climate.uvic.ca

1. Title: The Arctic Ocean and its role in past, present and future climate/climate variability

2. Type of report: Progress

3. PI name and address:

Andrew Weaver
Mailing Address:                          Courier Address:

School of Earth and Ocean Sciences        School of Earth and Ocean Sciences
University of Victoria                    at Gordon Head Complex, Room 296a
PO Box 3055 University of Victoria 
Victoria, BC, V8W 3P6                     3964 Gordon Head Road 
Canada                                    Victoria, BC, V8N 3X3, Canada

Tel:  (250) 472 4001
Fax:  (250) 472 4004

email: weaver@uvic.ca

URL climate.uvic.ca

4. Other participating researchers:

Marika Holland

National Center for Atmospheric Research
PO Box 3000
Boulder, CO 80307

Tel:  (303) 497 1734
Fax:  (303) 497 1700

email: mholland@ucar.edu

URL: MHolland@UCAR

5. Objectives, Methods, Main Results, References:

In this section I provide brief descriptions of the research that has either appeared in print, is in press or has been recently submitted. All references can be found in section 6.

The impact of rising atmospheric CO2 levels on low frequency North Atlantic climate variability

Marika M. Holland, Aaron J. Brasket and Andrew J. Weaver

Observations show that the North Atlantic climate system possesses pronounced interdecadal variability in its sea-ice, ocean and atmosphere components. The long timescale associated with this variability suggests that the ocean, and in particular its thermohaline component, may play an integral role in its mechanism. A question which naturally arises concerns how such variability will change under increased levels of atmospheric CO2. Several studies have examined the impact of increased atmospheric CO2 on climate variability through the use of atmospheric/ocean mixed layer model or full coupled models, although they have used fairly short integrations (20-80 years) and largely focused on changes in atmospheric variability. In this study we used a coupled ice/ocean/atmosphere model in an attempt to quantify potential changes in North Atlantic interdecadal climate variability under increased atmospheric CO2. We focused our attention on the variability of the thermohaline circulation induced by fluctuations in Arctic Ocean ice export to the North Atlantic. Under 2XCO2 conditions, the variance of the thermohaline circulation was reduced to 7% of its simulated value under present day forcing. This decrease was caused by relatively low ice export variability and changes in the primary ice melt location in the northern North Atlantic.

The role of ice ocean interactions in the variability of the North Atlantic thermohaline circulation

Marika M. Holland, Cecelia M. Bitz, Michael Eby and Andrew J. Weaver

The simulated influence of Arctic sea ice on the variability of the North Atlantic climate was examined in the context of a global coupled ice/ocean/atmosphere model. This coupled system incorporates a general circulation ocean model, an atmospheric energy moisture balance model and a dynamic/thermodynamic sea ice model. Under steady seasonal forcing, an equilibrium solution was obtained with very little variability. To induce variability in the model, daily varying stochastic anomalies were applied to the wind forcing of the northern hemisphere sea ice cover. These stochastic anomalies had observed spatial patterns but were random in time. Model simulations were run for 1000 years from an equilibrium state and the variability in the system was analyzed. The sensitivity of the system to the ice/ocean coupling of both heat and fresh water was also examined.

Under the stochastic forcing conditions, low amplitude (approximately 10% of the mean) variability in the thermohaline circulation (THC) occurs. This variability has significant spectral power at interdecadal timescales which are concentrated at approximately 20 years. It is forced by fluctuations in the export of ice from the Arctic into the North Atlantic. Large changes in sea-surface temperature and salinity are related to changes in the overturning circulation and the sea ice coverage in the northern North Atlantic. Additionally, the THC variability influences the Arctic basin through heat transport under the ice pack.

Results from sensitivity studies suggest that the fresh water exchange from the variable ice cover is the dominant process for forcing variability in the overturning. The simulated Arctic ice export provides a stochastic forcing to the northern North Atlantic which excites a damped oscillatory ocean-only mode. The insulating capacity of the variable sea ice has a negligible effect on the THC. Ice/ocean thermal coupling acts to preferentially damp THC variability with periods greater than 30 years, but has little influence on variability at higher frequencies.

