Climate Modelling Group
School of Earth and Ocean Sciences

International Arctic Research Center and Cooperative Institute for Arctic Research
University of Alaska Fairbanks

Global Change Research in the Arctic Research Opportunities Proposal

 

Title: The Arctic Ocean and its Role in Past, Present and Future Climate/ Climate Variability

Principal Investigator:

Andrew Weaver: School of Earth and Ocean Sciences, University of Victoria

PO Box 3055, Victoria, BC, V8W 3P6

Tel: (250) 472-4001; Fax: (250) 472-4004; email: weaver@uvic.ca

Co-Investigators:

Ken Caldeira: Lawrence Livermore National Laboratory, Climate System Modeling Group

PO Box 808, Livermore, CA, 94550

Tel: (925) 423—4191; Fax: (925) 422—6388; email: kenc@llnl.gov

Phil Duffy: Lawrence Livermore National Laboratory, Climate System Modeling Group

PO Box 808, Livermore, CA, 94550

Tel: (925) 422—3722; Fax: (925) 422—6388; email: pduffy@llnl.gov

Clara Deser: Climate and Global Dynamics Division, National Center for Atmospheric

Research, PO Box 3000, Boulder, CO, 80307—3000

Tel: (303) 497—1359; Fax (303) 497—1333; email: cdeser@ucar.edu

Marika Holland: National Center for Atmospheric Research

PO Box 3000, Boulder, CO, 80307—3000

Tel: (303) 497—1734; Fax (303) 497—1700; email: mholland@cgd.ucar.edu

2. Abstract

The proposal details research aimed at fulfilling four main objectives. The central unifying theme of the four objectives is to enhance our understanding of the Arctic Ocean and its role in climate change and climate variability. The first objective involves two phenomenological questions: 1) What processes drive interannual variability in the 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 radiative forcing, or is it simply the Arctic response to low frequency variations in atmospheric forcing? Our approach will involve the analysis of output from the Canadian Centre for Climate Modelling and Analysis (CCCma) coupled model, the National Center for Atmospheric Research (NCAR) climate system model (CSM), and ocean-ice coupled simulations using the NCAR ocean model (NCOM). The systematic testing of improved parameterizations and representations of sea ice and its interaction with the Arctic Ocean using the UVic coupled model.

The second objective involves a detailed sensitivity analysis of the sea ice component of the UVic coupled model. The goal of this analysis is to investigate what processes are important to resolve/include in more complicated coupled atmosphere ocean general circulation models used to project future climate. The third objective of this proposal is the development of a high resolution, control volume finite element model with which to investigate the freshwater balance and circulation of the Arctic Ocean. An adaptive agglomeration additive correction multigrid approach will be employed and the ocean model will eventually be coupled to the dynamic/ thermodynamic sea ice model used in addressing Objective 2. The freshwater budget of the Arctic Ocean is strongly influenced by transport through choke points such as Bering Strait, Fram Strait, Barents Sea and the Canadian Archipelago. The sophisticated numerical approach offered by the control volume finite element method is ideal for representing these features. In addition, converging meridians and irregular coastlines are easily handled.

The final objective of this proposal involves the incorporation of both land surface and ocean carbon cycle models into the UVic coupled model in order to investigate the causes and consequences of northern hemisphere glaciation. The UVic coupled model, with its ocean, atmosphere, sea ice and land ice sheet subcomponent models, is currently capable of maintaining the Laurentide and Fennoscandian ice sheets as well as simulating glacial termination. Our inability to capture glacial inception 116 kyr BP suggests a missing feedback within the coupled model and it is hoped that the inclusion of ocean carbon and especially land surface models will eliminate this problem. The goal of this objective is to develop an Earth System Climate Model with which to investigate future climate change with emphasis on the Arctic through interaction with research conducted in the first three objectives.

The modest funding requested through this proposal will have substantial leverage within Canada through funding from national organizations.

3 Project Description:

3.1 Introduction

The purpose of this proposal is to initiate new research into the role of the Arctic Ocean in climate change and climate variability. The proposal is broken down into four subsections describing research to be conducted under four separate objectives. The first objective (section 3.2) involves two phenomenological questions: 1) What processes drive interannual variability in the 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 radiative forcing, or is it simply the Arctic response to low frequency variations in atmospheric circulation?

The second objective (section 3.3) involves a detailed sensitivity analysis of the sea ice component of the UVic coupled model. The goal of this analysis is to investigate what processes are important to resolve/include in more complicated coupled atmosphere ocean general circulation models used to project future climate. The third objective (section 3.4) is the development of a high resolution, control volume finite element model with which to investigate the freshwater balance and circulation of the Arctic Ocean. An adaptive agglomeration additive correction multigrid approach will be employed and the ocean model will eventually be coupled to the dynamic/thermodynamic sea ice model detailed in section 3.3. The fourth objective (section 3.5) involves incorporating both land surface and ocean carbon cycle models into the UVic coupled model in order to investigate the causes and consequences of northern hemisphere glaciation.

