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On-line Supplementary Material
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Feedbacks and Interactions: From the Arctic cryosphere to the climate system
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Terry V. Callaghan, Margareta Johansson, Jeff Key, Terry Prowse, Maria
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Ananicheva, Alexander Klepikov
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Quantification of recent budget estimates
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Total freshwater inputs to the Arctic Ocean, calculated at about 8500 km3 y-1, are dominated by river flow (38%),
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inflow through the Bering Strait (30%), and precipitation-evaporation directly occurring on the Arctic Ocean (24%)
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(Serreze et al. 2006). Importantly, this estimate is an order of magnitude smaller than the total amount stored in
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the Arctic Ocean. Freshwater exports from the Arctic Ocean occur principally through the straits of the Canadian
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Arctic Archipelago (35%) and via Fram Strait as liquid (26%) and sea ice (25%). For such calculations, the
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volumes of freshwater are referenced to a mean ocean salinity of 34.8 ‰ (parts per thousand).
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Also recognizing that a steady state of the freshwater budget should not be expected, Serreze et al. (2006) noted
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that their values indicate larger freshwater inflow through Bering Strait and larger liquid freshwater outflow
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through Fram Strait than earlier estimates by others.
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Peterson (2006) conducted an analysis of changes in freshwater budget components for a broader ‘Arctic region’
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than Serreze et al. (2006), which included the additional large land-ocean catchment of Hudson Bay in North
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America as well as the Nordic Seas and North Atlantic subpolar basins. Increasing precipitation-evaporation over
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the marine environments and larger river flow, probably also tied to increases in high-latitude precipitation, was
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estimated to have contributed ~20 000 km3 of freshwater to the total region although these contributions varied
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from low values in the 1960s to high values in the 1990s.
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Notably, the river trend included a decline in flow for the Hudson Bay system. Sea-ice ablation added a slightly
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smaller amount of ~15 000 km3, and glacial melt added ~2000 km3. Due to the lack of complete mass-balance
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estimates for the Greenland Ice Sheet, its contributions were excluded from the latter amount but it was noted to
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have been ~80 km3 y-1 in the 1990s and to have recently increased to ~220 km3 y-1 (Peterson et al. 2006). The
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most recent estimate of current annual loss of ice from the Greenland Ice Sheet is similar at about 205 ± 50 Gt yr-
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(2005, 2006) (AMAP 2011).
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Freshwater budget components and changes
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Snowmelt and river discharge
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Compared to all other world oceans, the Arctic Ocean receives a disproportionately large amount of river runoff to
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its total volume via the Lena, Mackenzie, Ob, and Yenisey rivers that are dominantly nival rivers. River flow
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provides the largest input to the Arctic Ocean freshwater budget (Prowse and Flegg 2000). Although the
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seasonality of observed multi-decadal increases in total Eurasian river flow to the Arctic Ocean has not been fully
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examined, (AMAP 2011) note that some increases in the magnitude and advances in timing of the snowmelt
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freshet on northern rivers have been observed and greater changes are expected in the future. This will be due
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not only to changes in the size and seasonality of snowmelt and glacier melt but also to the effects of thawing
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permafrost in changing flow pathways and storage. Such changes in the timing and magnitude of flows are
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important to how river water is distributed, directed and/or stored in the Arctic Ocean (e.g. Cooper et al. 2008;
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Jones et al. 2008).
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Small mountain glaciers, ice caps, and the Greenland Ice Sheet
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The overall freshwater contribution from the glaciers (328 556 km2) in a broadly defined, pan-Arctic drainage
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basin, as well as small ice caps around the Greenland Ice Sheet (but not including the ice sheet itself), is ~1700
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km3 for the period 1961 to 2001 (Dyurgerov and Carter, 2004). Freshwater contributions from these glaciers
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varied annually from near zero to just under 200 km3, far less than contributed by the corresponding nine major
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rivers (5282 km3 y-1) within the same pan-Arctic region. However, there is a greater ‘positive change signal’ from
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the glaciers than river discharge (Dyurgerov and Carter, 2004). Partly included in this total are the glaciers from
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the Alaska-Yukon region, which contribute to the North Pacific Ocean waters near the principal Arctic Ocean
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inflow point, the Bering Strait. These glaciers have experienced some of the most rapid wastage. In the case of
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Alaska, for example, freshwater contributions have increased from 52 ± 15 km3 y-1 over the 1950s to 1990s
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period to ~96 km3 y-1 from the mid-1990s to 2001 (Arendt et al. 2002), and more recent sampling suggests
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increased ablation particularly at low elevations. In reference to the Bering Strait to which these glaciers
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contribute, flow volumes to the Arctic Ocean have been significantly revised upward (i.e. by ~50% to ~2500 km3 y-
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remains unknown.
