Title: The Arctic Ocean and its Role in Climate Change/Variability
Principal Investigator:Andrew Weaver: School of Earth and Ocean Sciences, University of Victoria, PO Box 3055, Victoria, BC, V8W 3P6
Ed Carmack: Institute of Ocean Sciences, PO Box 6000, Sidney BC, V8L 4B2
Tel: (250) 363-6585; Fax (250) 363-6746
Greg Flato: Canadian Centre for Climate Modelling and Analysis, University
of Victoria, PO Box 1700, Victoria, BC, V8W 2Y2
Tel: (250) 363-8233; Fax: (250) 363-8247
Lawrence Mysak: Department of Atmospheric & Oceanic Sciences, McGill University
805 Sherbrooke St. W., Montreal, QC, H3A 2K6
Tel: (514) 398 3768; Fax: 514-398-6115
The purpose of this proposal is to initiate new research into the role of the Arctic Ocean in climate change and climate variability. A team of researchers has been put together with expertise in the areas of ocean/ice/climate modelling, ocean data collection and data analysis to tackle the scientific questions outlined below. The theoretical understanding and modelling technology developed through the course of this project will benefit CCCma global climate modelling activities, and will compliment work being done in other CRN nodes and CRYSYS. For example, the ongoing CRYSYS collaborative project is focused primarily on sea-ice, its snow cover and the terrestrial cryosphere. The present proposal will provide a complimentary oceanographic perspective, especially with regard to coupled sea-ice and ocean processes. In addition, it will provide an additional Canadian contribution to the WCRP ACSYS and CLIVAR international programmes.
The proposal will have both a phenomenological and a technological component. The phenomenological component will be designed to address the following two scientific questions:
1: What processes drive interannual to interdecadal variability in Arctic freshwater export, and how does this variability affect the global ocean and climate?
2: Is the recent observation of warm, subsurface North Atlantic water intrusion into the Arctic consistent with the response expected from enhanced anthropogenic greenhouse gas forcing?
The technological component of the proposal will aim to:
1: Develop improved parameterisations of the Arctic Ocean, its sea ice cover and their coupling for potential use in the CCCma coupled model.
Approximately 10% of the world's river runoff, accounting for ~3300 km3/yr (with large seasonal and interannual variability Cattle, 1985), enters the Arctic Ocean, which occupies only ~5% of the total ocean surface area and ~1.5% of its volume. Bering Strait inflow represents the second largest freshwater source for the Arctic Ocean (~1670 km3/yr ), with precipitation minus evaporation (~900 km3/yr ) and the import of freshwater in the Norwegian coastal current (~330 km3/yr ) accounting for the remainder of the freshwater sources. The sources of freshwater for the Arctic are balanced primarily by the export of sea ice in the East Greenland Current, which accounts for a freshwater loss to the Arctic and a gain by the Greenland Sea of ~2800 km3/yr (Aagaard and Carmack 1989). The exchange of water through the Canadian Archipelago and Fram Strait results in a loss of approximately 900 km3/yr and 820 km3/yr of freshwater, respectively. At low temperatures the density of sea water is largely controlled by salinity. As such, variations in the freshwater exchange (via both ocean and sea ice transports) between the Arctic and Atlantic Oceans are likely to affect the formation of deep and intermediate water masses there. Indeed, modelling results from Mauritzen and Häkkinen (1997) show that the thermohaline circulation increased by 10-20% in response to a decrease in sea ice export of 800 km3. The relative strength of the freshwater sources to the Nordic and Labrador Seas from the Arctic will also likely influence the preferred location and relative strengths of deep water formation. From a relatively short timeseries (1979-1985), Steele et al. (1996) show that simulated interannual variability in the outflow through the Canadian Archipelago is anticorrelated with the outflow through Fram Strait, with the Fram Strait anomalies leading the Canadian Archipelago anomalies by one year. This may explain why deep water formation in the Nordic and Labrador Seas have been observed to be out of phase in the past few decades. Alternatively, Mysak et al. (1990) and Mysak and Venegas (1998) suggest that this out of phase relationship may be associated with Greenland Sea ice export anomalies which freshen the surface waters and result in subsequent Labrador Sea ice anomalies once the freshwater has reached that region.
