Info

Changes in both surface sources (P - E + R and sea ice melting) and lateral transports can lead to freshwater content changes in the Arctic Ocean. Surface fluxes are likely to be the dominant factors for future changes as global warming and the atmospheric hydrological cycle intensify. For the 20th century changes, lateral transports may have played a more important role (Wu et al. 2007). Proshutinsky et al. (2002) suggested that anomalous freshwater storage within the anticyclonic Beaufort Gyre can be potentially larger than changes in river runoff and sea ice export. Häkkinen and Proshutinsky (2004) and Hátún et al. (2005) emphasise the contribution of the Atlantic water inflow. Jungclaus et al. (2005), Wu and Wood (2007) have all realised the importance of freshwater exchange between the Arctic and the subpolar North Atlantic in affecting basin scale freshwater content changes. Wu and Wood (2007) have shown that anomalous atmospheric conditions such as the winter of 1971/72 may cause a circulation regime change within the Arctic/subpolar North Atlantic Ocean system that has a long lasting effect on water exchanges through the Greenland-Iceland-Scotland (GIS) straits. Freshwater redistribution following such circulation changes can lead to substantial freshwater content changes comparable to the recent freshening trend reported by Dickson et al. (2002), Curry and Mauritzen (2005). The GSA can now be well simulated in climate models (Haak et al. 2003; Koenigk et al. 2006a; Wadley and Bigg 2004, 2006). Haak et al. (2003) suggested from their model simulations that the GSA is linked to anomalous sea ice export through Fram Strait driven by anomalous atmospheric circulation. On the other hand Houghton and Visbek (2002), Wadley and Bigg (2006) have recently questioned the advective nature of the GSA.

15.7 Conclusions

Increasing greenhouse gas concentrations in the atmosphere have a disproportionate impact on polar climates relative to global warming. Enhanced warming due to polar amplification, first pointed out by Manabe and Stouffer (1980), is now a well recognised phenomenon (see, e.g., Holland and Bitz 2003; ACIA 2005). Additional freshwater input due to increased moisture transport from the subtropics and river discharges has made another distinction for the polar regions under an accelerating global hydrological cycle (Wu et al. 2005; Stocker and Raible 2005). Changes in the global freshwater cycle will directly affect the distribution of water resources worldwide (see, Oki and Kanae 2006, for a recent review). Changing patterns and severity of droughts and floods will be parts of its climate impact on regional scales. Extra freshwater input into the Arctic/subarctic oceans has another worrying consequence on the climate system. This is its potential of diluting the northern polar oceans where deep convection occurs and the associated weakening the Atlantic thermohaline circulation (THC, e.g. Vellinga and Wood 2002; Wu et al. 2004; Curry and Mauritzen 2005). Moreover, meltwater input from a disintegrating Greenland ice sheet could further accelerate the THC weakening (Fichefet et al. 2003; Jungclaus et al. 2006b).

There are already signs of systematic changes in the Arctic/subarctic freshwater cycle (Dickson et al. 2002; Curry et al. 2003; Curry and Mauritzen 2005; Peterson et al. 2006). In order to understand and attribute the observed changes to different causes, long climate records and comprehensive computer models are needed to expand our research into further depth and accuracy. Having described the progress in simulating the terms in the Arctic hydrological budget above, it is clear that there are weak areas in both observations and modelling. Because the polar regions are highly sensitive parts of the global hydrological cycle, we need observations to be more reliable, continuous with better coverage for monitoring global changes. We need climate models to resolve more detailed processes and feedbacks in simulating precipitation, evaporation, sea ice and land hydrology. We need better estimates of the magnitude and variability of the Arctic/subarctic hydrological budgets. As modellers, we would like to use increasingly more observational measurements to validate and constrain climate model simulations. In the meantime, we would also like to use our models to help understand the mechanisms of observed variability and change, to attribute them to different possible causes, and to use model projections to guide future observational efforts.