The influence of sea ice physics on simulations of climate change

Marika M. Holland, Cecilia M. Bitz, and Andrew J. Weaver

Sea ice cover is an important factor in the climate system due to feedback mechanisms associated with its influence on the surface albedo and ice-ocean-atmosphere exchange. However, sea ice models in GCMs typically used relatively crude physics. Single column and basin scale ice models have attempted to assess the importance of different physical parameterizations, however, this has often been done in uncoupled systems which means that various coupled feedback mechanisms are missing.

We examined the sensitivity of climate change simulations in a global coupled ice-ocean-atmosphere model to different sea ice physics. In particular, the influences of ice dynamics and a sub-gridscale ice thickness distribution were addressed. The importance of these parameterizations for the simulation of present-day climate conditions and the climate response to increasing atmospheric CO2 levels is discussed. Additionally, we examined the influence of the albedo feedback mechanism in climate change experiments.

As in several previous studies, we found that the sea-ice parameterizations have a significant influence on present-day climate simulations, modifying both the annual mean ice-ocean-atmosphere conditions and the seasonal variation of these properties. For example, in models with motionless sea ice (i.e., thermodynamic-only models) the ice volume increases significantly and undergoes a smaller seasonal cycle. Resolving the ice thickness distribution also increases the ice thickness, but acts to enhance the seasonal cycle. Additionally, the ocean circulation is modified due to different ice/ocean buoyancy fluxes, leading to different Antarctic Bottom Water formation rates.

The presence of ice dynamics and the sub-gridscale ice thickness distribution also influences the response of the system to climate perturbations. Under increased atmospheric CO2 forcing, simulating ice dynamics and the ice thickness distribution enhances the ice area response. However, the ice volume response is diminished when ice dynamics in included and enhances when the ice thickness distribution is resolved. The ocean response to global warming is also modified due to the changes in ice physics and the thermohaline circulation is less sensitive to climate change scenarios in models that resolve ice dynamics and the ice thickness distribution.

Additional simulations were performed to quantify the influence of the albedo feedback mechanism on climate change simulations. In increasing CO2 simulations, which neglected the influence of a changing surface albedo, amplified warming was still present (although reduced) at high latitudes due to the poleward retreat of the ice cover and larger ocean-atmosphere heat exchange. In these simulations, the albedo feedback has a considerable influence on the climate response to global warming, accounting for 17% of the global air temperature increase, 37% of the Northern Hemisphere ice area decrease and 31% of the Northern Hemisphere ice volume decrease.

The Canada Basin 1989-1995: Upstream events and far-field effects of the Barents Sea branch

Fiona McLaughlin, Ed Carmack, Robbie Macdonald, Andrew J. Weaver and J. Smith

Physical and geochemical tracer measurements were collected at one oceanographic station (Station A: 72 N 143W) in the southern Canada Basin from 1989 to 1995, along sections from the Beaufort Shelf to this station in 1993 and 1995, and along a section westward of Banks Island in 1995. These measurements were examined to see how recent events in three upstream Arctic Ocean sub-basins impacted upon Canada Basin waters. Upstream events included Atlantic layer warming, relocation of the Atlantic/Pacific water mass boundary, and increased ventilation of boundary current waters. Early signals of change appeared first in the Canada Basin in 1993 along the continental margin and, by 1995, were evident at Station A in the basin interior and farther downstream. Differences in physical and geochemical properties (nutrients, oxygen, 129l and CFCs) were observed throughout much of the water column to depths greater than 1600 m. In particular, the boundary distinguishing Pacific from Atlantic-origin water was found to be shallower and Atlantic-origin water occupied more of the Canada Basin water column. By 1995, Atlantic-origin water in the lower halocline at Station A was found to be colder and more ventilated. Likewise, within the Atlantic layer, Fram Strait Branch (FSB) water was colder, fresher, and more ventilated, and Barents Sea Branch (BSB) water was warmer, fresher, and more ventilated than during previous years. By comparing observations at Station A with eastern Nansen Basin observations, the main source of these changes was traced to dense water outflow from the Barents Sea. Studies indicated that in early 1989 Barents Sea waters were 2 Deg C warmer and that between 1988 and 1989, a large volume of dense water had left the shelf. These events coincided with an atmospheric shift to increased cyclonic circulation in 1989, a transition unprecedented in its magnitude, geographic reach, and apparent oceanographic impact. The effects of a large outflow of dense Barents Sea water were observed some 5000 km away downstream in the Canada Basin where the BSB component of the Atlantic layer had increased 20% by 1995.

