a gross over-simplification since, in reality, there are several different processes contributing to the THC and they occur in different areas. The THC can be subdivided into four steps (Fig. 9.4).

  1. Upper layer inflow of warm, saline Atlantic water into the [[Arctic (Arctic Ocean)] Ocean] and the Nordic Seas.
  2. Cooling and brine rejection making the incoming waters denser.
  3. Vertical transfer of near-surface waters to deeper layers.
  4. The overflow of the dense waters in the deep layers over the Greenland–Scotland Ridge and their return to the Atlantic.

Although these steps are linked by feedback loops that prevent strict causal relations, the primary processes driving the THC seem to be steps 2 and 3, which are termed thermohaline ventilation. By the action of the thermohaline ventilation, density and pressure fields are generated that drive horizontal exchanges between the Arctic Mediterranean and the Atlantic (steps 1 and 4). Box 9.2 illustrates the basic mechanisms of the thermohaline forcing.

Box 9.2.Thermohaline forcing of Atlantic inflow to the Arctic

The processes by which thermohaline ventilation induces Atlantic inflow to the Arctic Mediterranean can be illustrated by a simple model where the Arctic Mediterranean is separated from the Atlantic by a ridge (the Greenland–Scotland Ridge). South of the ridge, Atlantic water (red) with uniform temperature, salinity, and density (r) extends to large depths. North of the ridge, the deep layers (blue) are less saline, but they are also much colder than the Atlantic water and therefore denser (r+Dr). Above this deep, dense layer is the inflowing Atlantic water, which is modified by cooling and brine rejection to become increasingly similar to the deep layer as it proceeds away from the ridge.The causal links between the processes involved can be broken into three steps.

ACIA Box 9.2.png.jpeg

Thermohaline ventilation – Cooling and brine rejection make the inflowing Atlantic water progressively denser until it has reached the density of the deeper layer. At that stage, the upper-layer water sinks and is transferred to the deeper layer.This is equivalent to raising the interface between the two layers in the ventilation areas, which are far from the ridge.

Overflow – When ventilation has been active for some time, the interface will be lifted in the ventilation areas and will slope down towards the ridge. Other things being equal, this implies that the pressure in deep water will be higher in the ventilation areas than at the same depth close to the ridge. A horizontal internal (so-called baroclinic) pressure gradient will therefore develop which forces the deep water towards and across the ridge. In this simple model, the overflow is assumed to pass through a channel, sufficiently narrow to allow neglect of geostrophic effects. If the rate at which upper-layer water is converted to deeper-layer water is constant, the interface will rise until it can drive an overflow with a volume flux that equals the ventilation rate.

Sea-level drop – When thermohaline ventilation has initiated a steady overflow, there will be a continuous removal of water from the Arctic Mediterranean. Without a compensating inflow, the sea level would drop rapidly north of the ridge.Thus an uncompensated overflow of the present-day magnitude would make the average sea level in the Arctic Mediterranean sink by more than one meter a month. As soon as the water starts sinking north of the ridge, there will, however, develop a sea-level drop across the ridge. This sea-level drop implies that water in the upper layer north of the ridge will experience lower pressure than water at the same level in the Atlantic. A sea surface (so-called barotropic) pressure gradient therefore develops that pushes water northward across the ridge.The amount of Atlantic water transported in this way increases with the magnitude of the sea-level drop. In the steady state, the sea-level drop is just sufficient to drive an Atlantic inflow of the same volume flux as the overflow and the ventilation rate.

When upper-layer water is converted to deeper-layer water at a certain ventilation rate (in m3/s), an overflow and an Atlantic inflow are therefore generated which have the same volume flux on long timescales. In the present state, these fluxes must equal the estimated overflow flux of about 6 Sv. Simple, non-frictional, models indicate that the required interface rise is several hundred meters, as is observed, while the required sea-level drop is only of the order of 1 cm.

Thermohaline ventilation

The waters of the [[Arctic (Arctic Ocean)] Ocean] and the Nordic Seas are often classified into various layers and a large number of different water masses[16]. For the present assessment, it is only necessary to distinguish between "surface" (or upper layer) waters and "dense" waters, which ultimately leave the Arctic Mediterranean as overflow into the North Atlantic. The term "dense waters" is used to refer to deep and intermediate waters collectively and the term "thermohaline ventilation" is used as a collective term for the processes that convert surface waters to dense waters. Thermohaline ventilation is a two-step process that first requires cooling and/or brine rejection to increase the surface density and then a variety of processes that involve vertical transfer.