In 1999 the Canadian Institute for Climate Studies and the Atmospheric Environment Service created a new node of the Canadian Climate Research Network (CCRN) under the name: The Arctic Ocean and its Role in Climate Change/Climate Variability. A. Weaver is the principal investigator of this project which involves collaboration with co-investigators: E. Carmack (IOS), G. Flato (CCCma) and L. Mysak (McGill). In addition, a successful proposal was submitted by A. Weaver and G. Flato to the Canadian federal government Climate Change Action Fund entitled: Improved representation of sea ice in the CCCma global coupled climate model. These two projects provide most of support for the research conducted under Objective 1 although we are applying for support for M. Holland and C. Deser so that they can participate in this project.

The federal Natural Sciences and Engineering Research Council (NSERC – the Canadian equivalent of the NSF) also recently awarded multi-year funding to a national team of researchers under the theme: Climate System History and Dynamics. Support from this project will provide partial funding for the work detailed under Objective 4 below. In addition, NSERC has a Strategic Grant program which directly allows one to match research grants obtained from international, industry or non government organizations. A Weaver will approach this program in the second year of this project should the application be successful.

In summary, this proposal, while providing a detailed science plan for A. Weaver’s overall research in the next two years, is only requesting partial funding in order to meet its overall goals. In addition, it provides a mechanism to support collaborations between NCAR, LLNL and UVic.

3.2 Objective 1 – Low frequency Arctic/North Atlantic Interactions

This objective is 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 observed warm, subsurface North Atlantic water intrusion into the Arctic consistent with the response expected from enhanced anthropogenic greenhouse gas forcing, or is it simply the Arctic response to low frequency variations in atmospheric circulation?

3.2.1 What processes drive interannual to interdecadal variability in Arctic freshwater export, and how does this variability affect the global ocean and climate?

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 increases 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.

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) studies concluded that strong northerly winds (in the Fram Strait region) 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 Arctic freshwater export.

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.

Thompson and Wallace (1998) recently analysed the northern hemisphere (north of 20°N) sea-level pressure field and noted that the dominant EOF revealed a spatial pattern whose centre was over the Arctic. Over the Arctic this EOF appeared zonally-symmetric, although signals of opposite sign were located over the Atlantic and Pacific Ocean, breaking down the zonal-symmetry. They argued that land sea contrasts were the source of this asymmetry. They further termed this pattern, which revealed strong seasonal—interdecadal variability, the "Arctic Oscillation" (AO) and noted that it resembled the NAO in the North Atlantic, although the NAO had no signature in the Pacific. Correlations between the AO index and Eurasian surface air temperatures were substantially higher than similar correlations using the NAO index. Finally, they noted that the AO consisted of two components: the first being of equivalent barotropic nature and extending well into the stratosphere, giving rise to a zonally-symmetric pattern; the second being confined to the troposphere and of a baroclinic nature, leading to an asymmetric zonal circulation. This oscillation, which contains the NAO as an apparent subset, appears to have important links to the variability of the Arctic Ocean, its sea ice and hence freshwater export. Indeed the AO may be an important natural mode of the climate system whose low-frequency modulation may be inherently linked to the observed northern hemisphere decadal-interdecadal climate variability.

Our conjecture is that Arctic sea ice export (and its relationship to the NAO and the AO) 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 (Holland et al., 1999) 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 include 1) applying the anomalous wind forcing to certain regions only; 2) adding the anomalous wind forcing effects to the model calculation of latent and sensible heat fluxes; 3) changing the number of categories in the sea ice model; 4) removing the oceanic effects of brine rejection and sea ice melting; 5) modifying the ice/ocean/atmosphere coupling. 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.

The CCCma and NCAR have both conducted several multi-century climate simulations whose results will 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. Greg Flato (CCCma), a co-PI in the CCRN Arctic project, will be intimately involved (at no additional cost) 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 and NCAR CSM coupled general circulation models (GCMs), the UVic coupled model (with its simpler representation of the atmosphere) is viewed as an essential tool to further this understanding.

3.2.2 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, or is it simply the Arctic response to low frequency variations in atmospheric forcing?

In the Arctic Ocean, water of Atlantic origin is evident as a relatively warm and salty water-mass at intermediate depths (200—600m). Recent observations (e.g. Carmack et al. 1995; McLaughlin et al. 1996) indicate that this Atlantic layer 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.

Observations (Carmack et al., 1995; McLaughlin et al., 1996; Carmack et al., 1998a) have also shown that the recent changes in the Arctic, outlined above, are manifested by three processes:

a. signal propagation by narrow, topographically-steered currents which rapidly carry changes in water mass properties around the perimeter of the basin via the Arctic Circumpolar Boundary Current;

b. lateral (not quite isopycnal) transfer of properties by double-diffusive intrusions and caballing;

c. regime shifts in basin-wide water mass structure by the movement of topographically-locked fronts from one ridge and basin system to another. Such changes cause major variations in the distribution of heat, salt and nutrients within the Arctic Ocean.