The UVic Earth System Climate Model: Model Description, Climatology, and Applications to Past, Present and Future Climates.

Andrew J. Weaver, M. Eby, E. C. Wiebe, C. M. Bitz, P. B. Duffy, T.L. Ewen, A. F. Fanning, M. M. Holland, A. MacFadyen, O. Saenko, A. Schmittner, H. Wang, and M. Yoshimori

A new earth system climate model of intermediate complexity is developed and its climatology was compared against observations. The UVic Earth System Climate Model consists of a three-dimensional ocean general circulation model coupled to a thermodynamic/dynamic sea ice model, an energy-moisture balance atmospheric model with dynamical feedbacks, and a thermomechanical land ice model. In order to keep the model computationally efficient a reduced complexity atmosphere model is used. Atmospheric heat and freshwater transports are parametrised through Fickian diffusion, and precipitation is assumed to occur when the relative humidity reaches greater than 85%. Moisture transport can also be accomplished through advection if desired. Precipitation over land is assumed to instantaneously return to the ocean via one of 33 observed river drainage basins. Ice and snow albedo feedbacks are included in the coupled model by locally increasing the prescribed latitudinal profile of the planetary albedo. The atmospheric model includes a parametrisation of water vapour/planetary long wave feedbacks, although the radiative forcing associated with changes in atmospheric CO2 is prescribed as a modification of the planetary long wave radiative flux. A specified lapse rate is used to reduce the surface temperature over land where there is topography. The model uses prescribed present day winds in its climatology although a dynamical wind feedback is included which exploits a latitudinally-varying empirical relationship between atmospheric surface temperature and density.

The ocean component of the coupled model is based on the GFDL Modular Ocean Model 2.2, with a global resolution of a 3.6° (zonal) x 1.8° (meridional) and 19 vertical levels, that includes an option for a brine-rejection parametrisation. The sea ice component incorporates an elastic-viscous-plastic rheology to represent sea ice dynamics and various options for the representation of sea ice thermodynamics and thickness distribution. The systematic comparison of the coupled model with observations reveals good agreement, especially when moisture transport is accomplished through advection.

Global warming simulations conducted using the model to explore the role of moisture advection reveal a climate sensitivity of 3.0°C for a doubling of CO2, in line with other more comprehensive coupled models. Moisture advection, together with the wind feedback leads to a transient simulation in which the meridional overturning in the North Atlantic initially weakens, but eventually re-establishes to its initial strength once the radiative forcing is held fixed, as found in many coupled atmosphere GCMs. This is in contrast to experiments in which moisture transport is accomplished through diffusion whereby the overturning re-establishes to a strength that is greater than its initial condition.

When applied to the climate of the Last Glacial Maximum, the model obtains tropical cooling (30°N—30°S), relative to the present, of about 2.1°C over the ocean and 3.6°C over the land. These are generally cooler than CLIMAP estimates, but not as cool as some other reconstructions. This moderate cooling is consistent with alkenone reconstructions and a low to mid climate sensitivity to perturbations in radiative forcing. An amplification of the cooling occurs in the North Atlantic due to the weakening of North Atlantic Deep Water formation. Concurrent with this weakening is a shallowing and a more northward penetration of Antarctic Bottom Water.