Woodgate and Aagaard 2005), although the exact contribution of ablating glaciers to such volume increases
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Some freshwater budget analyses contain little discussion about the role of the Greenland Ice Sheet, despite its
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strategic placement as a freshwater source in the North Atlantic. In addition to freshwater volume, the location of
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the input may also be important (Randall et al. 2007) and meltwater runoff from the ice sheet is potentially a major
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source of freshening that has not yet been included in relevant models (Randall et al. 2007).
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Sea ice
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The Arctic Ocean is a salt rather than temperature stratified ocean and hence, sea ice growth/ablation and ocean
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dynamics can be greatly modified by changes in freshwater. It is the salt-stratified upper layers that provide the
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vertical stability to permit formation of an ice cover. Surface freshwater layers, however, also contribute with the
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halocline to thermally shield sea ice from bottom melt, which can be driven by the large quantities of heat
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contained in warmer, deeper water originating from the Atlantic Ocean. Carmack (2000) estimated that the
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volume of ice forming and melting each year amounted to a 1.45 m equivalent of freshwater (depth of freshwater
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across the ocean) assuming an un-deformed ice cover, and an additional 0.45 to 0.7 m of freshwater if the mass
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in ice ridges is also considered. However, sea ice has undergone significant changes in areal coverage and
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thickness.
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For some key episodic losses, sea ice-bottom melt has been linked to solar heating of the upper ocean (Perovich
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et al. 2008). Bottom melt can also result from the loss of thermal insulation from the warmer Atlantic water
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provided by the surface layers of freshwater and cold halocline. The stability of these upper layers, particularly
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with enhanced vertical mixing, has been identified as a ‘key wild card’ regarding future sea-ice loss (Serreze et al.
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2007). Sea ice is also exported through Fram Strait along with sea ice meltwater, but export of sea ice meltwater
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seems to be the least likely to influence thermohaline circulation (Jones et al. 2008). This could play a large role
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in changing ice conditions.
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Ocean storage and pathways
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The fate of sea ice meltwater and other forms of freshwater is not simply direct export because the Arctic Ocean
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also holds significant freshwater in storage with variable releases. While about one-quarter of the total is held on
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shelves, the majority is in the Eurasian and Canada basins, the latter being the largest single freshwater
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storehouse in the Arctic Ocean. This has been ascribed to the deep halocline of Canada Basin, which stores
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freshwater from sea ice meltwater, meteoric water (ocean precipitation and terrestrial runoff), and low-salinity
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Pacific water from the Bering Strait. Estimates of this storage (like the other freshwater budget terms) vary in the
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literature, and are primarily due to changes in import-export to Canada Basin and, in the accuracy and ability to
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measure its content. Despite variations in its estimated volume (e.g. ~46 000 km3; Carmack 2000) and 25 600
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km3 (Yamamoto-Kawai et al. 2008), it is generally accepted that the largest source of the average annual
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freshwater input (~3200 km3) to this and other freshwater storehouses in the Arctic Ocean (Yamamoto-Kawai et
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al. 2009) is river runoff – estimated by Yamamoto-Kawai et al. (2008) for Canada Basin to be 800 km3 y-1 and to
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be just slightly smaller than the amount removed by sea ice formation (900 km3 y-1). They further estimate that the
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average export of ice and liquid freshwater from Canada Basin contributes ~40% of the freshwater flux to the
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North Atlantic.
