climate.uvic.ca

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

2. Type of report: Final

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 Ian Stewart 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: M Holland@UCAR

5. Results:

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 are 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 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 resolves 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.

Importance of wind-driven sea ice motion for the formation of Antarctic Intermediate Water in a global climate model

Saenko, O. A. and A.J. Weaver

The UVic ocean-atmosphere-sea ice model was used to show the importance of wind-driven sea ice motion in the formation of low salinity Antarctic Intermediate Water (AAIW). The model was still able to reasonably simulate a tongue of relatively low salinity AAIW even when the direct momentum transfer from wind to the ocean was neglected, provided that the wind stress was applied to sea ice. In contrast, when the wind stress exclusively drove the ocean, the model failed to capture the properties of AAIW. The large-scale wind-driven sea ice motion preconditioned the growth of sea ice in locations different from regions of ice melt on the annual mean basis. Melting of sea ice then provided fresh water to feed AAIW, whereas its growth made near-surface Antarctic waters saltier, contributing to the formation of AABW. That is, the growth and subsequent offshore transport of sea ice acted as a freshwater conduit from near-shore regions, where AABW is formed, to subpolar regions, where AAIW is formed. Sea ice dynamics were also shown to be important in the simulation of a local salinity minimum at intermediate depths in the southern Indian Ocean and a local salinity maximum in the western Weddell Sea. We concluded that the proper representation of southern hemisphere ventilation processes in climate models requires the use of wind-driven sea ice dynamics

Evidence of change in the Sea of Okhotsk: Implications for the North Pacific

Hill, K.L., A.J. Weaver, H.J. Freeland and A. Bychkov

Russian data from 5 cruises during the period 1949 to 1952 were compared with observations taken during WOCE P1W in 1993 to examine changes which may have occurred in the Sea of Okhotsk during the latter half of the last century. A basinwide warming and freshening of the Sea of Okhotsk was found in the archived data. Since the Sea of Okhotsk is thought to be the major source region for North Pacific Intermediate water (NPIW), calculations were conducted to see whether or not this change in Sea of Okhotsk water properties was consistent with evidence of large-scale freshening of intermediate waters in the North Pacific. From several Okhotsk-to-Pacific salt flux calculations, we concluded that the Sea of Okhotsk was capable of causing the freshening noted in the NPIW over the past half century under certain assumed outflow conditions.

Pathways of freshwater transport in the Southern Ocean.

Saenko, O.A., A.J. Weaver and M.H. England

A region of enhanced northward transport of freshwater across 60 S was found in the UVic coupled model. The fresh water escapes the subantarctic region between about 100 W and the Antarctic Peninsula, rather than being transported to the north in a circumpolar manner. A majority of this freshwater is not of local origin. It is transported from the north-west to the south-east in the Pacific. This freshwater accumulates to the west of the Antarctic Peninsula, which can be seen in both the model-simulated and observed salinities. It then moves to the north in a rather localized region, contributing to the formation of Antarctic Intermediate Water (AAIW). Observations of zonal salinity gradients west of the Antarctic Peninsula suggest that our model results are consistent with AAIW pathways in the real ocean.

Tidally driven mixing in the oceanic general circulation.

Simmons, H.L., S.R. Jayne, L.C. St. Laurent and A.J. Weaver

A parameterization of tidally-driven mixing that evolves spatially and temporally was developed and incorporated into a global ocean model. At equilibrium, globally-averaged mixing had a profile ranging from 0.3 cm2/s at thermocline depths to 7.7 cm2/s in the abyss, with globally averaged values of 0.9 cm2/s, in close agreement with inferences from global balances (Munk, 1966). Water properties were strongly influenced by the combination of weak mixing in the upper ocean and enhanced mixing in the deep ocean. Climatological comparisons showed substantial reduction of temperature/salinity bias, relative to a control run with a uniform vertical mixing rate of 0.9 cm2/s. This suggests that bottom intensified mixing is an essential component of the balances required for maintenance of ocean stratification. We offer an energy-consistent and practical means of both improving the physical representation of ocean mixing processes in climate models and demonstrate the substantial improvements arising from this improved representation of ocean physics.