The large changes that occur in Arctic/North Atlantic freshwater exchange are epitomised by the Great Salinity Anomaly (GSA) of the late 1960s. This event freshened the upper 500 m of the northern North Atlantic with a freshwater excess of approximately 2000 km3 (or 0.032 Sv over a two year period). Dickson et al. (1988) traced this fresh anomaly as it was advected around the subpolar gyre for over 14 years. It originated north of Iceland in the late 1960s, moving southwestward into the Labrador Sea (1971-1973) and then proceeding across the north Atlantic, returning to the Greenland Sea in 1981-1982.
Several studies have examined the cause of the GSA and have generally determined that it was a result of Arctic/North Atlantic interactions. Both modelling (Häkkinen 1993) and observational (Walsh and Chapman 1990; Wohlleben and Weaver 1995; Mysak and Venegas 1998) studies concluded that strong northerly winds caused an increased sea ice export into the Greenland Sea. The large freshwater flux anomaly that was associated with this transport was likely enhanced by the relatively large advection of thick ice from north of Greenland. Additionally, as simulated by Häkkinen (1993), increased oceanic transport of freshwater from the Arctic occurred. This was caused by fresh anomalies within the Siberian Sea that were advected across the Arctic, entering the Greenland Sea approximately 4 years later. During the GSA, the anomalous sea ice and oceanic freshwater transports were coincident, resulting in a significant and persistent freshening of the north Atlantic. This appears to have resulted in a reduction of deep water formation with winter convection in the Labrador sea limited to the upper 200 m (compared to 1000-1500 m for 1971-1973) (Lazier 1980).
North Atlantic SST records for the past century reveal slowly varying basin-scale changes including cold anomalies prior to 1920, warming from 1930-1940, and cooling again in the 1960s. Kushnir (1994) described the SST pattern associated with these long-term changes as uni-polar with a strong maximum around Iceland and in the Labrador Sea and a weaker maximum in a band near 35°N across the central Atlantic. The atmospheric pattern associated with the cooling in the 1960s has a negative pressure anomaly to the east of positive SST anomalies (also see Deser and Blackmon 1993 who suggest the pattern resembles the North Atlantic Oscillation [NAO]). Because the SLP anomalies appear downstream of the SST anomalies, these authors suggest that the atmosphere is responding to the ocean on these timescales.
Evidence for changes in the subpolar North Atlantic Ocean over similar timescales, compiled by Dickson et al. (1996), indicates that synchronous with the cooling in the late 1960s, convective activity reached a maximum in the Greenland Sea and a minimum in the Labrador Sea. These convective extremes occurred at the approximate time of the GSA. Since the early 1970s, the Greenland Sea has become progressively more saline and warmer through horizontal exchange with the deep waters of the Arctic Ocean. At the same time, the Labrador Sea has become colder and fresher as a result of local deep convection. Reverdin et al. (1997) explored patterns associated with salinity anomalies and found a single pattern explains 70% of the variance of lagged salinity anomalies. The pattern represents a signal originating in the Labrador Sea that propagates from the west to the northeast in the subpolar gyre. The strong correlation between salinity and sea ice in the Labrador Sea lead Reverdin et al. (1997) to link the salinity anomalies to the export of Arctic freshwater.
Delworth et al. (1993) described the first coupled ocean-atmosphere GCM study of long-term thermohaline variability. They associated the variability primarily with oceanic processes. Later Delworth et al. (1997) found salinity anomalies in the surface layer of the Arctic Ocean precede anomalies of the thermohaline intensity by 10-15 years. In agreement with the proposed climate cycle of Wohlleben and Weaver (1995), these Arctic freshwater anomalies are connected to the North Atlantic through SLP anomalies in the Greenland Sea resembling the pattern that Walsh and Chapman (1990) report preceded the GSA. Weaver and Valcke (1998) gave further evidence to suggest that the GFDL model thermohaline variability is a mode of the fully coupled atmosphere-ocean-ice system.