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The storage values vary with time due to atmospheric circulation, which can control pathways of freshwater
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to/from storage basins as well as the storage/release from within the storage basins (i.e. Ekman pumping, which
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under anticyclonic [cyclonic] circulation stores [releases] freshwater). Most recently, measurements from Canada
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and Makarov basins indicate that there has been a freshwater storage increase of up to 8500 km3, and by
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extrapolation, almost 11 000 km3 in all the deep basins of the western Arctic. By contrast, the Eurasian Basin in
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the eastern Arctic and closer to the main export to the North Atlantic has experienced a loss of about 3300 km3,
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giving a net gain of 7700 km3 (McPhee et al. 2009). This is a significant increase being approximately four times
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the volume associated with the Great Salinity Anomaly (a near-surface pool of fresher-than-usual water tracked in
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the subpolar gyre currents from around 1968 to 1982, which affected regional climate) and similar in magnitude to
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the total 1981 to 1995 sea ice attrition (melt plus export) estimated in the above noted freshwater budget by
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Peterson et al. (2006).
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Model projections
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At the time of the Arctic Climate Impact Assessment (ACIA 2005), there was a concern about the intensification of
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the hydrological cycle at high latitudes and the effect this would have on the AMOC. ACIA noted that projections
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by atmosphere-ocean GCMs produced varying results with changes in the maximum strength of the AMOC by
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the end of the 21st century ranging from zero to a reduction of 30–50%. The suite of ACIA-selected GCMs
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(Kattsov and Källén 2005) did not include freshwater runoff from melting ice sheets and glaciers. Hence, it was
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surmised that the model AMOC sensitivity to global climate change might be too weak, although sensitivity
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experiments indicate freshwater runoff must be several times ‘present-day’ volumes to appreciably alter the
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AMOC.
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A strong scientific debate remains about the potential significance of freshwater effects on the AMOC (Randall et
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al. 2007). For example, Holland et al. (2007) noted that a constituent result among models for the period 1950 to
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2050 (observations and modeled results) is an acceleration of the hydrological cycle, including increased ocean
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net-precipitation, river runoff, and net sea-ice melt. They also noted, for liquid water, a larger export to lower
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latitudes, primarily through Fram Strait, and storage in the Arctic Ocean. By contrast, export and storage of
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freshwater in the form of sea ice decreases, although there is significant variability in sea-ice budget terms.
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Largely similar results were reported by Koenigk et al. (2007).
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A number of efforts are underway to more accurately define the role of freshwater in AMOC weakening, the
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Coupled Model Intercomparison Project (CMIP) and Paleoclimate Modelling Intercomparison Project (PMIP)
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coordinated effort being major examples. All models used in CMIP show that AMOC weakening projected for the
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21st century is caused more by changes in the surface heat flux than by freshwater, although its effect on high-
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latitude stratification plays a contributing role (Arzel et al. 2008). They further noted that interannual exchanges in
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freshwater between the GIN Seas and the North Atlantic have a major driving influence on the interannual
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variability of deep convection over the 21st century.
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References
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ACIA. 2005. Arctic Climate Impact Assessment. Cambridge University Press, 1042 pp.
AMAP. 2011. Snow, Water, Ice and Permafrost in the Arctic (SWIPA). Arctic Monitoring and Assessment Programme
(AMAP), Oslo.
Arendt, A.A., K.A. Echelmeyer, W.D. Harrison, C.S. Lingle and V.B. Valentine. 2002. Rapid wastage of Alaskan glaciers and
their contribution to rising sea level. Science 297: 382-386.
Arzel, O., T. Fichefet, H. Goosse and J-L. Dufresne. 2008. Causes and impacts of changes in the Arctic freshwater budget
during the twentieth and twenty-first centuries in an AOGCM. Climate Dynamics 30:37–58.
Carmack, E.C. 2000. The Arctic Ocean's freshwater budget: Sources, storage and export. In: The Freshwater Budget of the
Arctic Ocean, ed E.L. Lewis, E.P. Jones, P. Lemke, T.D. Prowse and P. Wadhams, p. 91-126., , Dordrecht: Kluwer
Academic Publishers.
Cooper, L.W., J.W. McClelland, R.M. Holmes, P.A. Raymond, J.J. Gibson, C.K. Guay and B.J. Peterson. 2008. Flowweighted values of runoff tracers (d18O, DOC, Ba, alkalinity) from the six largest Arctic rivers. Geophysical Research
Letters 35: L18606, doi:10.1029/2008GL035007.