Two additional processes may play important roles in the thermodynamics of the Arctic Ocean: thermobaricity and geothermal heating. Recent numerical experiments using non-hydrostatic models show that the differential compressibility of seawater (thermobaricity) exerts critical control on convective processes in high-latitude oceans. Such processes are thought to affect overturning in convective gyres and sinking plumes (Carmack and Killworth, 1978), and to exert a type of thermodynamic selective withdrawal of water masses from the Arctic Ocean (Aagaard et al., 1991). Oceanographic data from the Canada and Amundsen basins of the Arctic Ocean show that the lower thousand or so meters of these quasi-isolated basins are mixed by geothermal heating (Carmack et al., 1998b). It has also been suggested that geothermal heating in the Greenland-Iceland-Norwegian Sea system may lead to inter-decadal oscillations in the convective properties of these three basins. Neither of these processes are included in current ocean GCMs.

In some recent experiments using the UVic coupled model (Wiebe and Weaver 1998) which examined the transient 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). Such warm water intrusion only existed in the experiments which replaced the usual horizontal/vertical mixing scheme with either a mixing scheme with reduced near-surface diffusivities, or the Gent and McWilliams (1990) parameterization for mesoscale mixing (which include a rotation of the diffusion tensor to be aligned along isopycnals). In the usual horizontal/vertical mixing case, there was enhanced intrusion of the surface waters which dramatically melted back the ice edge, exposing the ocean surface to the cold polar air. Strong convection then eliminated the signature of the warm Atlantic waters entering the Arctic. During the transient CO2 increase, the ice edge in the other cases was not as greatly affected so that the subsurface signature of the intrusion of warm Atlantic waters survived.

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, it is interesting that the observed warming trend is consistent with our model response to increasing atmospheric CO2.

An alternative explanation is afforded by the compilation of observational and modelling results of Proshutinsky and Johnson (1997) and Grotefendt et al. (1998). They find that the wind-driven circulation of sea-ice and the upper-ocean tends to alternate between two distinct modes: one with a large Beaufort gyre and transpolar drift directed towards Fram Strait; one with a small Beaufort gyre and transpolar drift directed towards the Canadian Archipelago. Fluctuations between these two modes occur on roughly decadal timescales. Their circulation patterns are broadly consistent with the observed shift in freshwater export from Fram Strait to the Canadian Archipelago, a corresponding shift in the import of North Atlantic Water, and a resulting displacement of water mass boundaries.

Swift et al. (1998) show that the recent changes in the Arctic 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, and hence the AO, which corresponds to relatively warm air temperatures in the Greenland Sea region and thus a reduction in oceanic heat loss. 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). Nevertheless, it is clear that channel-flow approaches to the movement of water masses through the Arctic Archipelago are woefully inadequate. Model parameterizations must account for: buoyancy-boundary currents; sub-basin circulation; tidal mixing and frontogenesis; wind-forced and ice-forced surface flows; thermohaline circulation within channels.

This question poses two hypotheses to explain the observed subsurface intrusion of warm North Atlantic waters, together with the shift of the Atlantic layer from the Lomonosov to the Mendeleyev Ridges. Our analysis and model experiments will be aimed at testing these hypotheses.

As noted earlier, initial results (Wiebe and Weaver, 1998), which incorporate more realistic representations of ocean mixing into the UVic coupled model revealed the warm, subsurface intrusion of Atlantic waters into the Arctic Ocean under transient CO2-increase experiments. The transient CO2-increase experiments (Boer et al., 1998a,b) conducted using the present version of the CCCma coupled model (Flato et al., 1998) does not exhibit such a result. This is, however, entirely consistent with the results of Wiebe and Weaver (1998) in which we showed that when the usual horizontal/vertical subgrid scale ocean mixing is used, the increased retreat of sea ice (under transient CO2-increase), leads to the exposure of ocean surface waters to the cold polar air which in turn induces unrealistically strong oceanic convection, wiping out any subsurface signature of warm water intrusion.

The CCCma are presently undertaking new transient CO2-increase experiments with a better representation of the ocean which includes the Gent and McWilliams (1990) parameterization (although they have included a small explicit background lateral diffusivity – the next generation ocean component of the CCCma coupled model is the NCAR Ocean Model [NCOM]). The results from this experiment will be examined for evidence of subsurface intrusion of warm Atlantic water into the Arctic. The Arctic Ocean results from NCAR CSM simulations (which currently use the Gent and McWilliams parameterization) will also be examined. In addition, the UVic coupled model which now uses a rotated coordinate grid (allowing for better resolution of the Arctic), and includes sea ice dynamics will be integrated under increasing CO2 radiative forcing to reexamine the subsurface Arctic intrusion issue.

As also noted earlier, an alternative explanation for the observed changes in Arctic water mass structure is provided by low frequency shifts in atmospheric-forced ice and ocean circulation. Simulations using the UVic coupled model driven by long-term (multi-decadal) timeseries of wind fields (based on observations) will be analysed to asses the extent to which observed changes in the Arctic water masses are consistent with atmospheric circulation variations. Additionally, simulations using the NCAR ocean model which are driven by a 40 year timeseries (1958—1998) of atmospheric forcing will be examined. The results from these simulations will be directly comparable to observations of the Arctic subsurface ocean warming. Finally, it is anticipated that the ice-ocean coupled simulations using the parallel ocean program (POP) model, which uses a rotated coordinate grid and allows for oceanic exchange through the Arctic Archipelago, will be available for analysis during the second year of this proposal.