Climate models are usually evaluated by spinning them up under perpetual present-day forcing and comparing the model results with present-day observations. Implicit in this approach is the assumption that the present day observations are in equilibrium with the present day radiative forcing. The comparison of a long transient integration (starting at 6 KBP), forced by changing radiative forcing (solar, CO2, orbital), with an equilibrium integration revealed substantial differences. Relative to the climatology from the present-day equilibrium integration, the global mean surface air and sea surface temperatures (SSTs) were 0.74°C and 0.55°C colder, respectively, deep ocean temperatures were substantially cooler, and southern hemisphere sea ice cover was 22% larger, although the North Atlantic conveyor remained remarkably stable in all cases. The differences were due to the long timescale memory of the deep ocean to climatic conditions which prevailed throughout the late Holocene. It was also demonstrated that a global warming simulation that started from an equilibrium present-day climate (cold start) underestimated the global temperature increase at 2100 by 13% when compared to a transient simulation, under historical solar, CO2 and orbital forcing, that was also extended out to 2100. This was larger (13% compared to 9.8%) than the difference from an analogous transient experiment which did not include historical changes in solar forcing. These results suggest that those groups that do not account for solar forcing changes over the 20th century may slightly underestimate (~3% in our model) the projected warming by the year 2100.

Improved representation of sea ice processes in climate models

Saenko, O. A., G.M. Flato and A.J. Weaver

The apparent sensitivity of high latitudes to climate perturbations has spurred the development of global climate model components with improved parametrisations of sea-ice related processes. We focused on two of these. The first involved the ocean component in which we generalized a recently developed parametrisation of brine rejection during sea ice formation for use in a multi-category sea ice model (i.e. one that resolve the thickness distribution function). It employed explicit subsurface mixing of brine-enriched surface waters, resulting from sea ice growth. The parameterisation was implemented in the UVic coupled model, and numerical experiments were performed to highlight the physical processes and feedbacks involved. It was shown that a better representation of brine rejection improved the simulation of intermediate and deep ocean waters. Over the Arctic Ocean it also improves the simulation of the warm Atlantic Layer and sharpened the halocline.

The second part of this study focused on the sea-ice component. We performed a series of stand-alone sea-ice model experiments comparing a recently developed multi-layer energy-conserving thermodynamic scheme with the simplified scheme used in many existing climate models. Experiments were done with and without the inclusion of dynamic processes (ice motion and deformation). Of particular interest was the impact of changes in the representation of dynamic and thermodynamic processes on the response of sea ice to climate perturbations. This was accomplished by comparing results obtained with present-day and future climate forcing, the latter obtained from the CCCma coupled climate model. We found that the more sophisticated thermodynamic scheme increased the sensitivity of ice volume, but decreased the sensitivity of ice area. As in previous studies, the introduction of ice dynamics tended to reduce sensitivity relative to a thermodynamic-only model.

6. Publications resulting from direct IARC funding:

1. Holland, M.M., A.J. Brasket and A.J. Weaver, 2000: The impact of rising atmospheric CO2 on low frequency North Atlantic climate variability. Geophysical Research Letters, 27, 1519—1522.

2. Holland, M.M., C.M. Bitz, M. Eby and A.J. Weaver, 2001: The role of ice ocean interactions in the variability of the North Atlantic thermohaline circulation. Journal of Climate, 14, 656—675. 79.

3. Weaver, A.J., M. Eby, E. C. Wiebe, C. M. Bitz, P. B. Duffy, T. L. Ewen, A. F. Fanning, M. M. Holland, A. MacFadyen, O. Saenko, A. Schmittner, H. Wang and M. Yoshimori, 2001: The UVic Earth System Climate Model: Model description, climatology and application to past, present and future climates. Atmosphere-Ocean, in press.

4. Saenko, O., G. M. Flato and A. J. Weaver, 2001: Improved representation of sea-ice processes in climate models. Atmosphere-Ocean, in press.

5. McLaughlin, F. E. Carmack, R. Macdonald, A.J. Weaver and J. Smith, 2001: The Canada Basin 1989-1995: Upstream events and far-field effects of the Barents Sea branch. Journal of Geophysical Research., submitted.

6. Holland, M.M., C.M. Bitz and A.J. Weaver, 2001: The influence of sea ice physics on simulations of climate change. Journal of Geophysical Research, submitted.