Recent observations (e.g. Carmack et al. 1995; McLaughlin et al. 1996) indicate that the Atlantic layer within the Arctic Ocean has undergone large changes since 1990. These include a shift in the frontal structure which separates different Atlantic layer water masses (from the Lomonosov to the Mendeleyev ridge), and a significant warming of the Atlantic layer. By 1994, this warming extended across the Nansen, Amundsen and Makarov Basins. Swift et al. (1998) show that these changes are likely caused by an increase in the temperature of the Atlantic waters which enter the Arctic Basin through Fram Strait. The anomalous warmth of these waters appears to be correlated with the NAO which corresponds to relatively warm air temperatures in the Greenland sea region and thus a reduction in oceanic heat loss. The temperature signal of these waters is transported into the Arctic Ocean by topographically-steered boundary currents. It then enters the interior ocean through intrusive layers which extend laterally into the ocean basins (Carmack et al. 1998). An open question remains as to where the Arctic waters displaced through the intrusion of the Atlantic layer went, although enhanced transport through the Canadian Archipelago is plausible. This would be consistent with recent observations of anomalous cold and fresh waters in the Labrador Sea since the late 1980s (Dickson et al. 1996).
Outline of proposed research:
Based on the background analysis presented above, our conjecture is that Arctic sea ice export (and its relationship to the NAO) plays an integral role in decadal-interdecadal North Atlantic ocean/climate variability.
The impact of ice export on climate variability will initially be addressed by applying an anomalous wind stress forcing to the UVic coupled model. The first 20 EOFs from NCEP reanalysis SLP data have been used to generate a synthetic anomalous wind stress field which has been applied over the North Atlantic Ocean. Initial results are extremely encouraging as the model reveals decadal-interdecadal variability around the North Atlantic which is intimately linked to the export of sea ice from the Arctic. Nevertheless, much analysis remains to isolate the dominant mechanism and timescale for the variability and to unequivocally prove that sea ice dynamics are crucial to the oscillation. These sensitivity analyses will involve 1) applying the anomalous wind forcing only over ice; 2) shutting off the oceanic ice advection; 3) adding the anomalous wind forcing effects to the model calculation of latent and sensible heat fluxes; 4) changing the number of categories in the thermodynamic component of the sea ice model; 5) removing the oceanic effects of brine rejection and sea ice melting; 6) using climatological (from the spin up of the coupled model) freshwater fluxes or heat fluxes to determine whether heat or freshwater flux changes amplify or diminish the variability. In addition, it will be important for us to determine, through sensitivity analyses to internal sea ice parameters (e.g., number of categories, shear strength, vertical temperature resolution, ice-ocean coupling parameter, albedo), whether or not the UVic coupled model allows self-sustained oscillations within a particular parameter range.
In addition, Mysak and his group will use the Tremblay and Mysak (1997) sea ice model, in a limited ocean domain, to investigate the issue of Arctic freshwater export and its effects on low frequency North Atlantic climate variability. Their results will be compared with those obtained using the UVic coupled model. We will also couple the Tremblay and Mysak (1997) sea ice model into the UVic coupled model to conduct further intercomparisons as to the effects of sea ice dynamics on Arctic freshwater export to the North Atlantic. Mysak and his team will also undertake a detailed statistical analysis of long records of climate data within the Arctic region to further the ideas of Mysak and Venegas (1998). In particular, they will look for modes of climate variability within the Arctic that affect the export of sea ice into the North Atlantic.
The CCCma has conducted several multi-century climate simulations whose results could be analysed with regard to a variety of processes, such as: Arctic freshwater export, sea-ice export, and variations in ocean temperature/salinity structure and circulation. Flato will be intimately involved in the efforts conducted using the UVic coupled model, and is currently conducting a range of sensitivity studies related to the role of sea ice dynamics in climate variability and change. Since it is impossible to conduct the large number of experiments needed to understand the sensitivity of sea ice models and their parameterisations in the CCCma coupled general circulation model, the UVic coupled model (with its simpler representation of the atmosphere) is viewed as an essential tool to further this understanding.
In order to facilitate close collaboration and smooth technology transfer, one of the post-doctoral fellows supported under this proposal would work directly with the CCCma and their coupled model, implementing improved parameterizations of ice-ocean processes and conducting process studies. Examples of the sort of parameterizations which might be pursued include: vertical heat transfer in sea-ice, sub-grid-scale variability of ice surface properties and heat exchange, sea-ice rheology, and oceanic mixing of brine rejected by ice growth. By providing explicitly for the development and study of such parameterizations in the CCCma model, in collaboration with the broader range of process studies to be undertaken within the proposed Node, technological developments will be directly available within the CCCma modelling environment.
