Abstract

Arctic Ocean freshwater budgets are examined from ten models participating in the Intergovernmental Panel on Climate Change Fourth Assessment Report. This includes an analysis of sea ice transport and storage, ocean transport and storage, and net surface flux exchange. Simulated budgets for the late 20th century are compared to available observations, followed by an analysis of simulated changes from 1950-2050. The consistent theme over this period is an acceleration of the Arctic hydrological cycle which is expressed as an increase in the flux of water passing through the hydrologic elements. Increased freshwater inputs to the ocean from net precipitation, river runoff, and net ice melt result. While generally attended by a larger export of liquid freshwater to lower latitudes, primarily through Fram Strait, liquid freshwater storage in the Arctic Ocean increases. In contrast, the export and storage of freshwater in the form of sea ice decreases. The qualitative agreement between models for which the only common forcing is rising greenhouse gas concentrations implicates this greenhouse gas loading as the cause of the change. Although the models perform quite well in their simulations of net precipitation over the Arctic Ocean and terrestrial drainage, they differ significantly regarding the magnitude of the trends and their representation of contemporary mean ocean and sea ice budget terms. To reduce uncertainty in future projections of the Arctic freshwater cycle, the climate models as a group require considerable improvement in these aspects of their simulations.

Figure caption: The decadal changes in the multi-model ensemble mean Arctic Ocean freshwater budget terms from 1950-2050, expressed as anomalies with respect to the 1950-2050 mean. The terms are the net precipitation over the Arctic Ocean, net precipitation over the terrestrial drainage assumed as equivalent to river runoff, the total ice transport through Fram Strait and the Barents Sea, the total ocean freshwater exchange with the North Atlantic (Fram Strait+Barents Sea contributions) and the Bering Strait transport. The sign convention is such that a positive anomaly is an increasing source (or decreasing sink) of freshwater for the Arctic Ocean. Transports through the Canadian Arctic Archipelago are not included since most models do not have an open strait there.

Support: NSF, NASA.

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Finnis, J., M.M. Holland, M.C. Serreze, and J.J. Cassano 2007: Response of Northern Hemisphere Extratropical Cyclone Activity and Associated Precipitation to Climate Change, as Represented by CCSM3. J. Geophys. Res., in press.

Abstract

The projected effects of rising CO2 levels on Northern Hemisphere extratropical cyclone activity and cyclone-associated precipitation are examined for September through May, using output from version 3 of the Community Climate System Model (CCSM3). A cyclone identification algorithm was applied to a 5 member ensemble of CCSM3 20th and 21st century output, along with a method of isolating precipitation produced by each cyclone. Mean seasonal statistics describing cyclone activity and the character of associated precipitation were calculated over several study regions for twenty year periods. The dominant change in cyclone activity is a marked mid-latitude decrease in frequency during autumn, winter, and spring. Few significant shifts in storm tracks or cyclone intensity were identified. Total daily precipitation from these events is found to increase into the 21st century, largely due to increases in available atmospheric moisture with rising temperatures. This thermodynamic increase in precipitation leads to large rises in total seasonal cyclone-associated precipitation over high latitudes, while over mid-latitudes the thermodynamic increase is offset by the dynamic effect associated with decreased cyclone frequency.

Figure caption: Change in the number of wet days (at least 1 mm of precipitation in a day), presented as the 20th century ensemble mean subtracted from 21st century ensemble mean.

Support: NSF, NASA.

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White, D., L Hinzman, L Alessa, J Cassano, M Chambers, K Falkner, J Francis, W.J. Gutowski, Jr., M Holland, R. M Holmes, H Huntington, D Kane, A Kliskey, C Lee, J McClelland, B Peterson, T. S Rupp, F Straneo, M Steele, R Woodgate, D Yang, K Yoshikawa, T Zhang, 2007: The Arctic Freshwater System: Changes and Impacts. J Geophys. Res., in press.

Abstract

Dramatic changes have been observed in the Arctic over the last century. Many of these involve the storage and cycling of fresh water. On land, precipitation and river discharge, lake abundance and size, glacier area and volume, soil moisture, and a variety of permafrost characteristics have changed. In the ocean, sea ice thickness and areal coverage has decreased, and water mass circulation patterns have shifted, changing freshwater pathways and sea ice cover dynamics. Precipitation onto the ocean surface has also changed. Such changes are expected to continue, and perhaps accelerate, in the coming century, enhanced by complex feedbacks between the oceanic, atmospheric, and terrestrial freshwater systems. Change to the arctic freshwater system heralds changes for our global physical and ecological environment as well as human activities in the Arctic. In this paper we review observed changes in the arctic freshwater system over the last century in terrestrial, atmospheric, and oceanic systems.