There are competing sources of freshwater adding to the Arctic/subarctic oceans as global warming continues. At the present, there are considerable uncertainties even for the climatological means for the individual contributors from both observational estimates and climate model simulations (see Fig. 15.1). Large differences also exist between model simulated budget terms and observationally based estimates, as well as among different models. Those uncertainties in the means will undoubtedly overshadow any predicted budget and trends. We should aim to achieve an observationally constrained, multi-model ensemble prediction of the Arctic freshwater budget such as the one shown in Table 15.1 in the near future. It will enable us to answer the following questions: What is the likely upper bound of freshwater input into the Arctic/subarctic oceans? How much of that is likely to be realised over the next 50 or 100 years? Which component is likely to play a leading role? Besides, how can we best use the Arctic as an indicator for monitoring the global hydrological cycle? To complete these tasks will require concerted efforts from both the observational and the modelling communities.

Acknowledgements Peili Wu and Richard Wood are funded by the UK Department of Environment, Food and Rural Affairs under the Climate Prediction Programme (PECD/7/12/37). Tore Furevik has been supported by the Norwegian Research Council through the NoClim project. We thank Jeff Ridley and Jonathan Gregory for helpful comments on the review of Greenland ice sheet.

References

Aagaard K, Carmack EC (1989) The role of sea ice and other fresh-water in the Arctic circulation. J Geophys Res, 4 (C10):14485-14498.

ACIA (2005) Arctic Climate Impact Assessment: Scientific report. Cambridge University Press, Cambridge.

Allen MR, Ingram WJ (2002) Constraints on the future changes in climate and the hydrological cycle. Nature, 419:224-232.

Arnell NW (2005) Implications of climate change for freshwater inflows to the Arctic Ocean. J Geophys Res, 110:D07105.

Arzel O, Fichefet T, Goosse H (2006) Sea ice evolution over the 20th and 21st centuries as simulated by current AOGCMs. Ocean Modelling, 12 (3-4):401-415.

Belkin IM, Levitus S, Antonov J (1998) Great salinity anomalies in the North Atlantic, Prog Oceanogr, 41:1-68

Bethke I, Furevik T, Drange H (2006) Towards a more saline North Atlantic and a fresher Arctic under global warming. Geophys Res Lett, 33:L21712.

Blindheim J (1989) Cascading of Barents Sea bottom water into the Norwegian Sea, Rapports et Procès-Verbaux des Réunions/ Conseil Permanent International pour l'Exploration del la Mer, 17:161-189

Carmack EC (2000) The Arctic oceans freshwater budget: Sources, storage and export. In: Lewis EL et al. (eds.), The Freshwater Budget of the Artic Ocean, Kluwer, Dordrecht, The Netherlands, pp 91-126.

Chen JL, Wilson CR, Tapley BD (2006) Satellite gravity measurements confirm accelerated melting of Greenland ice sheet. Science, 313:1958-1960.

Comiso JC (2002) A rapidly declining perennial ice cover in the Arctic. Geophys Res Lett, 29:1956.

Cullather RI, Bromwich DH, Serreze MC (2000) The atmospheric hydrologic cycle over the Arctic basin from reanalyses. Part I: Comparison with observations and previous studies. J Clim, 13:923-937.

Curry R, Mauritzen C (2005) Dilution of northern North Atlantic Ocean in recent decades. Science, 308:1772-1774.

Curry R, Dickson B, Yashayaev I (2003) A change in the freshwater balance of the Atlantic Ocean over the past four decades. Nature, 426:826-829.

Dery SJ, Wood EF (2005) Decreasing river discharge in northern Canada. Geophys Res Lett, 32: L10401.

Dery SJ, Stieglitz M, McKenna EC, Wood EF (2005) Characteristics and trends of river discharge into Hudson, James, and Ungava Bays, 1964-2000. J Clim, 18:2540-2557.