Other publications by A. Weaver during IARC funding period.

1. Weaver, A.J., P.B. Duffy, M. Eby and E.C. Wiebe, 2000: Evaluation of ocean and climate models using present-day observations and forcing. Atmosphere-Ocean, 38, 271—301.

2. Stone, D.A., A.J. Weaver and F.W. Zwiers, 2000: Trends in Canadian precipitation intensity. Atmosphere-Ocean, 38, 321—347.

3. Flato, G.M., G.J. Boer, W.G. Lee, N.A. McFarlane, D. Ramsden, M.C. Reader and A.J. Weaver, 2000: The Canadian Centre for Climate Modelling and Analysis global coupled model and its climate. Climate Dynamics, 16, 451—467.

4. Rutter, N.W., A.J. Weaver, D. Rokosh, A.F. Fanning and D.G. Wright, 2000: Data-model comparison of the Younger Dryas event. Canadian Journal of Earth Sciences, 37, 811—830.

5. Weaver, A.J., and F.W. Zwiers, 2000: Uncertainty in climate change Nature, 407, 571-572..

6. Zwiers, F.W., and A. J. Weaver, 2000: The causes of 20th century warming, Science, 290, 2081-2082.

7. Weaver, A.J. and H. Raptis, 2001: Gender differences in introductory atmospheric and oceanic science exams: Multiple choice versus constructed response questions. Journal of Science Education and Technology, 10, 115-126.

8. Duffy, P.B., M. Eby and A.J. Weaver, 2001: Climate model simulations of effects of increased atmospheric CO2 and loss of sea ice on ocean salinity and tracer uptake. Journal of Climate, 14, 520—532.

9. Bitz, C.M., M.M. Holland, A.J. Weaver and M. Eby, 2001: Simulating the ice-thickness distribution in a coupled climate model. Journal of Geophysical Research, 106, 2441—2463.

10. Schmittner, A. and A.J. Weaver, 2001: Dependence of multiple climate states on ocean mixing parameters. Geophysical Research Letters, 28, 1027—1030.

11. Hillaire-Marcel, C., A. de Vernal, G. Bilodeau and A.J. Weaver, 2001: Absence of deep-water formation in the Labrador Sea during the last interglacial period. Nature, 410, 1073—1077.

12. Yoshimori, M., A.J. Weaver, S.J. Marshall and G.K.C. Clarke, 2001: Glacial termination: Sensitivity to orbital and CO2 forcing in a coupled climate system model. Climate Dynamics, 17, 571-588.

13. McAvaney, B.J., C. Covey, S. Joussaume, V. Kattsov, A. Kitoh, W. Ogana, A.J. Pitman, A.J. Weaver, R.A. Wood, and Z.-C. Zhao, 2001: Model evaluation. In: Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, England.

14. Stone, D.A., A.J. Weaver and R.J. Stouffer, 2001: Projection of climate change onto modes of atmospheric variability. J. Climate, in press.

15. Yoshimori, M., M.C. Reader, A.J. Weaver and N.A. MacFarlane, 2001: On the causes of glacial inception at 116KaBP. Climate Dynamics, in press.

16. Claussen, M., L. A. Mysak, A. J. Weaver, M. Crucifix, T. Fichefet, M.-F. Loutre, S. L. Weber, J. Alcamo, V.A. Alexeev, A. Berger, R. Calov, A. Ganopolski, H. Goosse, G. Lohman, F. Lunkeit, I.I. Mohkov, V. Petoukhov, P. Stone and Z. Wang, 2001: Earth System Models of Intermediate Complexity: Closing the gap in the spectrum of climate system models. Climate Dynamics, submitted.

17. Schmittner, A., K.J. Meissner, M. Eby and A. J. Weaver, 2001: Forcing of the deep ocean circulation in simulations of the Last Glacial Maximum. Paleoceanography, submitted.

18. Meissner, K.J., A. Schmittner, E.C. Wiebe and A.J. Weaver, 2001: Simulations of Heinrich Events in a coupled ocean-atmosphere-sea ice model. Geophysical Research Letters, submitted.


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