Cooling and brine rejection

Production of dense waters in the arctic Nordic Seas is due initially to atmospheric cooling, and then to brine rejection during sea-ice formation[17]. The waters flowing into the Nordic Seas from the Atlantic exhibit a range of [[temperature]s] depending on location and season. On average, their temperature is close to 8° C, but it decreases rapidly after entering the Nordic Seas. The temperature decrease is especially large in the southern Norwegian Sea. The simultaneous salinity decrease indicates that some of the temperature decrease may be due to admixture of colder and less saline adjacent water masses. Except for relatively small contributions of freshwater from river inflow and the Pacific-origin waters flowing along the east coast of Greenland, the adjacent water masses are predominantly of Atlantic origin. Thus, atmospheric cooling in the Nordic Seas is the main cause of the decreasing temperature of the inflowing Atlantic water.

Attempts have been made to calculate the heat loss to the atmosphere from climatological data, but the sensitivity of the results to different parameterizations of the heat flux makes these estimates fairly uncertain[18]. Most of the heat loss from the ocean to the atmosphere occurs in ice-free areas of the Nordic and Barents Seas[19].

Brine rejection, however, is intimately associated with sea-ice formation[20]. When ice forms at the ocean surface, only a small fraction of the salt follows the freezing water into the solid phase, the remainder flowing into the underlying water. Brine also continues to drain from the recently formed ice. Both processes increase the salinity, and therefore density, of the ambient water. In a stationary state, the salinity increase due to brine rejection in cold periods is compensated for by freshwater input from melting ice in warm periods, but freezing and melting often occur in different regions. For example, on the shallow shelves surrounding the arctic basins rejected brine results in shelf waters sufficiently dense to drain off the shelves, thus becoming separated from the overlying ice[21]. Winds can also remove newly formed ice from an area while leaving behind the high salinity water.

Vertical transfer of water

Fig. 9.6. Three of the thermohaline ventilation processes that occur in the Arctic Mediterranean: boundary current deepening, open-ocean convection, and shelf convection.

The second step in thermohaline ventilation is the vertical descent of the surface waters made denser by cooling and brine rejection. Several processes contribute to the transfer. These include the sinking of the boundary current as it flows around the Arctic Mediterranean, open-ocean convection, and shelf convection as well as other ventilation processes (Fig. 9.6).

  1. The boundary current enters the Arctic Mediterranean as pure Atlantic water with relatively high temperature (>8° C) and salinity (>35.2). It enters mainly through the Faroe–Shetland Channel and within the Channel joins with part of the Iceland–Faroe Atlantic inflow. Part of the boundary current continues as an upper-layer flow along the continental slope to Fram Strait. There, one branch moves toward Greenland while the other enters the [[Arctic (Arctic Ocean)] Ocean] and flows sub-surface along its slope to join the first branch as it exits again through Fram Strait. The flow continues as a subsurface boundary current over the slope off East Greenland all the way to Denmark Strait with the core descending en route[22]. While circulating through the Arctic Mediterranean, boundary current waters experience a large temperature decrease, much of it during the initial flow along the Norwegian shelf. While the associated density increase is partly offset by a salinity decrease, there is still a considerable net density increase. After passing Fram Strait, both branches are submerged without direct contact to the atmosphere such that temperature and salinity changes occur mainly through isopycnal mixing with surrounding waters. Isopycnal mixing occurs between waters of the same density but different temperatures and salinities.
  2. Open-ocean convection is very different from boundary current deepening, being essentially a vertical process. After a pre-conditioning phase in which the waters are cooled and mixed, further intensive cooling events may trigger localized intense descending plumes or eddies with horizontal scales of the order of a few kilometers or less[23]. They have strong vertical velocities (of the order of a few hundredths of a meter per second), but do not represent an appreciable net volume flux since they induce upward motion in the surrounding water[24]. They do, however, exchange various properties (such as CO2) between the deep and near-surface layers as well as to the atmosphere. They also help maintain a high density at depth. Open-ocean convection is assumed to occur to mid-depths in the Iceland Sea[25]. In the Greenland Sea, convective vortices have been observed to reach depths of more than 2,000 m[26] and it is assumed that convection in earlier periods penetrated all the way to the bottom layers to produce the very cold Greenland Sea Deep Water, as observed in 1971[27].
  3. Shelf convection results from brine rejection and convection, and can lead to the accumulation of high salinity water on the shelf bottom[28]. Freezing of surface waters limits the temperature decrease, but if [[wind]s] or other factors remove the sea ice while leaving the brine-enriched water behind, prolonged cooling can produce a high salinity water mass close to the freezing point. Eventually, gravity results in this saline, dense water mass flowing off the shelf and sinking into the arctic abyss. As it sinks, it entrains ambient waters and its characteristics change[29]. Shelf convection is the only deep-reaching thermohaline ventilation process presumed to enter the [[Arctic (Arctic Ocean)] Ocean] and hence is responsible for local deep-water formation.