Dickson B, Yashayaev I, Meincke J, Turrel B, Dye S, Holfort J (2002) Rapid freshening of the deep North Atlantic Ocean over the past four decades. Nature, 416:832-837.

Dickson R, Rudels B, Dye S, Karcher M, Meincke J, Yashayaev I (2007) Current estimates of freshwater flux through Arctic and subarctic seas. Prog Oceanogr, 73:210-230.

Dickson RR, Meincke J, Malmberg SA, Lee AJ (1988) The Great Salinity Anomaly in the northern North Atlantic. Prog Oceanogr, 20:103-151.

Fahrbach E, Schauer U, Rohard G, Meincke J, Osterhus S (2003) How to measure oceanic fluxes from North Atlantic through Fram Strait. ASOF Newsletter, 1: 3-7 (unpublished manuscript).

Fichefet T, Poncin C, Goosse H, Huybrechts P, Janssens I, Le Treut H (2003) Implications of changes in freshwater flux from the Greenland ice sheet for the climate of the 21st century. Geophys Res Lett, 30:1911.

Furevik T, Bentson M, Drange H, Kindem I, Kvamsto N, Sorteberg A (2003) Description and evaluation of the Bergen Climate Model: ARPEGE coupled with MICOM. Climate Dyn, 21(1):27-51.

Gregory JM, Huybrechts P (2006) Ice-sheet contributions to future sea-level change. Phil Trans R Soc Lond, A(364):1709-1731.

Gregory JM, Stott PA, Cresswell DJ, Rayner NA, Gordon C, Sexton DMH (2002) Recent and future changes in Arctic sea ice simulated by the HadCM3 AOGCM. Geophys Res Lett, 29:2175.

Gregory JM, Huybrechts P, Raper SCB (2004) Threatened loss of the Greenland ice-sheet. Nature, 428:616.

Haak H, Jungclaus J, Mikolajewicz U, Latif M (2003) Formation and propagation of great salinity anomalies. Geophys Res Lett, 30:1473.

Haak H, Jungclaus JH, Koenigk T, Sein D, Mikolajewicz U (2005) Arctic Ocean freshwater budget variability. ASOF Newsletter, 3:6-8 (unpublished manuscript).

Hanna E, Huybrechts P, Mote TL (2002) Surface mass balance of the Greenland ice sheet from climate-analysis data and accumulation/runoff models. Ann Glaciol, 35:67-72.

Hakkinen S, Proshutinsky A (2004) Freshwater content variability in the Arctic Ocean. J Geophys Res, 109 (C3):C03051.

Hatun H, Sand0 AB, Drange H, Hansen B, Valdimarsson H (2005) Influence of the Atlantic subpolar gyre on the thermohaline circulation. Science, 16:1841-1844.

Holland MM, Bitz CM (2003) Polar amplification of climate change in the Coupled Model Intercomparison Project. Climate Dyn, 21:221-232.

Houghton RW, Visbek M (2002) Qasi-decadal salinity fluctuations in the Labrador Sea. J Phys Oceanogr, 32:687-701.

IPCC (2001) Climate Change. The Scientific Basis. Houghton JT, Ding Y, Griggs DJ, Noguer M, van der Linden PJ, Dai X (eds.), Cambridge University Press, Cambridge, pp 525-582.

Johannessen OM, Shalina EV, Miles MW (1999) Satellite evidence for an Arctic sea ice cover in transformation. Science, 286:1937-1939.

Johannessen OM et al. (2004) Arctic climate change: Observed and modelled temperature and sea ice variability. Tellus, 56A:328-341.

Johannessen OM, Khvorostovsky K, Miles MW, Bobylev LP (2005) Recent ice-sheet growth in the interior of Greenland. Science, 310:1013-1016.

Johns TC et al. (2006) The new Hadley Centre climate model (HadGEM1): Evaluation of coupled simulations. J Clim, 19:1327-1353.