The investigation of the dynamics of thermohaline intrusions in the Arctic Ocean will involve the analysis and interpretation of hydrographic data. The observational results will be used for the development and improvement of theoretical models of thermohaline intrusions. A number of specific questions will be addressed. For example, how do the intrusions change as they advance from the boundaries into the interior of the ocean basins? How is the shape of the intrusions (i.e., sawtooth structure in temperature-salinity space) related to the vertical fluxes of heat and salt by double-diffusion? How can theories of thermohaline intrusions be improved to better match the observations?

This project will be extended to investigate the effects of mixing processes (i.e., thermohaline intrusions and diffusive-convection) on the larger-scale dynamics of the Arctic Ocean. This will involve the development and improvement of mixing parameterizations for inclusion into ocean GCMs. Model simulations will then be performed with the UVic coupled model to determine how the large-scale dynamics are affected by the smaller-scale mixing processes. A number of specific questions will be addressed: Given a change in the Atlantic water inflow, how long does it take for this signal to propagate through the Arctic system? Given the observed change at the boundaries, are the lateral fluxes associated with thermohaline intrusions large enough to produce the observed warming in the ocean interior? At what rate is the warming transmitted vertically into the upper ocean by diffusive-convection?

3.3 Objective 2 – Sensitivity analysis of dynamic/thermodynamic sea ice processes in a coupled atmosphere-ocean-sea ice model

The Arctic Ocean, its sea-ice cover, and their coupling are poorly represented in coupled atmosphere-ocean general circulation models, and our conjecture is that significant improvements can be made with little additional computational cost.

All coupled models assume that during the process of sea ice formation, brine rejection occurs on the scale of the ocean grid. This assumption is known to cause excessive convection and subsequent vertical mixing and oceanic heat loss (e.g. Duffy and Caldeira, 1997; Caldeira and Duffy, 1998). In reality, brine rejection occurs on very small spatial scales (hundreds of metres – Denbo and Skyllingstad, 1996). To account for this effect Duffy et al. (1998) recently included a more realistic parameterization for brine rejection into the UVic coupled model. In this parameterization rejected salt is mixed to a depth which is calculated based on a prescribed density contrast relative to the surface, prior to the initiation of convection. This approach has the realistic property that rejected salt is mixed more deeply in regions where the vertical density stratification is weak, and less deeply in regions where the stratification is strong. The results from the inclusion of this parameterization, together with the Gent and McWilliams (1990) parameterization for mixing associated with mesoscale eddies, were dramatic and extremely encouraging. Spurious southern ocean convection was eliminated; the formation and representation of Antarctic intermediate Water was substantially enhanced; salinities were more realistic (both globally and locally); the overcooling problem of deep ocean temperatures when the Gent and McWilliams (1990) parameterisation is used was eliminated; sea ice extents were also improved.

As noted above, the present representation of the Arctic Ocean, its sea ice cover and their interaction in most current coupled models is rather crude. In collaboration with G. Flato we propose to undertake a number of sensitivity experiments with both the UVic and the CCCma coupled models to determine whether or not the inclusion of more physically-realistic sea ice processes is climatically important.

The UVic coupled mode currently allows for two types of sea ice thickness-distribution (ITD). The first is the traditional ‘fixed-category’ representation of the ITD used in Hibler (1980) and Flato and Hibler (1995). The second has a ‘Lagrangian’ delta-function representation similar to those used in one-dimensional model studies of Bjork (1992) and Schramm et al. (1997). The ‘Lagrangian’ delta-function ITD method has the additional option allowing for interior temperature resolution which gives the sea ice thermal inertia (unlike zero-heat capacity models) and more realistic heat conduction. The thermodynamic model also explicitly parameterises the effects of brine pockets as in Maykut and Untersteinter (1971). We use the elastic viscous plastic rheology for ice dynamics from Hunke and Dukowicz (1997), which approximates the Hibler (1979) viscous plastic model on synoptic and longer timescales.

The sensitivity to the different ice model components will be examined in the context of the global coupled ice/ocean/atmosphere model. This will include studies on the importance of the ice thickness distribution, the presence of vertical temperature resolution in the ice, the ice/ocean/ atmosphere coupling parameterisations, and the presence of ice dynamics. The influence of these processes on determining the ice mass balance and ice/ocean/atmosphere exchange will be studied in simulations of present day climate and under transient CO2-increase conditions. This will allow us to examine how improvements in sea ice physics modify the simulated climate and its response to perturbations in the climate system.

