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CGD Research Catalog
Dr. Marika Holland
Abstract: Processes in the climate system that can either amplify or dampen the climate response to an external perturbation are referred to as climate feedbacks. Climate sensitivity estimates depend critically on radiative feedbacks associated with water vapor, lapse rate, clouds, snow, and sea ice, and global estimates of these feedbacks differ among general circulation models. By reviewing recent observational, numerical, and theoretical studies, this paper shows that there has been progress since the Third Assessment Report of the Intergovernmental Panel on Climate Change in (i) the understanding of the physical mechanisms involved in these feedbacks, (ii) the interpretation of intermodel differences in global estimates of these feedbacks, and (iii) the development of methodologies of evaluation of these feedbacks (or of some components) using observations. This suggests that continuing developments in climate feedback research will progressively help make it possible to constrain the GCMs range of climate feedbacks and climate sensitivity through an ensemble of diagnostics based on physical understanding and observations. Support: National Science Foundation. Figure. (High resolution figure.) The normalized zonally averaged surface air temperature change from 17 models participating in the AR4 of the IPCC. The temperature change is computed as the 2080-99 average from the so-called SRES AlB scenario minus the 1980-99 average from climate of the twentieth-century simulations. The zonally averaged change is normalized by the global average surface air temperature change. The amplified warming in the Arctic is suggestive of the important positive feedbacks associated with ice and snow cover at these latitudes.
Abstract: We present the sea ice simulation of the CCSM3 T42-gx1 and T85-gx1 control simulations and examine the influence of the parameterized sea ice thickness distribution (ITD) on polar climate conditions. This includes an analysis of the change in mean climate conditions and simulated sea ice feedbacks when an ITD is included. We find that including a representation of the sub-gridscale ITD results in larger ice growth rates and thicker sea ice. These larger growth rates represent a higher heat loss from the ocean-ice column to the atmosphere, resulting in warmer surface conditions. Ocean circulation, most notably in the southern hemisphere is also modified by the ITD because of the influence of enhanced high latitude ice formation on the ocean buoyancy flux and resulting deep water formation. Changes in atmospheric circulation also result, again most notably in the southern hemisphere. There are indications that the ITD also modifies simulated sea ice related feedbacks. In regions of similar ice thickness, the surface albedo changes at 2XCO2 conditions are larger when an ITD is included, suggesting an enhanced surface albedo feedback. The presence of an ITD also modifies the ice thickness-ice strength relationship and the ice thickness-ice growth rate relationship, both of which represent negative feedbacks on ice thickness. The net influence of the ITD on polar climate sensitivity and variability results from the interaction of these and other complex feedback processes. Support: National Science Foundation. Figure. (High resolution figure.) The change in surface albedo per surface air temperature change at 2XCO2 conditions as a function of initial summer minimum sea ice thickness for the northern hemisphere. Regions with initial sea ice thinner than 0.5m are not used in this analysis. Values are shown for a simulation with a resolved ice thickness distribution (red), a single ice thickness category simulation (black) and a single ice thickness category tuned run (green). The resolved ice thickness distribution simulation typically has a larger surface albedo change per temperature change for ice of the same thickness, indicating a larger surface albedo feedback.
Abstract: We examine the Arctic Ocean freshwater budgets in climate model integrations of the 20th and 21st century. An ensemble of 6 members of the Community Climate System Model version 3 (CCSM3) is used for the analysis allowing us to assess the anthropogenically forced trends over the integration length. 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) Sea properties are discussed. Over the 20th and the 21st centuries, the Arctic freshens due to increased river runoff, net precipitation, and decreased ice growth. For many of the budget terms, the maximum 50 year trends in the timeseries occur from approximately 1975-2025, 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 20th and 21st 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 Sea. Nevertheless, reductions in GIN Sea ice melt do result from the reduced Fram Strait transport 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 deepwater formation. Support: National Science Foundation. Figure. (High resolution figure.) The 50 year running trend for the major components of the Arctic ocean freshwater budget, including the total ocean transport, the total ice transport, the river runoff and the net precipitation (P-E) over the 20th and 21st centuries. The sign convention is such that a source of freshwater for the Arctic is a positive value, so a positive trend indicates an increasing source (or decreasing sink) of freshwater for the Arctic Ocean. The 95% confidence intervals are indicated by the grey shading. The values are plotted relative to the middle year used in the trend analysis, so the values for 1925 show the trend for 1900-1950.