Jungclaus JH, Haak H, Latif M, Mikolajewicz U (2005) Arctic-North Atlantic interactions and multidecadal variability of the meridional overturning circulation. J Clim, 18:4013-4031.

Jungclaus JH, Botzet M, Haak H, Keenlyside N, Luo JJ, Latif M, Marotzke J, Mikolajewicz U, Roeckner E (2006a) Ocean circulation and tropical variability in the coupled model ECHAM5/ MPI-OM. J Clim, 19:3952-3972.

Jungclaus JH, Haak H, Esch M, Roeckner E, Marotzke J (2006b) Will Greenland melting halt the thermohaline circulation? Geophys Res Lett, 33:L17708.

Karcher M, Gerdes R, Kauker F, Koberle C, Yashayaev I (2005) Arctic Ocean change heralds North Atlantic freshening. Geophys Res Lett, 32, L21606.

Kattsov VM, Walsh JE (2000) Twentieth-century trends of arctic precipitation from observational data and a climate model simulation. J Clim, 13:1362-1370.

Krabill W et al. (2004) Greenland ice sheet: increased coastal thinning. Geophys Res Lett, 31, L24402.

Koenigk TU, Mikolajewicz U, Haak H, Jungclaus J (2006a) Variability of Fram Strait sea ice export: Causes, impacts and feedbacks in a coupled climate model. Climate Dyn, 26(1):17-34.

Koenigk TU, Mikolajewicz U, Haak H, Jungclaus J (2007) Arctic freshwater export and its impact on climate in the 20th and 21st Century. J Geophys Res, 112, G04S41, doi:10.1029/2006JG000274.

Lambert FH, Stott PA, Allen MR, Palmer MA (2004) Detection and attribution of changes in 20th century land precipitation. Geophys Res Lett, 31, L10203.

Lammers AR, Shiklomanov AI, Vorosmarty CJ, Fekete BM, Peterson BJ (2001) Assessment of contemporary Arctic river runoff based on observational discharge records. J Geophys Res, 106(D4):3321-3334.

Lunt DJ, de Noblet-Ducoudre N, Charbit S (2004) Effects of a melted Greenland ice sheet on climate, vegetation, and the cryosphere. Climate Dyn, 23 (7-8): 679-694.

Manabe S, Stouffer RJ (1980) Sensitivity of a global climate model to an increase of CO2 concentration in the atmosphere. J Geophys Res, 85:5529-5554.

McClelland JW, Holmes RM, Peterson BJ (2004) Increasing river discharge in the Eurasian Arctic: Consideration of dams, permafrost thaw, and fires as potential agents of change. J Geophys Res, 109:D18102.

McClelland JW, Dery SJ, Peterson BJ, Holmes RM, Wood EF (2006) A pan-arctic evaluation of changes in river discharge during the latter half of the 20th century. Geophys Res Lett, 33: L06715.

Oki T, Kanae S (2006) Global hydrological cycles and world water resources. Science, 313:1068-1072.

Pardaens AK, Banks HT, Gregory JM, Rowntree PR (2003) Freshwater transports in HadCM3. Climate Dyn, 21:177-195.

Peterson BJ, Holmes RM, McClelland JW, Vorosmarty CJ, Lammers RB, Shiklomanov AI, Shiknomanov IA, Rahmstorf S (2002) Increasing river discharge to the Arctic Ocean. Science, 298:2171-2173.

Peterson BJ, McClelland J, Curry R, Holmes RM,Walsh JE, Aagaard K (2006) Trajectory shifts in the Arctic and subarctic freshwater cycle. Science, 313:1061-1066.

Prinsenberg SJ, Hamilton J (2004) The Oceanic fluxes through Lancaster Sound of the Canadian Archipelago, ASOF Newsletter No 2 (unpublished manuscript).

Proshutinsky A, Bourke R, MacLaughlin F (2002) The role of the Beaufort gyre in Arctic climate variability: seasonal to decadal climate scales. Geophys Res Lett, 29:2100.