Eby and Holloway (1995) proposed a complicated method for ocean grid rotation so that the Arctic could be better resolved in global ocean models. In their approach, a separate North Atlantic model, in which the North Atlantic and the Arctic were rotated so that the Equator of the ocean grid ran through the pole, was connected to a second global (less the North Atlantic) model along the original Atlantic equator. This approach required the continuous switching of boundary conditions between the two models. We have recently implemented a simpler grid rotation which rotates the grid globally and hence locates the singularity of the North Pole over central Greenland (for numerical purposes). Internationally, a grid rotation scheme has already been included in the French LMD global coupled model, while alternative schemes are being assessed for inclusion into the NCAR model. We will conduct sensitivity studies using both unrotated and rotated grid versions of the UVic coupled model to determine whether or not the rotation of the grid allows for a more realistic representation of the Arctic Ocean. In addition, through the careful comparison of experiments which use both rotated atmospheric and oceanic components, we will determine whether the interpolation of air-sea fluxes produces any significant differences.

Finally, we have already demonstrated the importance of the parameterization of local effects of brine rejection within the context of the UVic coupled model (Duffy et al. 1998). The improvements realised with this parameterisation were so great that we will conduct an experiment with the CCCma coupled model which includes this effect. The substantial reduction of spurious ocean convection may further reduce, if not eliminate, the need for large flux adjustments in the Southern Ocean domain of the CCCma coupled model and will also reduce them substantially near the ice edge in the north Atlantic.

3.4 Objective 3 –Development of a high resolution, control volume finite element model of the Arctic Ocean

The freshwater budget of the Arctic Ocean is strongly influenced by transport through choke points such as Bering Strait, Fram Strait, Barents Sea and the Canadian Archipelago. These choke points also represent the locations through which the Arctic communicates with the rest of the global ocean. As such, their resolution is important if one wishes to capture the interaction between the Arctic Ocean and the rest of the worlds ocean and in particular to investigate the role of the Arctic in climate change/climate variability. To this end, a Ph.D. student M. Roth will work with A. Weaver at UVic to develop a high resolution control volume finite element model of the Arctic Ocean. The model will be global in domain although high resolution will be reserved for the Arctic Ocean. It is anticipated that in order to avoid difficulties with the specification of surface boundary conditions, the ocean model will be coupled to the sea ice and energy/moisture balance atmosphere model discussed in sections 3.3 and 3.5, respectively.

The control volume based, finite element hybrid method is ideal for handling complicated coastlines and converging meridians. It was developed to meet two requirements: 1)– to conserve mass, momentum, energy, or any scalar such as turbulent kinetic energy and dissipation (k-e) and 2)– to allow complex geometries e.g. coastlines. The geometry is first split into an unstructured set of finite elements. Control volumes are formed from the portions of finite elements surrounding each node. If fluxes between adjacent control volumes are forced to be equal, global conservation is ensured. First developed in a 2D Navier-Stokes framework (Schneider and Raw 1987), the method has since been extended to flow at all speeds (Karimian 1994), three-dimensional flow (Roth 1997), and is the basis for the commercial code TASCFlow.

The difficulty with the control volume finite element approach lies in the representation of the fluxes or integration point values. As discussed by Stubley et al. (1980), a simple central differencing scheme, while formerly 2nd order accurate, leads to negative coefficients in high Peclet number flows resulting in divergent or oscillatory solutions. That is, central differencing can be said to have a good "profile" but a bad "influence". Conversely, upwinding ensures positive coefficients (good influence) but suffers from 1st order accuracy (bad profile). In practice, a parameter is calculated based on local Peclet number which blends the two

An additional difficulty is the coupling of velocity and pressure. Pressure does not appear explicitly in the continuity equation. Without further treatment, solutions can exhibit the classic "chequerboard" pressure solution (Patankar 1980). In the staggered-grid arrangement first introduced by Harlow and Welch (1965), and later popularized under the label SIMPLE (Patankar 1980), all flow variables are located on separate grids. While eliminating the pressure problem, implementation into unstructured grids was difficult at best. The control-volume-based, finite element method is co-located in that all flow variables are associated with the same node.

The great computational bottleneck in the process is the solver. The multigrid method (Briggs 1987) solves systems of equations on successively coarser grids. The additive correction multigrid (ACM) strategy (Hutchinson and Raithby 1986) forms new grids by combining fine grid control volume equations to form coarse grid control volume equations. These coarse grids are solved and the corrections transmitted back to the fine grid. The adaptive agglomeration ACM (Elias et al., 1997), which we shall use, improves by combining control volumes in an unstructured way based upon local coefficients.

Michael Roth, the Ph.D. student involved in this project, developed a 3-D control volume finite element model for the Navier Stokes equations for his M.Sc. thesis. His applications were to fluid flow within a mechanical engineering framework. As such, for his Ph.D. he will be required to implement a treatment of the Coriolis term, oceanic convection, spherical coordinates as well as islands. He will also need to consider ways of coupling his ocean model to the sea ice and atmospheric models discussed earlier.

3.5 Objective 4 – Incorporation of Land Surface and Carbon Cycle Models in the UVic Coupled Model to Investigate the Causes and Consequences of Northern Hemisphere Glaciation.