Figure caption: The times series of (a) September and (b) March ice extent from 1979-2005. The thin line indicates the linear trend in the time series.

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Stroeve, J., M.M. Holland, W. Meier, T. Scambos, and M.C. Serreze, 2007: Arctic Sea Ice Decline: Faster than Forecast, Geophys. Res. Lett., 34, L09501, doi: 10.1029/2007GL029703, 2007.

Abstract

From 1953 to 2006, Arctic sea ice extent at the end of the melt season in September has declined sharply. All models participating in the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) show declining Arctic ice cover over this period. However, depending on the time window for analysis, none or very few individual model simulations show trends comparable to observations. If the multi-model ensemble mean time series provides a true representation of forced change by greenhouse gas (GHG) loading, 33 - 38% of the observed September trend from 1953 - 2006 is externally forced, growing to 47 - 57% from 1979 - 2006. Given evidence that as a group, the models underestimate the GHG response, the externally forced component may be larger. While both observed and modeled Antarctic winter trends are small, comparisons for summer are confounded by generally poor model performance.

Figure caption: Arctic September sea ice extent (× 106 km2) from observations (thick red line) and 13 IPCC AR4 climate models, together with the multi-model ensemble mean (solid black line) and standard deviation (dotted black line). Models with more than one ensemble member are indicated with an asterisk. Inset shows 9-year running means.

Support: National Science Foundation and NASA.

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Tremblay, L.B., M.M. Holland, I.V. Gorodetskaya, and G.A. Schmidt, 2007: An Ice-Free Arctic? Opportunities for computational science, Journal of Computing in Science and Engineering, 9, 65-74.

Abstract

In this article, the authors discuss future projections of the Arctic sea-ice cover from sophisticated General Circulation Model (GCMs), the uncertainties associated with these projections, and how the use of simpler component models can help in the interpretation of complex GCMs.

Figure caption: Arctic Ocean surface circulation. Red arrows indicate warm Atlantic Ocean currents and blue arrows indicate cold Arctic surface currents. North Atlantic drift waters entering the Arctic west of Svalbard flow counterclockwise at depth (the warm core is at roughly 300 meters) and exit through the Fram Strait (not shown).

Support: NSERC, NSF.

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Hunke, E.C. and M.M. Holland, 2007: Global atmospheric forcing data for Arctic ice-ocean modeling, J. Geophys. Res., 12, C04S14, doi:10.1029/2006JC003640.

Abstract

We compare three forcing data sets, all variants of National Centers for Environmental Prediction (NCEP) forcing, in global ice-ocean simulations and evaluate them for use in Arctic model studies. The data sets include the standard Arctic Ocean Model Intercomparison Project (AOMIP) protocol, standard NCEP forcing fields, and the data set of Large and Yeager (2004). We explore their performance in Arctic simulations using a global, coupled, sea ice-ocean model, and find that while these forcing data sets have many similarities, the resulting simulations present significant differences, most notably in ice thickness and ocean circulation. This underscores the sensitivity of Arctic sea ice and ocean to slight changes in environmental forcing parameters. This study also highlights the difficulties faced by the model intercomparison community attempting to disentangle simulation differences due to model physics from those caused by small differences in forcing parameters. Assessing the simulation uncertainty due to inaccuracies in the forcing data provides context for the simulation uncertainty associated with model physics.

Figure caption: Downwelling (a) longwave and (b) shortwave radiation (W m-2), averaged over the Arctic for 1981-2000.

Support: DOE, NSF.

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Serreze, M.C., M.M. Holland, and J. Stroeve, 2007: Perspectives on the Arctic's shrinking sea-ice cover, Science, 315, 1533-1536.

Abstract

Linear trends in arctic sea-ice extent over the period 1979 to 2006 are negative in every month. This ice loss is best viewed as a combination of strong natural variability in the coupled ice-ocean-atmosphere system and a growing radiative forcing associated with rising concentrations of atmospheric greenhouse gases, the latter supported by evidence of qualitative consistency between observed trends and those simulated by climate models over the same period. Although the large scatter between individual model simulations leads to much uncertainty as to when a seasonally ice-free Arctic Ocean might be realized, this transition to a new arctic state may be rapid once the ice thins to a more vulnerable state. Loss of the ice cover is expected to affect the Arctic's freshwater system and surface energy budget and could be manifested in middle latitudes as altered patterns of atmospheric circulation and precipitation.