Rayner NA, Parker DE, Horton EB, Folland CK, Alexander V, Rowell DP, Kent EC, Kaplan A (2003) Global analyses of sea surface temperature, sea ice, and night marine air temperature since the late nineteenth century. J Geophys Res, 108(D14):4407.

Ridley JK, Huybrechts P, Gregory JM, Lowe JA (2005) Elimination of the Greenland ice sheet in a high CO2 climate. J Clim, 18:3409-3427.

Rignot E, Kanagaratnam P (2006) Changes in the velocity structure of the Greenland ice sheet. Science, 311:986-990.

Rothrock DA,Yu Y, Maykut GA (1999) Thinning of the Arctic sea ice cover. Geophys Res Lett, 26:3469-3472.

Serreze MC, Barry RG (2005) The Arctic Climate System. Cambridge University Press, Cambridge.

Serreze MC, Barrett AP, Lo F (2005) Northern high-latitude precipitation as depicted by atmospheric reanalyses and satellite retrievals. Mon Wea Rev, 133:3407-3430.

Serreze MC, Barrett AP, Slater AG, Woodgate RA, Aagaard K, Lammers RB, Steele M, Moritz M, Meredith M, Lee CM (2006) The large-scale freshwater cycle of the Arctic. J Geophys Res, 111 (C11), Art No C11010.

Stocker TF, Raible CC (2005) Water cycle shifts gear. Nature, 434:830-832.

Stott PA, Tett S, Jones G, Allen M, Mitchell J, Jenkins GJ (2000) External control of 20th century temperature by natural and anthropogenic focings. Science, 290:2133-2137.

Toniazzo T, Gregory JM, Huybrechts P (2004) Climatic impact of a Greenland deglaciation and its possible irreversibility. J Clim, 17:21-33.

Vellinga M, Wood RA (2002) Global climatic impacts of a collapse of the Atlantic thermohaline circulation. Climatic Change, 54:251-267.

Wadley MR, Bigg GR (2004) "Great Salinity Anomalies" in a coupled climate model. Geophys Res Lett, 31, L18302.

Wadley MR, Bigg GR (2006) Are "Great Salinity Anomalies" advective? J Clim, 19:1080-1088.

Walsh JE, Kattsov VM, Chapman WL, Govorkova V, Pavlova T (2002) Comparison of Arctic climate simulations by uncoupled and coupled global models. J Clim, 19:1429-1446.

Winsor P (2001) Arctic sea ice thickness remained constant during the 1990s. Geophys Res Lett, 28:1039-1041.

Woodgate RA, Aagaard K (2005) Revising the Bering Strait freshwater flux into the Arctic Ocean. Geophys Res Lett, 32:L02602.

Wu P, Wood R (2007) Convection induced long term freshening of the subpolar North Atlantic Ocean. submitted.

Wu P, Wood R, Stott P (2004) Does the recent freshening trend in the North Atlantic indicate a weakening thermohaline circulation? Geophys Res Lett, 31:L02301.

Wu, P, Wood R, Stott P (2005) Human influence on increasing Arctic river discharges. Geophys Res Lett, 32:L02703.

Wu, P, Wood R, Stott P, Jones GS (2007) Deep North Atlantic freshening simulated in a coupled climate model. Prog Oceanogr, 73:370-383.

Zhang X, Walsh JE (2006) Towards a seasonally ice-covered Arctic Ocean: Scenarios from the IPCC AR4 model simulations. J Clim, 19:1730-1747.

Zhang X, Zhang J (2001) Heat and freshwater budgets and pathways in the Arctic Mediterranean in a coupled ocean/sea ice model, J Oceanogr, 57:207-234.

Zhang X, Ikeda M, Walsh JE (2003) Arctic sea ice and freshwater changes driven by the atmospheric leading mode in a coupled sea ice-ocean model. J Clim, 16:2159-2177.

Was this article helpful?

0 0

Post a comment