This objective is centred around three main goals:

1: Was the closure of the Isthmus of Panama (IP) responsible for the onset of northern hemisphere glaciation?

2: Can the transition between the last glacial maximum and the present be reproduced in a coupled atmosphere-ocean-sea ice-continental ice sheet - carbon cycle model?

3: Do the inclusion of land surface feedbacks allow for glacial inception at 116 kyr BP ?

Approximately 3 million years ago the Late Pliocene event occurred, marked by northern hemisphere ice sheet formation and the onset of glacial/interglacial cycles (see Kennett, 1982 for a review). Luyendyk et al. (1972) and Berggren & Hollister (1974) hypothesized that oceanic circulation changes coincident with the IP final elevation were the probable cause. Berggren (1982) suggested that upon IP closure, increased amounts of warm subtropical waters moved northward in the Atlantic. He hypothesized that the stronger and warmer Gulf Stream after closure could have lead to intensified evaporation at midlatitudes and hence precipitation over eastern Canada, Greenland and Western Europe, providing the moisture required for glaciation. Ruddiman et al. (1980) suggested that an intensified, warmer Gulf Stream could have contributed to conditions favorable to ice sheet growth by causing a vigorous meridional atmospheric circulation associated with a strong temperature gradient at the eastern coast of North America. Murdock et al (1997) recently examined the effects of IP closure using the UVic coupled model and did indeed find that deep water formation in the North Atlantic initiated upon IP closure, leading to a warmer North Atlantic. Associated with the warmer North Atlantic was increased area-averaged evaporation by approximately 1.0 cm/yr from 23°N-49°N and precipitation by 0.4 cm/yr from 49°N-88°N. Thus their results were consistent with the Berggren hypothesis but due to their lack of a continental ice sheet model, were unable to quantitatively verify it.

Over the last few years A. Weaver and his group have invested a large amount of effort to develop an atmospheric model suitable for coupling to ocean models, for the purpose of undertaking the long-timescale integrations required to investigate climate variability on the decadal-millennial timescale. To this end they have coupled an ocean GCM to an energy-moisture balance atmosphere model (EMBM), into which a dynamic/ thermodynamic sea ice model has been incorporated (referred to as the UVic coupled model). Atmospheric dynamical feedbacks are included through the Boussinesq assumption and interactive water vapour feedbacks are also parameterized. The original version of the UVic coupled model did not include topography on land. This has been rectified in the latest version of the model and we now also allow for moisture advection. The virtue of the atmospheric component of the coupled model is that we do not need to employ explicit flux adjustments to keep the simulation of the present climate stable. Thus, it is ideally suited for both climate and paleoclimate modelling.

The Marshall and Clarke (1997a,b) UBC Ice Sheet model has also been included into our coupled model and we are able to capture the essential processes involved in glacial termination 21,000 kyr BP. In addition, when forced under perpetual present day atmospheric CO2 levels and orbital parameters, the UVic coupled model produces realistic ice sheets over Greenland and Antarctica. A very challenging and outstanding problem concerning our inability to capture glacial inception 116 kyr BP remains. Since our goal is to determine whether or not the closure of the Isthmus of Panama was responsible for the onset of northern hemisphere glaciation ~ 3 Ma, it is very important that we attempt to resolve the glacial inception issue at 116 kyr BP.

In order to address why our model (as do others) does not capture glacial inception we are taking two approaches. In the first, we are determining whether or not our simple atmosphere is missing some important process. As such, several equilibrium experiments have been conducted under past atmospheric CO2 and orbital conditions, and the sea surface temperature and sea ice mask fields have been passed to the CCCma atmospheric GCM. We are now integrating the CCCma atmospheric GCM to see whether or not our SST/sea ice boundary condition leads to the presence or absence of perpetual snow cover in northern middle to high latitudes.

In the second approach we are attempting to determine whether certain feedbacks are missing in our model that are necessary for us to capture glacial inception. We are now including land surface (BIOME 3) and carbon cycle (in collaboration with researchers at Lawrence Livermore National Laboratories – LLNL) models as well as a parameterization for cloud feedbacks, with the goals of developing an Earth System Climate Model (ESCM).

The capturing of millennial timescale variability and its packaging into Bond Cycles in cold climates, its association with Heinrich events, and its dependence on the mean climatic state remains one of the greatest challenges for paleoclimate modelers. These projects must be viewed as extremely ambitious and high risk. If successful, the knowledge gained from them would be fundamental and of utmost importance.