Reference:

Munk, W. H., Abyssal recipes, Deep Sea Res., 13, 707-730, 1966.

North Atlantic response to the above-normal export of sea ice from the Arctic

Saenko, O.A., E.C. Wiebe and A. J. Weaver

The response of the thermohaline circulation (THC), as well as the freshwater and heat budgets of the northern North Atlantic, to above-normal sea ice export from the Arctic were examined using the UVic coupled model. Two cases were considered: a pulse-like and a persistent above-normal export of sea ice from the Arctic. In the pulse-like case, the export of ice was doubled and sustained at that level for a specified period of time, ranging from one to five years. We showed that, depending on the cumulative ice flux, the strength of the THC and the heat transport from the subtropics to the subpolar North Atlantic decreased by 5-20%. It took 15-20 years for the extra sea ice to convert into the freshwater anomaly and propagate towards and then within the North Atlantic water column of deep water formation, from the surface to the depths below 1000 m. About the same time was needed for the THC to return to its normal (control) state.

In the case of a persistent above-normal export of sea ice from the Arctic, the THC did not collapse, at least within the range of the ice export increase (1.5 to 3 times) used. Rather, after about 15-20 years the THC showed a tendency for returning back to its normal (control) state. Two factors were involved in this process. First, the internal (to the coupled system) redistribution of freshwater between the Arctic and North Atlantic, associated with the enhanced export of sea ice, made the North Atlantic fresher and Arctic Ocean saltier. This, if persistent, decreases the amount of freshwater leaving the Arctic towards the North Atlantic in a liquid form. Second, because the THC did not collapse, the freshwater anomaly propagates downward in the North Atlantic, removing the excess of buoyancy from the surface.

We suggested that the decadal time scale of 15-20 years for North Atlantic THC variability is linked to the variability of sea ice export on different time scales. The variability of sea ice export produces freshwater anomalies within the Arctic Ocean and North Atlantic of opposite sign. It then takes about the same time (15-20 years) for the freshwater anomalies to both propagate horizontally from the Arctic Ocean interior to the North Atlantic region of deep water formation and downward within the North Atlantic water column.

Freshwater transport through the southwest Canadian Arctic Archipelago due to buoyancy and wind forcing: Dolphin and Union Strait to Queen Maud Gulf

Arfeuille, G., A.J. Weaver, E.C. Carmack, F.A. McLaughlin and G.M. Flato

The input of fresh water from the Arctic into the North Atlantic via the transport of sea ice and low salinity waters is an important component of the global climate system through its effects on deepwater formation. Part of this fresh water is transported through the Canadian Arctic Archipelago (CAA) by shallow, coastal-trapped buoyancy-driven boundary currents (BBCs). CTD data collected between Dolphin and Union Strait and Queen Maud Gulf in the summers of 1995, 1999, and 2000 were examined to show the structure and behaviour of BBCs. These hydrographic data were also supplemented with traditional knowledge. The presence of driftwood originating from the Mackenzie River, predominantly on the south side of channels, supports an eastward transport through the region. The presence of BBCs on both sides of the channels appears to be a frequent occurrence, but with the fresher water being more often present on the south shore. Some data from the summer 2000 showed a different feature with much fresher water on the north side. A subsequent strong wind event created a complete reversal of this situation, setting up a strong cross-channel horizontal salinity gradient and an amplified BBC on the south shore. BBCs in the CCA, away from the mouth of major rivers or along ice edges, are likely transient features that respond strongly to local wind stress changes. Still, the net transport of these BBCs is eastward associated with the mean eastward wind stress. Freshwater transport estimates of 30 to 585m3/s (0.95 to 18.5 km3/yr) were found for a variety of BBCs.