There are at least two additional sinking mechanisms (not included in Fig. 9.6) that may transfer dense water downward; isopycnal sinking and frontal sinking. Overflow water is often defined as water denser than ?=27.8[30] and such water is widely found in the Arctic Ocean and the Nordic Seas, close to the surface. During winter, mixing and cooling result in surface densities up to and above this value. This water can therefore flow over the ridge, sinking below the top of the ridge but without crossing isopycnals. This is termed "isopycnal sinking". A somewhat-related mechanism has been termed "frontal sinking", which indicates that near-surface water from the dense side of a front can sink in the frontal region and flow under the less dense water. In the Nordic Seas, this has been observed in the form of low-salinity plumes sinking at fronts between Arctic and Atlantic waters[31].

Horizontal water exchange

The Nordic Seas and the Arctic Ocean are connected to the rest of the World Ocean through the Canadian Archipelago, across the Greenland–Scotland Ridge, and through the Bering Strait, and they exchange water and various properties with the World Ocean through these gaps. Four exchange branches can be distinguished (Fig. 9.7). The near-surface outflow from the Arctic Ocean through the Canadian Archipelago and Denmark Strait, and the Bering Strait inflow to the Arctic Ocean from the Pacific are important in connection with freshwater flow through the Arctic Ocean and the Nordic Seas. For the THC, the overflow of cold and dense water from the Nordic Seas into the Atlantic and the inflow of Atlantic water to the Nordic Seas and the Arctic Ocean are the most important factors.

Overflow

The term overflow is used here to describe near-bottom flow of cold, dense (? < 27.8)[32] water from the Arctic Mediterranean across the Greenland–Scotland Ridge into the Atlantic. It occurs in several regions. In terms of volume flux, the most important overflow site is the Denmark Strait, a deep channel between Greenland and Iceland with a sill depth of 620 m. The transport in this branch is estimated at 3 Sv, or about half the total overflow flux[33]. Mauritzen C.[34] and Rudels et al.[35] argue that water from the East Greenland Current forms the major part of this flow. Other sources contribute, however[36]; some researchers suggest the Iceland Sea as the primary source for the Denmark Strait overflow[37].

The Faroe Bank Channel is the deepest passage across the Greenland–Scotland Ridge and the overflow through the channel is estimated to be the second largest in terms of volume flux, approximately 2 Sv[38]. Owing to the difference in sill depth, the deepest water flowing through the Faroe Bank Channel is usually colder than water flowing through the Denmark Strait and the Faroe Bank Channel is thus the main outlet for the densest water produced in the Arctic Mediterranean.

Overflow has also been observed to cross the Iceland–Faroe Ridge at several sites, as well as the Wyville–Thomson Ridge, but more intermittently. The total overflow across these two ridges has been estimated at slightly above 1 Sv, but this value is fairly uncertain compared to the more reliable estimates for the Denmark Strait and Faroe Bank Channel overflow branches[39].

As the overflow waters pass over the ridge, their temperature varies from about -0.5° C upward. A large proportion of the water is significantly colder than the 3° C value often used as a limit for the overflow (approximately equivalent to ? >27.8). After crossing the ridge, most of the overflow continues in two density- driven bottom currents that are constrained by the effects of the earth’s rotation (i.e., the Coriolis force) to follow the topography, although gradually descending. The bottom current waters undergo intensive mixing and entrain ambient waters from the Atlantic Ocean, which increases the water temperature. When the Denmark Strait and Faroe Bank Channel overflow waters join in the region southeast of Greenland, they have been warmed to 2 to 3° C, typical of the North Atlantic Deep Water. Through entrainment, enough Atlantic water is added to approximately double their volume transport.

Atlantic inflow

Inflow of Atlantic water to the Nordic Seas occurs across the Greenland–Scotland Ridge along its total extent except for the westernmost part of the Denmark Strait. Iceland and the Faroe Islands divide this flow into three branches (Fig. 9.7); the Iceland branch[40], the Faroe branch[41], and the Shetland branch[42]. There is a gradual change in water mass characteristics with the most southeastern inflow being the warmest (and most saline). There is also a difference in the volume fluxes, with that for the Iceland branch being much less than for the other two, which are similar in magnitude.