3.6 Relationship of Objectives 1—4 to the IARC/CIFAR Joint Announcement of Opportunity

This proposal bridges four of the 8 themes from the IARC/CIFAR Joint Announcement of Opportunity although theme III Atmosphere-ice-land-ocean interactions and feedbacks in the Arctic that affect change, including observations and modelling is perhaps its central focus. Our proposition to investigate Arctic/North Atlantic interactions, as well as the recent observation of subsurface warm water intrusion into the Arctic and our proposal to develop a control volume finite element ocean model are central to this theme. Our detailed sensitivity analysis of processes captured in the sea ice component of our coupled model is also directly related to the development of an understanding of atmosphere-ice-ocean interactions and albedo feedbacks. The paleo modelling effort is both important within the context of theme III as well as providing a tool with which to quantitatively examine hypotheses for paleo Arctic climate change. The results from these paleo experiments would be available for comparison with the proxy record as produced under theme II Arctic paleoclimatic reconstructions from ice cores, tree rings, permafrost, lake and ocean sediments. Indeed we would welcome the opportunity to discuss our paleo modelling results with researchers funded under theme II at the annual PI meetings in Fairbanks.

The detection of contemporary climate change necessarily requires an estimate of the background natural climate variability. Our proposal to examine low frequency variability in and around the Arctic is therefore relevant to theme I Detection of contemporary climate change in the Arctic by ground observations, remote sensing and climate "fingerprinting". We have also proposed to conduct transient climate change experiments with which to examine projected Arctic change. This analysis is central to this theme as well as to theme V Impacts and consequences of global climate change, including effects on biota and ecosystems in the Arctic.

4 Data Plan

All model output is routinely archived on our local backup system and are readily available for those who wish access to it. In addition, the UVic coupled model is available on the world wide web for use by anyone requesting it. There currently are users in Chile, US, Canada, Korea, and England.

5 References Cited (not including those listed in the PI biographies)

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Berggren WA. 1982. Role of ocean gateways in climate change, in Climate in earth history, Berger WH, Crowell JC, Eds. pp. 118-125, National Academy Press, Washington, DC.

Berggren WA, Hollister CD. 1974. Paleogeography paleobiogeography and the history of circulation in the Atlantic Ocean. In: Studies in Paleooceanography. Hay WW, Ed. pp. 126—186, Soc. Econ. Paleontol. Mineral. Spec. Publ. 20.

Bjork G. 1992. On the response of the equilibrium thickness distribution of sea ice to export, mechanical deformation, and thermal forcing with application to the Arctic Ocean. J. Geophys. Res. 97:11,287-11,298.

Boer GJ, Flato GM, Reader MC, Ramsden D. 1998. A transient climate change simulation with historical and projected greenhouse gas and aerosol forcing: experimental design and comparison with the instrumental record for the 20th century. Clim. Dyn., submitted.

Boer GJ, Flato GM, Ramsden D. 1998. A transient climate change simulation with historical and projected greenhouse gas and aerosol forcing: projected climate for the 21st century. Clim. Dyn., submitted.

Briggs WL. 1987. A Multigrid Tutorial. Soc. Indust. App. Math.

Carmack EC, Killworth P. 1978. Formation and inter-leaving of abyssal water masses off Wilkes Land, Antarctica. Deep-Sea Research, 25:357-369.

Carmack EC, Macdonald RW, Perkin RG, McLaughlin FA, Pearson RJ. 1995. Evidence for warming of Atlantic water in the southern Canadian Basin of the Arctic Ocean: Results from the Larsen-93 expedition. Geophys. Res. Let. 22:1061-1064.

Carmack EC, Aagaard K, Swift JH, MacDonald RW, McLaughlin FA, Jones EP, Perkin RG, Smith JN, Ellis KM, Killius LR. 1998a. Changes in temperature and tracer distributions within the Arctic Ocean: results from the 1994 Arctic Ocean section. Deep-Sea Res. 44:1487-1502

Carmack EC, Matear R, Perkin RG, McLaughlin FA, MacDonald RW, Aagaard K. 1998b. Combined influence of geothermal heating and biological regeneration on the relict deep waters of the southern Canadian Basin. J. Geophys. Res. submitted.

Cattle H. 1985. Diverting Soviet rivers: Some possible repercussions for the Arctic Ocean. Polar Rec. 22:485- 498.

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Eby M, Holloway G. 1994. Grid transform for incorporating the Arctic in a global ocean model. Clim. Dyn. 10:241-247.

Elias S, Stubley G, Raithby G. 1997. An adaptive agglomeration method for additive correction multigrid. Int. J. Num. Meth. Eng. 40:887—903

Flato GM, Hibler III WD. 1995. Ridging and strength in modeling the thickness distribution of Arctic Sea ice. J. Geophys. Res., 100:18,611—18,626.

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Harlow F, Welch J. 1965. Numerical calculation of time-dependent viscous incompressible flow of fluid with free surface. Phys. Fluid. 8:2182—2189.

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6 Annual Milestones

Year 1:

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.

Year 2:

a) Development of improved parameterisations of sea ice/ocean coupling.

b) Completion of sea ice sensitivity analysis in UVic coupled model and analysis of role of Arctic freshwater forcing on North Atlantic decadal-interdecadal climate variability.

c) Assessment of the hypotheses regarding the observed subsurface Arctic intrusion of warm North Atlantic waters.

d) Incorporation of carbon cycle and land surface models into UVic coupled model1

e) Completion of initial experiments with control volume finite element Arctic ocean model2

1 It is expected that the paleo modelling experiments concerning glacial inception at 116 kyr BP, the role of the opening of the Isthmus of Panama in initiating northern hemisphere glaciation and the transition from the last glacial maximum to the present will be completed mid way into year 3 of this project. Funding support to see the project through to completion will come from the Canadian NSERC Strategic programme which will be approached for matching funds to this IARC proposal.