Dyurgerov, M.B. and C.L. Carter. 2004. Observational evidence of increases in freshwater inflow to the Arctic Ocean. Arctic,
Antarctic and Alpine Research 36: 11s7-122.s
Holland, M.M., J. Finnis, A.P. Barrett and M.C. Serreze. 2007. Projected changes in Arctic Ocean freshwater budgets.
Journal of Geophysical Research 112: G04S55, doi:10.1029/2006JG000354.
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174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
Jones, E.P., L.G. Anderson, S. Jutterström, L. Mintrop and J.H. Swift. 2008. Pacific freshwater, river water and sea ice
meltwater across Arctic Ocean basins: Results from the 2005 Beringia Expedition. Journal of Geophysical Research
113: C08012, doi:10.1029/2007JC004124.
Kattsov, V.M. and E. Källén. 2005. Future climate change: Modeling and scenarios for the Arctic. Chapter 4 in Arctic Climate
Impact Assessment, pp 99-150, Cambridge, UK: Cambridge University Press.
Koenigk, T., U. Mikolajewicz, H. Haak and J. Jungclaus. 2007. Arctic freshwater export in the 20th and 21st centuries. Journal
of Geophysical Research 112: GS04S41, doi:10.1029/2006JG000274.
McPhee, M.G., A. Proshutinsky, J.H. Morison, M. Steele and M. B. Alkire. 2009. Rapid change in freshwater content of the
Arctic Ocean. Geophysical Research Letters 36: L10602, doi:10.1029/2009GL037525.
Perovich, D.K., J.A. Richter-Menge, K.F. Jones and B. Light. 2008. Sunlight, water, and ice: Extreme Arctic sea melt during
the summer of 2007. Geophysical Research Letters 35: L11501, doi:10.1029/2008GL034007.
Peterson, B.J., J. McClelland, R. Curry, R.M. Holmes, J.E. Walsh and K. Aagaard. 2006. Trajectory shifts in the Arctic and
subarctic freshwater cycle. Science 313: 1061-1066.
Prowse, T.D. and P.O. Flegg. 2000. The magnitude of river flow to the Arctic Ocean: dependence on contributing area.
Hydrological Processes 14(16-17): 3185-3188.
Randall, D.A., R.A. Wood, S. Bony, R. Colman, T. Fichefet, J. Fyfe, V. Kattsov, A. Pitman, et al. 2007. Climate Models and
Their Evaluation. In: Climate Change 2007: the Physical Science Basis. Contribution of Working Group I to the Fourth
Assessment Report of the Intergovernmental Panel on Climate Change, ed Solomon, S., D. Quin, M. Manning, Z.
Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller. United Kingdom and New York, NY, USA: Cambridge
University Press, Cambridge, , pp. 589-662.
Serreze, M.K., A.P. Barrett, A.G. Slater, R.A. Woodgate, K. Aagaard, R.B. Lammers, M. Steele, R. Moritz, M. Meredith and
C.M. Lee. 2006. The large-scale freshwater cycle of the Arctic. Journal of Geophysical Research 111: C11010.
doi:10,1029/2005JC003424,
Serreze, M.C., M.M. Holland and J. Stroeve. 2007. Perspectives on the Arctic’s shrinking sea-ice cover. Science 315: 15331536.
Woodgate, R.A. and K. Aagard. 2005. Revising the Bering Strait freshwater flux into the Arctic Ocean. Geophyscial Research
Letters 32: L02602. doi:10.1029/2004GL021747.
Yamamoto-Kawai, M., F.A. McLaughlin, E.C. Carmack, S. Nishino and K. Shimada. 2008. Freshwater budget of the Canada
Basin, Arctic Ocean, from salinity, δ180, and nutrients. Journal of Geophysical Research 113:C01007.
doi:10.1029/2006JC003858.
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215
216
217
218
Yamamoto-Kawai, M., F.A. McLaughlin, E.C. Carmack, S. Nishino, K. Shimada and N. Kurita. 2009. Surface freshening of
the Canada Basin, 2003-2007: River runoff versus seas ice meltwater. Journal of Geophysical Research 114:
C00A05. doi:10.1029/2008JC005000.
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