Fig. 9.7. The Arctic Mediterranean has four current branches that import water into the upper layers; three from the Atlantic (the Iceland, Faroe, and Shetland branches), and one from the Pacific. The outflow occurs partly at depth through the overflows and partly as surface (or upper-layer) outflow through the Canadian Archipelago and the East Greenland Current. The numbers indicate volume flux in Sverdrups (106 m3/s) rounded to half-integer values and are based on observations, with the exception of the surface outflow, which is adjusted to balance[1].

The Iceland branch flows northward on the eastern side of the Denmark Strait. North of Iceland, it turns east and flows toward the Norwegian Sea, but the heat and salt content of this branch are mixed with ambient water of polar or [[Arctic (Arctic Ocean)] Ocean] origin and freshwater runoff from land. By the time it reaches the east coast of Iceland it has lost most of its Atlantic character. The Faroe and Shetland branches flow directly into the Norwegian Sea. On their way they exchange water, but still appear as two separate current branches off the coast of northern Norway. Their relative contribution to various regions is not clarified in detail but the Barents Sea is clearly most affected by the inner (Shetland) branch, while the western Norwegian Sea and the Iceland Sea receive most of their Atlantic water from the outer (Faroe) branch.

Budgets

The horizontal exchanges between the Arctic and oceans to the south transfer water, heat, salt, and other properties such as nutrients and CO2. Since typical [[temperature]s], salinities, and concentrations of various properties are known, quantifying the exchanges is mainly a question of quantifying volume fluxes.

The water budget for the Arctic Ocean and the Nordic Seas as a whole is dominated by the Atlantic inflow and the overflow (Fig. 9.7). The Bering Strait inflow is fairly fresh (S<33) and most of it can be assumed to leave the Arctic Mediterranean in the surface outflow[43]. The deeper overflow is formed from Atlantic water, which means that 75% of the Atlantic inflow is ventilated in the Arctic Ocean and the Nordic Seas. Errors in the flux estimates may alter this ratio somewhat, but are not likely to change the conclusion that most of the Atlantic inflow exits via the deep overflow rather than in the surface outflow.

The question as to how the thermohaline ventilation is split between the Nordic Seas and the [[Arctic (Arctic Ocean)] Ocean] and its shelves can be addressed in different ways. One method is to measure the fluxes of the various current branches that flow between these two ocean areas; another is to estimate the amount of water produced by shelf convection. Both methods involve large uncertainties, but generally imply that most of the ventilation occurs in the Nordic Seas with perhaps up to 40% of the overflow water produced in the Arctic Ocean[44]. That most of the heat loss also appears to occur in the Nordic and Barents Seas[45] highlights the importance of these areas for the THC.

What drives the Atlantic inflow to the Arctic Mediterranean? (9.2.3.5)

The Atlantic inflow is responsible for maintaining high [[temperature]s] in parts of the marine Arctic and potential changes in the Atlantic inflow depend on the forces driving the flow. The few contributions to this discussion to be found in the literature[46] generally cite direct forcing by wind stress, estuarine circulation, or thermohaline circulation as being the main driving forces.

The freshwater input combined with entrainment generates southward outflows from the Arctic Mediterranean in the upper layers, which for continuity reasons require an inflow (estuarine circulation). Similarly, thermohaline ventilation generates overflows, which also require inflow (thermohaline circulation). If inflows do not match outflows, sea-level changes are induced, which generate pressure gradients that tend to restore the balance. To the extent that the water budget (Fig. 9.7) is reliable, it is therefore evident that the processes that generate the estuarine circulation can account for 2 Sv of the Atlantic inflow, whereas thermohaline ventilation is responsible for an additional 6 Sv. This has led some researchers to claim thermohaline ventilation as the main driving force for the Atlantic inflow[47].

Wind affects both the estuarine and the thermohaline circulation systems in many different ways (e.g., through entrainment, cooling, brine rejection, flow paths). Direct forcing by wind stress has also been shown to affect several current branches carrying Atlantic water[48], but there is no observational evidence for a strong direct effect of wind stress on the total Atlantic inflow to the Nordic Seas. On the contrary, Turrell et al.[49] and Hansen et al.[50] found that seasonal variation in the volume flux for the two main inflow branches (the Faroe Branch and Shetland Branch on Fig. 9.7) was negligible, in contrast to the strong seasonal variation in the wind stress. Thermohaline ventilation is also seasonal, but its effect is buffered by the large storage of dense water in the Arctic Mediterranean, which explains why the total overflow and hence also thermohaline forcing of the Atlantic inflow has only a small seasonal variation[51]. In a recent modeling study, Nilsen et al.[52] found high correlations between the North Atlantic Oscillation (NAO) index and the volume flux of Atlantic inflow branches, but that variations in the total inflow were small in relation to the average value.