2 The complete development of a control volume finite element Arctic ocean model will most likely take three years. Funding support to see the project through to completion will come from the Canadian NSERC Strategic programme which will be approached for matching funds to this IARC proposal.

7 Statement of project responsibilities for each PI and participant

As noted earlier, this proposal describes the overall two year research plan for A. Weaver and his research group. The present proposal is designed to add substantial value to the three research projects initiated by Canadian national funding agencies: The Canadian Climate Research Network (CCRN) Arctic node whose PI is A. Weaver (with co-investigators E. Carmack, G. Flato and L. Mysak); the Climate Change Action Fund (CCAF – with co-investigator G. Flato); the NSERC Climate System History and Dynamics (CSHD) project (with co-investigators across Canada). In addition, new research has also been proposed for funding.

Below is a brief table listing the researchers at UVic who will be working on this project. The sources used to provide their support are also listed with proposed IARC funding indicated in bold (see UVic budget for a discussion).

Research Associates/ Start Date Funding Source

Postdoctoral Fellows

Andreas Schmittner January 1, 2000 CSHD Paleo

Katrin Messiner January 1, 2000 CCRN Arctic

Harper Simmons March 31, 2000 (tentative) CCAF

Michael Eby Currently here NSERC1/IARC

Edward Wiebe Currently here NSERC1/IARC

Ph.D. Students

Tracy Ewen Currently here IARC

Michael Roth Currently here IARC

Daithi Stone Currently here NSERC2

Masakazu Yoshimori Currently here CSHD Paleo

Melanie Cottet Puinel January 1, 2000 CCRN Arctic

Fiona McLaughlin3 Currently here IOS

Gilles Arfeuille4 Currently here CCRN Arctic

M.Sc. Students

Linda Waterman September 30, 1999 IARC

Katy Hill January 1, 2000 NSERC2

1 NSERC provides most of the salary support for M. Eby and E. Wiebe. Only partial support is requested here.

2 NSERC provides additional operating support for my research.

3 F. McLaughlin is a full time employee at the Institute of Ocean Sciences conducting part time doctoral research.

4 G. Arfeuille’s primary supervisor is E. Carmack; A. Weaver is his co-supervisor.

A. Weaver will be active in all components of this proposal and will ensure their project goals are attained. His primary responsibility will be analysing the output from experiments conducted under Objectives 1 and 2. Tracy Ewen who just arrived at UVic will be involved in collaborative work with LLNL researchers to incorporate a carbon cycle model into the UVic coupled model (Objective 4) as part of her thesis. The land surface model component of Objective 4 will be dealt with by postdocs funded off other sources. Michael Roth who also recently arrived will be developing the finite element model for his thesis (Objective 3). Linda Waterman who will arrive from CSIRO Australia by Oct. 1, 1999 will focus her thesis on the sensitivity analysis mentioned in Objective 3. Mike Eby is the person responsible for code development and will assist all researchers in ensuring that the land surface and ocean carbon models are compatible with the UVic coupled model. E. Wiebe is the UVic Climate Modelling Lab Systems Analyst.

M. Holland and C. Deser will focus on the first two objectives of this proposal. This will include the analysis of climate variability and climate change simulations in the UVic and CCCma models. Particular attention will be given to the Arctic/North Atlantic fresh water exchange variability, its impact on the North Atlantic deep water formation, and the possible propagation of anomalous oceanic conditions into the interior of the Arctic Ocean. M. Holland will also analyze the results from the sea ice sensitivity simulations and work to develop improved parameterizations of the ice cover and ice/ocean/atmosphere exchange.

Drs. Duffy and Caldeira will participate in several aspects of the proposed work. First, they contribute to the implementation of the LLNL brine-plume parameterization in the CCCma model by analyzing simulations performed with and without this parameterization. Once the implementation of the parameterization has been validated, subsequent simulations will be performed to evaluate the effect of the parameterization on the simulated fresh-water budget of the Arctic. Second, they will complete development of a version of the UVic EMBM capable of running on massively parallel computers. This parallel EMBM, when coupled to a parallel-capable ocean-sea ice model, will result in dramatically increased execution speeds. Third, Duffy and Caldeira will perform simulations to evaluate the effect of adding advective transport of moisture to the EMBM on simulated climate variability. This improvement to the EMBM has already been implemented, and was shown to produce dramatic improvements in the model's time-averaged simulated precipitation. The work proposed here will evaluate the effect of this model improvement on simulated natural climate variability. Finally, Duffy and Caldeira will collaborate with A. Weaver and T. Ewen to add an ocean carbon cycle modeling capability to the UVic model by coupling the LLNL ocean biogeochemistry model to the ocean, atmosphere, sea ice and land ice components of the UVic model.