Abstract: We examine the representation of the mean state and interannual variability of Antarctic sea ice in six simulations of the 20th century from coupled models participating in the Intergovernmental Panel on Climate Change fourth assessment report. The simulations exhibit a largely seasonal southern hemisphere ice cover, as observed. There is considerable scatter in the monthly simulated climatological ice extent among different models, but no consistent bias when compared to observations. The scatter in maximum winter ice extent among different models is correlated to the strength of the climatological zonal winds suggesting that wind forced ice transport is responsible for much of this scatter. Observations show that the leading mode of southern hemisphere ice variability exhibits a dipole structure with anomalies of one sign in the Atlantic sector associated with anomalies of the opposite sign in the Pacific sector. The observed ice anomalies also exhibit eastward propagation with the Antarctic Circumpolar Current, as part of the documented Antarctic Circumpolar Wave phenomenon. Many of the models do simulate dipole-like behavior in sea ice anomalies as the leading mode of ice variability, but there is a large discrepancy in the eastward propagation of these anomalies among the different models. Consistent with observations, the simulated Antarctic dipole-like variations in the ice cover are led by sea level pressure anomalies in the Amundsen/ Bellingshausen Sea. These are associated, to different degrees in different models, with both the Southern Annular Mode and the El Nino-Southern Oscillation (ENSO). There are indications that the magnitude of the influence of ENSO on the southern hemisphere ice cover is related to the strength of ENSO events simulated by the different models. Support: National Science Foundation. Figure. (High resolution figure.) The first empirical orthogonal function (EOF) of winter sea ice concentration from the model wsimulations from 1960 to 1999 using linearly detrended data. Shown are a) CCSM3, b) CSIRO-Mk3.0, c) GFDL-CM2.1, d) GISS-ER, e) MIROC3.2(hires) and f) UKMO-HadCM3. The contour interval is 5%, the zero contour is omitted and negative values are shaded.
Abstract: We compare three forcing data sets, all variants of 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 datasets 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. Support: National Science Foundation. Figure. (High resolution figure.) The (a) shortwave, (b) longwave and (c) sensible heat flux averaged over the Arctic for 1982 in W m2 for a number of different forcing datasets. Only summer values are shown in (a) in order to magnify differences in the data during these months. |
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Bony, S., R. Colman,
V. M. Kattsov, R. P. Allan, C. S. Bretherton, J. Dufresne, A. Hall, S.
Hallegatte, M. M. Holland, W. Ingram, D. A. Randall, B. J. Soden, G. Tselioudis,
and M. J. Webb, 2006: How well do we understand and evaluate climate change
feedback processes?, Journal of Climate, 19, 3445-3482.
Holland, M. M., C.
M. Bitz, E. C. Hunke, W. H. Lipscomb, and J. L. Schramm, 2006: Influence
of the sea ice thickness distribution on polar climate in CCSM3. Journal
of Climate, 19, 2398-2414.
Holland, M. M., J.
Finnis, and M. C. Serreze, 2006: Simulated Arctic Ocean freshwater budgets
in the 20th and 21st centuries. Journal of Climate, in press.
Holland, M. M., and
M. Raphael, 2006: Twentieth century simulation of the Southern Hemisphere
in coupled models. Part II: Sea ice conditions and variability. Climate
Dynamics, 26, 229-245.
Hunke, E. C., and
M. M. Holland, 2006: Global atmospheric forcing data for ice-ocean modeling.
Journal of Geophysical Research, accepted.