These studies indicate that the Atlantic inflow to the Arctic Mediterranean is mainly driven by thermohaline and estuarine forcing, but that fluctuations at annual and shorter timescales are strongly affected by wind stress. Variations in wind stress also have a large influence on how the Atlantic water is distributed within the Arctic Mediterranean.

Chapter 9: Marine Systems
9.1. Introduction (Ocean processes of climatic importance in the Arctic)
9.2. Physical oceanography
9.2.1. General features (Ocean processes of climatic importance in the Arctic)
9.2.2. Sea ice (Sea ice effect on marine systems in the Arctic)
9.2.3. Ocean processes of climatic importance
9.2.4. Variability in hydrographic properties and currents
9.2.5. Anticipated changes in physical conditions
9.3. Biota
9.3.1. General description of the Arctic biota community
9.3.2. Physical factors mediating ecological change
9.3.3. Past variability – interannual to decadal
9.3.4. Future change – processes and impacts on biota
9.4. Effects of changes in ultraviolet radiation
9.5. The carbon cycle and climate change
9.6. Key findings (Ocean processes of climatic importance in the Arctic)
9.7. Gaps in knowledge and research needs

References

Citation

Committee, I. (2012). Ocean processes of climatic importance in the Arctic. Retrieved from http://editors.eol.org/eoearth/wiki/Ocean_processes_of_climatic_importance_in_the_Arctic
  1. Seager, R., D.S. Battisti, J. Yin, N. Gordon, N. Naik, A.C. Clement and M.A. Cane, 2002. Is the Gulf Stream responsible for Europe's mild winters? Quarterly Journal of the Royal Meteorological Society, 128:2563–2586.
  2. Lewis, E.L., E.P. Jones, P. Lemke,T.D. Prowse and P. Wadhams (eds.), 2000. The Freshwater Budget of the Arctic Ocean. Kluwer Academic Press, 623 pp.
  3. Aagaard, K. and E.C. Carmack, 1989. The role of sea ice and other fresh-water in the Arctic circulation. Journal of Geophysical Research - Oceans, 94(C10):14485–14498.–Carmack, E.C., 2000. The Arctic Ocean's freshwater budget: sources, storage and export. In E.L. Lewis, E.P. Jones, P. Lemke, T.D. Prowse and P. Wadhams (eds.). The Freshwater Budget of the Arctic Ocean, pp. 91–126. Kluwer Academic Press.
  4. Rudels, B., 1989. The formation of polar surface water, the ice export and the exchanges through the Fram Strait. Progress in Oceanography, 22:205–248.
  5. Carmack, E.C., 2000. The Arctic Ocean's freshwater budget: sources, storage and export. In E.L. Lewis, E.P. Jones, P. Lemke, T.D. Prowse and P. Wadhams (eds.). The Freshwater Budget of the Arctic Ocean, pp. 91–126. Kluwer Academic Press.–Rudels, B., 1993. High latitude ocean convection. In: D.B. Stone and S.K. Runcorn (eds.). Flow and Creep in the Solar System: Observations, Modeling and Theory, pp. 323–356. Kluwer Academic Press.
  6. Zimov, S.A.,Y.V.Voropaev, I.P. Semiletov, S.P. Davidov, S.F. Prosiannikov, F.S. Chapin III, M.C. Chapin, S.Trumbore and S.Tyler, 1997. North Siberian lakes: A methane source fueled by Pleistocene carbon. Science, 277:800–802.
  7. Hansen, B. and S. Østerhus, 2000. North Atlantic–Nordic Seas exchanges. Progress in Oceanography, 45:109–208.
  8. Morison, J.H. and J.D. Smith, 1981. Seasonal variations in the upper Arctic Ocean as observed at T-3. Geophysical Research Letters, 8:753–756.
  9. McPhee, M.G. and J.H. Morison, 2001. Turbulence and diffusion: Under-ice boundary layer. In: J. Steele, S. Thorpe and K. Turekian (eds.). Encyclopedia of Ocean Sciences, pp 3071–3078. Academic Press.
  10. Aagaard, K., 1970. Wind-driven transports in the Greenland and Norwegian seas. Deep-Sea Research and Oceanographic Abstracts, 17:281–291.–Isachsen, P.E., J.H. LaCasce, C. Mauritzen and S. Häkkinen, 2003. Wind-driven variability of the large-scale recirculating flow in the Nordic Seas and Arctic Ocean. Journal of Physical Oceanography, 33:2534–2550.–Jónsson, S. 1991, Seasonal and interannual variability of wind stress curl over the Nordic Seas. Journal of Geophysical Research, 96(C2):2649–2659.