In some recent experiments using the UVic coupled model (Wiebe and Weaver 1998) to examine the transient ocean response of the climate system to increasing anthropogenic CO2, we found that subsurface warm waters intruded into the Arctic. Once in the Arctic, these waters were slowly advected and diffused throughout the basin, filling the middle layers with an anomalously warm water mass of Atlantic origin (see http://wikyonos.seos.uvic.ca/climate-lab/movies.html). While this warming of the mid-depth Arctic Ocean is qualitatively similar to the aforementioned recent observed patterns of subsurface warming, there are several discrepancies, including a spreading timescale which is too slow and a maximum warming which is slightly too deep. The lack of a proper representation of the Arctic Ocean in the Wiebe and Weaver (1998) version of the coupled model severely limited their ability to quantitatively capture the dynamics of the region. Nevertheless this intriguing finding warrants further exploration and we propose to do this with the more recent version of the UVic coupled model which now uses a rotated coordinate grid (allowing for better resolution of the Arctic), and includes sea ice dynamics.
Carmack will focus on ocean field programmes in the Arctic to aid in ocean/ice model development and validation. This focus takes advantage of his access to Canadian Coast Guard icebreakers and arctic logistic bases. His work will entail: a) observing change within the Arctic system; b) measuring the transport of freshwater components through the Canadian Arctic Archipelago; and c) analysing data related to T/S intrusions as an agent of thermohaline transition. The latter work will be carried out together with a postdoctoral research associate, in order to determine what these imply in terms of model transports and diffusivities
a) Initial sensitivity analysis of various sea ice models conducted in UVic coupled model.
b) Initial analysis of T/S intrusions and their implications for oceanic advection and diffusion.
c) Two manuscripts to be written discussing the role of Arctic Freshwater export (via the ocean and sea ice) on the decadal-interdecadal variability of the North Atlantic Climate.
d) Initial analysis of statistical relationships between Arctic climate indices.
a) Development of improved parameterisations of sea ice/ocean coupling.
b) Completion of sea ice sensitivity analysis in UVic coupled model including results from Tremblay and Mysak model.
c) Detailed understanding as to whether or not the observed subsurface Arctic intrusion of warm North Atlantic waters is consistent with anthropogenic response due to increasing greenhouse gases.
d) Analysis of freshwater export and role of sea ice dynamics from CCCma model.
e) Completion of analysis of statistical relationships between Arctic climate indices.
Transfer of technology to CCCma in terms of:
(i) Improved parameterisation of processes important for the Arctic Ocean, its ice cover and their coupling.
(ii) Completion of analysis of role of Arctic freshwater forcing on North Atlantic decadal-interdecadal climate variability.
Training of Highly Qualified Personnel:
The training of highly qualified personnel is an integral part of this project and the majority of the requested funds would be used to support salary costs for 4 postdocs and 5 graduate students, whose supervision will be distributed amongst the investigators. These young researchers would be exposed to a dynamic research environment which transcends traditional disciplines. Carmack is committed to providing opportunities for students and postdocs (both modellers and data analysts) to participate in Arctic field experiments aboard Canadian Coast Guard icebreakers in order to provide young investigators with an understanding of the nature of the Arctic and its environment, as well as the meaning and limits of data collected in remote regions.
The proposed budget shown below is for each of the 1999-00, 2000-01 and 2001-02 fiscal years.
1) A total of 4 postdoctoral research associates would be supported at a rate of 40,000 per annum each (including benefits).
All postdoctoral research associate would be expected to interact with all investigators in this project. One postdoc would be located at McGill while the rest would be located in Victoria.
2) A total of 5 MSc and PhD students would be funded on an ongoing basis (at an NSERC rate of 16,500 p.a.). Based on the experience of the PIs, such an initiative will also attract excellent scholarship students.
All UVic PIs would be on the supervisory committees for each student. It is expected that Dr. Mysak would act as an external examiner for UVic MSc students and conversely Drs. Carmack, Flato and Weaver would act as an external for McGill MSc students.
3) Travel expense for students, research associates and university investigators to the annual CMOS meeting each year. Travel is also to be used for Mysak and his team to participate in annual meetings in Victoria.
4) Publication charges and other operating costs for university participants.