On the role of wind-driven sea ice motion on ocean ventilation

Saenko, O.A., A. Schmittner and A.J. Weaver

Simulations with the UVic coupled model were used to investigate the role of wind-driven sea ice motion on ocean ventilation. Two model experiments were analyzed in detail: one including and the other excluding wind-driven sea ice transport. Model simulated concentrations of chlorofluorocarbons (CFCs) were compared with observations from the Weddell Sea, the southeastern Pacific and the North Atlantic. We showed that the buoyancy fluxes associated with sea ice divergence controlled the sites and rates of deep and intermediate water formation in the Southern Ocean. Divergence of sea ice along the Antarctic perimeter facilitated bottom water formation in the Weddell and Ross Seas. Neglecting wind-driven sea ice transport resulted in unrealistic bottom water formation in the Drake Passage and too strong convection along the Southern Ocean sea ice margin, whereas convection in the Weddell and Ross Seas was suppressed. The freshwater fluxes implicitly associated with sea ice export also determined the intensity of the gyre circulation and the rate of downwelling in the Weddell Sea.

In the North Atlantic, the increased sea ice export from the Arctic weakened and shallowed the meridional overturning cell. This resulted in a decreased surface flux of CFCs around 65 N by about a factor of two. At steady state, convection in the North Atlantic was found to be less affected by the buoyancy fluxes associated with sea ice divergence compared to that in the Southern Ocean.

Distinguishing the influences of heat, freshwater and momentum fluxes on ocean circulation and climate

Saenko, O.A., J.M. Gregory, A.J. Weaver and M. Eby

The separate and combined effects of windstress and freshwater forcing on the ocean circulation and on ocean transports of heat and freshwater were analyzed using the UVic coupled model. Suppressing the freshwater flux weakened the north Atlantic meridional overturning by 15% of its control value. With thermal forcing (no freshwater or momentum fluxes), it falls by only 20%. Thermal forcing is therefore dominant, in contradiction to the suggestion that freshwater forcing (net evaporation in the Atlantic) is the major driving force of this circulation. In the north Pacific, the meridional overturning intensified, resulting in the appearance of a deep western boundary current there. Suppressing the momentum flux (windstress) eliminated the subtropical barotropic gyres and reduced the flow through the Drake Passage by 65%, but did not lead to a substantial weakening of the deep outflow from the Atlantic at 30 S. However, with thermal forcing only, the outflow was reduced by 75%, suggesting that in our model the outflow was controlled by thermohaline rather than windstress forcing. Ocean meridional heat transport was somewhat sensitive to the removal of freshwater and momentum forcing, but freshwater transport was not. We showed that gyre transport cannot be attributed uniquely to windstress forcing, and argued that the question remains open as to whether the thermohaline "conveyor" transports freshwater into or out of the Atlantic.

The North Atlantic Oscillation in the CCSM2 and its influence on Arctic climate variability

Holland, M.M.

Recent observations suggest that large and widespread changes are occurring in the Arctic climate system. Many of these are associated with the North Atlantic Oscillation (NAO). The Arctic climate and its response to the NAO was examined in a control simulation of the newly released Community Climate System Model, version 2 (CCSM2). Variability in the atmosphere, ocean, and land systems were considered and the physical mechanisms that drive the variations were examined. It was found that the model realistically simulated the spatial structure and variance of the sea level pressure, surface air temperature, and precipitation associated with the NAO. The sea ice and Arctic ocean response to the NAO were also realistic, although they varied considerably over the length of the timeseries. This was related to variations in the spatial structure of the sea level pressure anomalies associated with the NAO over the timeseries. The model results suggested that these variations, which were similar to changes that occurred over the observed record, were common and part of the natural variability of the system. However, the magnitude of the observed trends over the last forty years in the NAO index were never realized in the model simulations, suggesting that these trends may be associated with changes in anthropogenic forcing, which the simulation examined did not include.

Improving the climate model representation of ocean mixing associated with summertime leads - results from a SHEBA case study

Holland, M.M.