  11. Ingvaldsen, R., H. Loeng and L. Asplin, 2002. Variability in the Atlantic inflow to the Barents Sea based on a one-year time series from moored current meters. Continental Shelf Research, 22:505–519.– Morison, J.H., 1991. Seasonal fluctuations in the West Spitsbergen Current estimated from bottom pressure measurements. Journal of Geophysical Research, 96 (C10):18381–18395.–Orvik, K.A. and Ø. Skagseth, 2003. The impact of the wind stress curl in the North Atlantic on the Atlantic inflow to the Norwegian Sea toward the Arctic. Geophysical Research Letters, 30(17), doi: 10.1029/2003GL017932.–Roach, A.T., K. Aagaard, C.H. Pease, S.A. Salo, T. Weingartner, V. Pavlov and M. Kulakov, 1995. Direct measurements of transport and water properties through the Bering Strait. Journal of Geophysical Research, 100(C9):18443–18458.
  12. Maslowski, W., B. Newton, P. Schlosser, A. Semtner and D. Martinson, 2000. Modelling recent climate variability in the Arctic Ocean. Geophysical Research Letters, 27(22):3743–3746.–Maslowski, W., D.C. Marble, W. Walczowski and A.J. Semtner, 2001. On large scale shifts in the Arctic Ocean and sea-ice conditions during 1979–98. Annals of Glaciology, 33:545–550.–Zhang, J., D. Rothrock and M. Steele, 2000. Recent changes in arctic sea ice: The interplay between ice dynamics and thermodynamics. Journal of Climate, 13(17):3099–3114.
  13. Carmack, E.C. and D. Chapman, 2003. Wind-driven shelf/basin exchange on an Arctic shelf: The joint roles of ice cover extent and shelf-break bathymetry. Geophysical Research Letters, 30:1778, doi:10.1029/2003GL017526.
  14. Broecker, W.S., D.M. Peteet and D. Rind, 1985. Does the ocean-atmosphere system have more than one stable mode of operation? Nature, 315:21–26.
  15. Seager, R., D.S. Battisti, J. Yin, N. Gordon, N. Naik, A.C. Clement and M.A. Cane, 2002. Is the Gulf Stream responsible for Europe's mild winters? Quarterly Journal of the Royal Meteorological Society, 128:2563–2586.
  16. Carmack, E.C., 1990. Large-scale physical oceanography of polar oceans. In: W.O. Smith Jr. (ed.). Polar Oceanography, Part A, Physical Science, pp. 171–222. Academic Press.–Hopkins, T.S., 1991.The GIN Sea – A synthesis of its physical oceanography and literature review 1972–1985. Earth-Science Reviews, 30:175–318.
  17. Aagaard, K., J.H. Swift and E.C. Carmack, 1985. Thermohaline circulation in the Arctic Mediterranean seas. Journal of Geophysical Research - Oceans, 90:4833–4846.
  18. Simonsen, K. and P.M. Haugan, 1996. Heat budgets of the Arctic Mediterranean and sea surface heat flux parameterizations for the Nordic Seas. Journal of Geophysical Research, 101(C3):6553–6576.
  19. Simonsen, K. and P.M. Haugan, 1996. Heat budgets of the Arctic Mediterranean and sea surface heat flux parameterizations for the Nordic Seas. Journal of Geophysical Research, 101(C3):6553–6576.
  20. Carmack, E.C., 1986. Circulation and mixing in ice-covered waters. In: N. Untersteiner (ed.). Geophysics of Sea Ice, NATO ASI Series B, 146:641–712.
  21. Anderson, L.G., E.P. Jones and B. Rudels, 1999. Ventilation of the Arctic Ocean estimated by a plume entrainment model constrained by CFCs. Journal of Geophysical Research - Oceans, 104:13423–13429.
  22. Rudels, B., E. Fahrbach, J. Meincke, G. Budéus and P. Eriksson, 2002. The East Greenland Current and its contribution to the Denmark Strait overflow. ICES Journal of Marine Science, 59:1133–1154.