5) Partial salary for secretary (W. Lewis) to work at Secretariat as well as communication charges (courier, phone fax etc.)
The team of researchers are well linked with related international research efforts which are summarised below:
Weaver is currently a member of the Steering Committee for the Arctic System Science/Ocean-Atmosphere Interactions component of the NSF Arctic Systems Science Program. He is also a member of the US National Academy of Sciences Committee on Major Ocean Programs and served on the local organising committee for the WCRP ACSYS 2nd Scientific Conference held in Orcas Island in November 1997. He is currently a Lead Author on Chapter 8 of the IPCC 3rd Scientific Assessment.
Carmack is and has been heavily involved in national and international committees dealing with Arctic climate issues. He was a charter member of the ACSYS Scientific Steering Group, and Chair of the Canadian ACSYS committee which produced the Canadian ACSYS Science Plan. He is a past member of the U.S. Polar research board and RSC Canadian Arctic Panel and has served as Chief Canadian Scientist for the 1994 Arctic Ocean Section and the 1997/98 JOIS/SHEBA Program.
Flato is currently a member of the ACSYS Numerical Experimentation Group and its Sea-Ice Model Intercomparison Project, the PI of the WCRP/CMIP cryosphere subproject, and a Co-I in CRYSYS. In addition, Flato is a co-developer of the CCCma global coupled model.
Mysak is a Fellow of the Royal Society of Canada and Member of the Order of Canada. He currently is involved in the Canadian CRYSYS programme and is on the Scientific Advisory Committee for the International Arctic Research Centre in Fairbanks, Alaska.
Weaver has already secured matching NSERC Strategic support for the three year initial duration of this project. In Year 3 we anticipate approaching NSERC for a new strategic initiative to support Canadian university participation in the international WCRP ACSYS programme, which will be in place until 2004 as WCRP CLIC (Cryosphere and Climate) programme is developed. In addition, a collaboration with Phil Duffy (Lawrence Livermore National Laboratories) will be continued.
Relationship to ACSYS and CLIVAR:
This proposal is directly related to the ocean and sea ice components of the ACSYS modelling programme (WCRP 1994). Our sea ice modelling sensitivity analysis directly addresses most of the questions on page 49 of WCRP (1994) in the Arctic Numerical Experimentation Programme. Our attempts to develop better ice-ocean coupling techniques is also central to the Ocean Process Models (page 51) of the ACSYS Science plan. Finally, the contribution by Carmack will directly address (page 56) the data requirements for Arctic model initialisation, forcing and validation.
As noted by WCRP (1994) on page 57: "Two sets of problems concerning the role of the Arctic in the climate system can be addressed with coupled atmosphere-ocean circulation models: the influence of the Arctic on natural climate variations and the impact on the response of the climate system to global warming. These model investigations will be promoted and co-ordinated by the WCRP research programme on climate variability (CLIVAR). The contribution of ACSYS to these activities should be to ensure that polar processes are properly represented in the coupled model. This includes the delivery of an optimised dynamic-thermodynamic sea-ice model and accurate ocean physics, tailored for the Arctic area.". Thus our two central scientific questions are key to one of the central goals of ACSYS and CLIVAR. Indeed, as a group, we will be positioned to make a major contribution to WCRP activities in this area. Given Canada's abundance of Arctic territory and its attempts to reinforce sovereignty of this area, we believe it is in the nation's interest to undertake research in this region.
During the course of the above collaborative research, various model enhancements and process parameterisations will necessarily be developed, particularly with regard to representation of Arctic Ocean circulation, water mass formation and mixing, ice-ocean coupling, and sea-ice processes. These will be available for use in the CCCma global model. The method for technology transfer to the CCCma will come directly via Greg Flato who is a co-PI of our project.
We anticipate setting up an advisory panel (which will necessarily include members from CCCma) to ensure that this research contribution directly and successfully to AES/CCCma coupled modelling activities.
The Secretariat for this group will be located in room 296b of the Gordon Head Complex. Wanda Lewis is in place to handle all correspondence associated with this project as well as distributing funds and maintaining the budget. The group will meet twice a year. One of these meetings will be at the annual CMOS congress with an additional meeting alternating between Montreal and Victoria. We have already requested a special Arctic session at the 2000 CMOS conference in Victoria, B.C.
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