Ice/ocean mixed layer model simulations were run for a SHEBA field project case study from July, 5-August, 8 1998. Observations indicate that, during this time, calm winds occurred and coincided with a rapid warming and freshening of the surface of a lead near the SHEBA camp. A subsequent storm mixed down this warm, fresh water. Simulations of this event were performed to isolate properties of the ocean system which must be accounted for to improve the climate model representation of mixing in summertime leads. A traditional method of simulating the ice/ocean system in which a single ocean mixed layer calculation is forced with fluxes aggregated over the ice and open water portions of the domain was compared to simulations in which separate mixed layer calculations were done for the lead and under-ice ocean systems. It was found that, not only are multiple ocean mixed layer calculations needed to improve the simulations, but the surface of the lead must be realistically embedded within the ice cover. When stable conditions occur, the lead surface remains isolated from the under-ice system. Using this method considerably improves simulated lead vertical temperature and salinity profiles, allowing the lead surface to freshen to 20 ppt and reach temperatures greater than 1.4oC above freezing. This modifies the ice mass budgets, increasing lateral melt rates, open water formation, and the amount of absorbed solar radiation. This has implications for the accurate simulation of climate change due to its effects on the albedo feedback mechanism.

6. Publications (all acknowledge IARC funding support):

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.

3. Stone, D.A., A.J. Weaver and R.J. Stouffer, 2001: Projection of climate change onto modes of atmospheric variability. Journal of Climate, 14, 3551—3565.

4. 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, 106, 19,639—19,655.

5. Saenko, O., and A. J. Weaver, 2001: Importance of wind-driven sea ice motion for the formation of Antarctic Intermediate Water in a global climate model. Geophysical Research Letters, 28, 4147—4150.

6. 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, H. D. Matthews, K. J. Meissner, 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, 39, 361—428.

7. Saenko, O., G. M. Flato and A. J. Weaver, 2002: Improved representation of sea-ice processes in climate models. Atmosphere-Ocean, 40, 21—43.

8. McLaughlin, F.A., E. Carmack, R. Macdonald, A.J. Weaver and J. Smith, 2002: The Canada Basin 1989-1995: Upstream events and far-field effects of the Barents Sea. Journal of Geophysical Research, 107(C7), 19:1—19:20 (10.1029/2001JC000904).

9. Saenko, O.A., E.C. Wiebe, and A.J. Weaver, 2002: North Atlantic response to the above-normal export of sea-ice from the Arctic. J. Geophys. Res., in press.

10. Saenko, O.A., A. Schmittner, and A.J. Weaver, 2002: On the role of wind-driven sea ice motion on ocean ventilation. Journal of Physical Oceanography, in press.

11. Saenko, O.A., J.M. Gregory, A.J. Weaver and M. Eby, 2002: Distinguishing the influences of heat, freshwater and momentum fluxes on ocean circulation and climate. Journal of Climate, in press.

12. Hill, K.L., A.J. Weaver, H.J. Freeland and A. Bychkov, 2002: Evidence of change in the Sea of Okhotsk: Implications for the North Pacific. Atmosphere-Ocean, submitted.

13. Simmons, H.L., S.R. Jayne, L.C. St. Laurent and A.J. Weaver, 2002: Tidally driven mixing in the oceanic general circulation. Ocean Modelling, submitted.

14. Saenko, O.A., A.J. Weaver and M.H. England, 2002: Pathways of freshwater transport in the Southern Ocean. Journal of Physical Oceanography, submitted.

15. Arfeuille, G., A.J. Weaver, E.C. Carmack, F.A. McLaughlin, and G.M. Flato, 2002: Freshwater transport through the southwest Canadian Arctic Archipelago due to buoyancy and wind forcing: Dolphin and Union Strait to Queen Maud Gulf. Atmosphere-Ocean, submitted.

16. Holland, M.M., 2002: Improving the climate model representation of ocean mixing associated with summertime leads - results from a SHEBA case study. J. Geophys. Res., submitted.

17. Holland, M.M., 2002: The North Atlantic Oscillation in the CCSM2 and its influence on Arctic climate variability, J. Climate, submitted.


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