  23. Budéus, G.,W. Schneider and G. Krause, 1998. Winter convective events and bottom water warming in the Greenland Sea. Journal of Geophysical Research - Oceans, 103:18513–18527.–Gascard, J.-C., A.J. Watson, M.-J. Messias, K.A. Olsson, T. Johannessen and K. Simonsen, 2002. Long-lived vortices as a mode of deep ventilation in the Greenland Sea. Nature, 416:525–527.–Marshall, J. and F. Schott, 1999. Open-ocean convection: Observations, theory, and models. Reviews of Geophysics, 37(1):1–64.–Watson, A.J., M.-J. Messias, E. Fogelqvist, K.A.Van Scoy, T. Johannessen, K.I.C. Oliver, D.P. Stevens, F. Rey,T. Tanhua, K.A. Olsson, F. Carse, K. Simonsen, J.R. Ledwell, E. Jansen, D.J. Cooper, J.A. Kruepke and E. Guilyardi, 1999. Mixing and convection in the Greenland Sea from a tracer release experiment. Nature, 401:902–905.
  24. Marshall, J. and F. Schott, 1999. Open-ocean convection: Observations, theory, and models. Reviews of Geophysics, 37(1):1–64.
  25. Swift, J.H. and K. Aagaard, 1981. Seasonal transitions and water mass formation in the Iceland and Greenland seas. Deep-Sea Research A, 28(10):1107–1129.
  26. Gascard, J.-C., A.J. Watson, M.-J. Messias, K.A. Olsson, T. Johannessen and K. Simonsen, 2002. Long-lived vortices as a mode of deep ventilation in the Greenland Sea. Nature, 416:525–527.
  27. Malmberg, S.-A., 1983. Hydrographic investigations in the Iceland and Greenland Seas in late winter 1971- "Deep Water Project." Jokull, 33:133–140.
  28. Jones, E.P., B. Rudels, and L.G. Anderson, 1995. Deep waters of the Arctic Ocean: origins and circulation. Deep-Sea Research I, 42:737–760.–Rudels, B., E.P. Jones, L.G. Anderson and G. Kattner, 1994. On the intermediate depth waters of the Arctic Ocean. In: O.M. Johannessen, R.D. Muench, and J.E. Overland (eds.). The Polar Oceans and their Role in Shaping the Global Environment. Geophysical Monograph Series, 85:33–46. American Geophysical Union, Washington D.C.–Rudels, B., H.J. Friedrich and D. Quadfasel, 1999. The Arctic Circumpolar Boundary Current. Deep-Sea Research II, 46:1023–1062.
  29. Jones, E.P., B. Rudels, and L.G. Anderson, 1995. Deep waters of the Arctic Ocean: origins and circulation. Deep-Sea Research I, 42:737–760.–Quadfasel, D., B. Rudels and K. Kurz, 1988. Outflow of dense water from a Svalbard fjord into the Fram Strait. Deep-Sea Research A, 35:1143–1150.–Rudels, B., 1986. The theta-S relations in the northern seas: Implications for the deep circulation. Polar Research, 4:133–159.–Rudels, B., E.P. Jones, L.G. Anderson and G. Kattner, 1994. On the intermediate depth waters of the Arctic Ocean. In: O.M. Johannessen, R.D. Muench, and J.E. Overland (eds.). The Polar Oceans and their Role in Shaping the Global Environment. Geophysical Monograph Series, 85:33–46. American Geophysical Union, Washington D.C.
  30. Dickson, R.R. and J. Brown, 1994. The production of North Atlantic Deep Water: sources, rates and pathways. Journal of Geophysical Research, 99(C6):12,319–12,342.
  31. Blindheim, J. and B. Ådlandsvik, 1995. Episodic formation of intermediate water along the Greenland Sea Arctic Front. ICES CM 1995/Mini:6. International Council for the Exploration of the Sea, Copenhagen, 11pp.
  32. Dickson, R.R. and J. Brown, 1994. The production of North Atlantic Deep Water: sources, rates and pathways. Journal of Geophysical Research, 99(C6):12,319–12,342.
  33. Dickson, R.R. and J. Brown, 1994. The production of North Atlantic Deep Water: sources, rates and pathways. Journal of Geophysical Research, 99(C6):12,319–12,342.
  34. Mauritzen, C., 1996. Production of dense overflow waters feeding the North Atlantic across the Greenland-Scotland Ridge. Deep-Sea Research I, 43(6):769–835.
  35. Rudels, B., E. Fahrbach, J. Meincke, G. Budéus and P. Eriksson, 2002. The East Greenland Current and its contribution to the Denmark Strait overflow. ICES Journal of Marine Science, 59:1133–1154.
  36. Strass, V.H., E. Fahrbach, U. Schauer and L. Sellmann, 1993. Formation of Denmark Strait Overflow Water by mixing in the East Greenland Current. Journal of Geophysical Research, 98(C4):6907–6919.