Figure caption: Time series of arctic sea-ice extent for alternate months and least-squares linear fit based on satellite-derived passive microwave data from November 1979 through November 2006. Listed trends include (in parentheses) the 95% confidence interval of the slope. Ice extent is also declining for the six months that are not shown, ranging from -2.8 ± 0.8% per decade in February to -7.2 ± 2.3% per decade in August.

Support: NSF, NASA, NOAA.

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Holland, M.M., C.M. Bitz, and B. Tremblay, 2006: Future abrupt reductions in the Summer Arctic sea ice, Geophys. Res. Lett., 33, L23503, doi:10.1029/2006GL028024.

Abstract

We examine the trajectory of Arctic summer sea ice in seven projections from the Community Climate System Model and find that abrupt reductions are a common feature of these 21st century simulations. These events have decreasing September ice extent trends that are typically 4 times larger than comparable observed trends. One event exhibits a decrease from 6 million km2 to 2 million km2 in a decade, reaching near ice-free September conditions by 2040. In the simulations, ice retreat accelerates as thinning increases the open water formation efficiency for a given melt rate and the ice-albedo feedback increases shortwave absorption. The retreat is abrupt when ocean heat transport to the Arctic is rapidly increasing. Analysis from multiple climate models and three forcing scenarios indicates that abrupt reductions occur in simulations from over 50% of the models and suggests that reductions in future greenhouse gas emissions moderate the likelihood of these events.

Figure caption: (a) Northern Hemisphere September ice extent for Run 1 (black), the Run 1 five-year running mean (blue), and the observed five-year running mean (red). The range from the ensemble members is in dark grey. Light grey indicates the abrupt event. (b) The Run 1 (black) and observed (red) 1990s averaged September ice edge (50% concentration) and Run 1 conditions averaged over 2010-2019 (blue) and 2040-2049 (green). The Arctic region used in our analysis is shown in grey.

Support: National Science Foundation, NASA.

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Holland, M.M., J. Finnis, and M.C. Serreze, 2006: Simulated Arctic Ocean freshwater budgets in the 20th and 21st centuries, J. Climate, 19, 6221-6242.

Abstract

The Arctic Ocean freshwater budgets in climate model integrations of the twentieth and twenty-first century are examined. An ensemble of six members of the Community Climate System Model version 3 (CCSM3) is used for the analysis, allowing the anthropogenically forced trends over the integration length to be assessed. Mechanisms driving trends in the budgets are diagnosed, and the implications of changes in the Arctic-North Atlantic exchange on the Labrador Sea and Greenland-Iceland-Norwegian (GIN) Seas properties are discussed. Over the twentieth and the twenty-first centuries, the Arctic freshens as a result of increased river runoff, net precipitation, and decreased ice growth. For many of the budget terms, the maximum 50-yr trends in the time series occur from approximately 1975 to 2025, suggesting that we are currently in the midst of large Arctic change. The total freshwater exchange between the Arctic and North Atlantic increases over the twentieth and twenty-first centuries with decreases in ice export more than compensated for by an increase in the liquid freshwater export. Changes in both the liquid and solid (ice) Fram Strait freshwater fluxes are transported southward by the East Greenland Current and partially removed from the GIN Seas. Nevertheless, reductions in GIN sea ice melt do result from the reduced Fram Strait transport and account for the largest term in the changing ocean surface freshwater fluxes in this region. This counteracts the increased ocean stability due to the warming climate and helps to maintain GIN sea deep-water formation.

Figure caption: The time series of (a) river runoff, (b) net precipitation, (c) sea ice export, and (d) ocean export in Sverdrups of freshwater over the twentieth and twenty-first centuries. The sign convention is such that a source of freshwater for the Arctic Ocean is a positive value. The ensemble mean is shown by the black line, and the gray envelope about the mean indicates the range in the individual ensemble members. The thin blue line shows results from a preindustrial (1870) control integration from years 450-650. These provided the initial conditions for the twentieth-century integrations, with different twentieth-century ensemble members starting from different years of the 1870 run.

Support: NSF.