  37. Jónsson, S., 1999. The circulation in the northern part of the Denmark Strait and its variability. ICES CM 1999/L:06. International Council for the Exploration of the Sea, Copenhagen, 9pp.–Swift, J.H. and K. Aagaard, 1981. Seasonal transitions and water mass formation in the Iceland and Greenland seas. Deep-Sea Research A, 28(10):1107–1129.
  38. Saunders, P.M., 2001. The dense northern overflows. In: G. Siedler, J. Church and J. Gould (eds.). Ocean Circulation and Climate: Observing and Modelling the Global Ocean, pp. 401–417. Academic Press.
  39. Hansen, B. and S. Østerhus, 2000. North Atlantic–Nordic Seas exchanges. Progress in Oceanography, 45:109–208.
  40. Jónsson, S. and Briem, J., 2003. Flow of Atlantic Water west of Iceland and onto the North Icelandic Shelf. ICES Marine Science Symposia, 219:326–328.
  41. Hansen, B., S. Østerhus, H. Hátún, R. Kristiansen and K.M.H. Larsen, 2003. The Iceland-Faroe inflow of Atlantic Water to the Nordic Seas. Progress in Oceanography, 59:443–474.
  42. Turrell, W.R., B. Hansen, S. Hughes and S. Østerhus, 2003. Hydrographic variability during the decade of the 1990s in the Northeast Atlantic and southern Norwegian Sea. ICES Marine Science Symposia, 219:111–120.
  43. Rudels, B., 1989. The formation of polar surface water, the ice export and the exchanges through the Fram Strait. Progress in Oceanography, 22:205–248.
  44. Rudels, B., H.J. Friedrich and D. Quadfasel, 1999. The Arctic Circumpolar Boundary Current. Deep-Sea Research II, 46:1023–1062.
  45. Simonsen, K. and P.M. Haugan, 1996. Heat budgets of the Arctic Mediterranean and sea surface heat flux parameterizations for the Nordic Seas. Journal of Geophysical Research, 101(C3):6553–6576.
  46. Hopkins, T.S., 1991.The GIN Sea – A synthesis of its physical oceanography and literature review 1972–1985. Earth-Science Reviews, 30:175–318.
  47. Hansen, B. and S. Østerhus, 2000. North Atlantic–Nordic Seas exchanges. Progress in Oceanography, 45:109–208.
  48. Ingvaldsen, R., H. Loeng and L. Asplin, 2002. Variability in the Atlantic inflow to the Barents Sea based on a one-year time series from moored current meters. Continental Shelf Research, 22:505–519.–Isachsen, P.E., J.H. LaCasce, C. Mauritzen and S. Häkkinen, 2003. Wind-driven variability of the large-scale recirculating flow in the Nordic Seas and Arctic Ocean. Journal of Physical Oceanography, 33:2534–2550.–Morison, J.H., 1991. Seasonal fluctuations in the West Spitsbergen Current estimated from bottom pressure measurements. Journal of Geophysical Research, 96 (C10):18381–18395.–Orvik, K.A. and Ø. Skagseth, 2003. The impact of the wind stress curl in the North Atlantic on the Atlantic inflow to the Norwegian Sea toward the Arctic. Geophysical Research Letters, 30(17), doi: 10.1029/2003GL017932.
  49. Turrell, W.R., B. Hansen, S. Hughes and S. Østerhus, 2003. Hydrographic variability during the decade of the 1990s in the Northeast Atlantic and southern Norwegian Sea. ICES Marine Science Symposia, 219:111–120.
  50. Hansen, B., S. Østerhus, H. Hátún, R. Kristiansen and K.M.H. Larsen, 2003. The Iceland-Faroe inflow of Atlantic Water to the Nordic Seas. Progress in Oceanography, 59:443–474.
  51. Dickson, R.R. and J. Brown, 1994. The production of North Atlantic Deep Water: sources, rates and pathways. Journal of Geophysical Research, 99(C6):12,319–12,342.–Hansen, B., W.R. Turrell and S. Østerhus, 2001. Decreasing overflow from the Nordic seas into the Atlantic Ocean through the Faroe Bank channel since 1950. Nature, 411:927–930.–Jónsson, S., 1999. The circulation in the northern part of the Denmark Strait and its variability. ICES CM 1999/L:06. International Council for the Exploration of the Sea, Copenhagen, 9pp.
  52. Nilsen, J.E.O., Y. Gao, H. Drange, T. Furevik and M. Bentsen, 2003. Simulated North Atlantic-Nordic Seas water mass exchanges in an isopycnic coordinate OGCM. Geophysical Research Letters, 30(10):1536, doi: 10.1029/2002GL016597.