Climate Modelling Group
School of Earth and Ocean Sciences


7.7.1. Title

The role of the North Atlantic Thermohaline Circulation in Climate/Climate Variability

7.7.2. Investigator

Name: Andrew Weaver
Institution: University of Victoria
% Research Time: 80%

7.7.3. Progress during WOCE-2

The Principal Investigator, Dr. A. Weaver, received research grant funding under WOCE-2 for his project entitled: Ocean Modelling/Model Development. Numerous projects were completed and are either in print, in press or at the submitted stage of the publication process. Details of all the accomplishments may be found in the attached Progress Reports for the 1993-94 and 1994-95 fiscal years. Below, a few selected accomplishments have been highlighted.

The references listed above may all be found in the NSERC Personal Data Form 100 for Dr. A. Weaver.

7.7.4. Summary

In this proposal climate models will be developed to study natural climate variability in the North Atlantic and its global teleconnections on the decadal-to-century timescale. To begin with, an approach will be used analogous to the methodology involved in numerical weather prediction (NWP). Since timescales of variability in the ocean are of the order of months or longer, it is common in NWP to parameterize the ocean as a simple mixed layer. Conversely, since the timescale of variability of atmospheric processes is short compared to the timescales of interest in this proposal (decades to century), it is appropriate, as a first step, to couple a simple EMBM to an OGCM. Since the timescale for variability of the cryosphere lies between that for the atmosphere and ocean, a TIM will be used.

Towards the end of this project simplified dynamics will be added to the EMBM to improve the realism of the atmospheric component of the coupled model. This AGCM will be based on a simplified version of the CCC AGCM and will allow for a better representation of atmospheric transport processes and surface winds.

Before using the coupled model to investigate climate variability it, and its component models, must be validated against observations. The climatology of the OGCM and its sub-grid scale mixing schemes, will be quantitatively validated against WOCE Freon observations. The EMBM will be validated using observed surface fluxes and poleward transports of heat and freshwater while the TIM will make use of observed sea-ice extents.

Through a systematic comparison of simple process-oriented models and three-dimensional models (in idealized, North Atlantic and global domains) a quantitative understanding of decadal-century climate/ climate variability will be obtained. This understanding will be fundamental in developing models for the purpose of climate prediction.

7.7.5. Objectives

The specific objectives of this proposal are to:
1. understand processes of decadal-interdecadal variability in the North Atlantic using a coupled OGCM-EMBM-TIM. The OGCM will have variable resolution ranging from 4 deg x 4 deg to 1/4 deg x 1/4 deg.

2. understand the role of eddies in transporting heat and salt in the North Atlantic Ocean.

3. use Freon as a tracer to validate the climatology of both North Atlantic and global ocean models as well as their sub-grid scale mixing parameterizations.

4. use a global OGCM and coupled OGCM-EMBM-TIM to examine teleconnections associated with processes in the North Atlantic.

5. develop a simplified dynamical AGCM which will be coupled to the OGCM and TIM to investigate processes of decadal-interdecadal variability in the North Atlantic.

7.7.6. Relationship to Canadian WOCE goals

The specific objectives outlined in section 7.7.5 are fundamentally linked to the goals of Canadian WOCE. In addition, the global OGCM mentioned in objective 4 will be provided to Canadian WOCE researchers as a community model (see section 2.2) to facilitate strong collaborations within WOCE-3.

Objectives 1 and 5 are centrally linked with the overall scientific objective, and in particular the fourth sub-objective, of Canadian WOCE. As discussed in section 7.7.7 and 7.7.8 below, a quantitative understanding of decadal-interdecadal climate variability in the North Atlantic can only be achieved through the use of fully coupled atmosphere-ice-ocean models. Objectives 1 and 5 offer a hierarchical approach in the modelling with the initial use of a simplified EMBM and the eventual inclusion of atmospheric dynamics.

Objective 2 will arise as an offshoot of objective 1. This too is centrally linked to the overall scientific objective and fourth sub-objective of Canadian WOCE since it is still an open question as to whether or not eddies are important in the poleward transport of heat and salt. Furthermore, comparisons will be done between idealized geometry simulations conducted at the University of Victoria and more realistic simulations conducted under the community eddy-resolving model effort discussed in section 2.1.

The third objective in section 7.7.5 is a necessary step if one is to have confidence in the simulations which will be undertaken to meet objectives 1, 2, 4, 5. WOCE Freon section data will be used to validate both the climatology and sub-grid scale processes of the North Atlantic and global models. As such, this objective also directly relates to the second sub-objective of Canadian WOCE in quantitatively analyzing the role of mixing in determining the characteristics of water masses.

In the fourth objective, both a coupled and an uncoupled global ocean model will be used in to examine the global teleconnections associated with North Atlantic decadal-interdecadal variability. This objective will allow for a quantitative understanding of how the North Atlantic influences the global ocean circulation (and climate). In addition, it will allow for the examination of how processes occurring in other oceans (e.g., Pacific, Southern) affect the North Atlantic.

7.7.7. Introduction

This section has been partitioned into a number of subsections, each of which deals with a particular subcomponent of the project. Throughout the section scientific questions are posed and a brief description of how these questions will be addressed is also presented. More details of the methodology to be used can be found in section 7.7.8.

7.7.7.1. Decadal-interdecadal variability

As discussed earlier in this document, there is substantial evidence for decadal climate variability in the air-sea-ice climate system. Recent hypotheses suggest that decadal variability in the Pacific may be either linked to changes in the El Niño/La Niña signal in the equatorial Pacific (Trenberth and Hurrell, 1994) or to midlatitude air-sea instabilities (Latif and Barnett, 1994). It is not clear whether similar mechanisms exist in the Atlantic or whether there is a relationship between the Pacific and Atlantic modes of variability. Below recent evidence is presented to show that decadal variability can exist in uncoupled ocean models in basins where deep water formation occurs.

Under mixed boundary conditions self-sustained internal variability on the decadal-interdecadal timescale can exist in ocean models (e.g., Weaver and Sarachik, 1991a; Weaver et al., 1991). Often this variability is linked to the turning on and shutting off of high latitude convection and the subsequent generation and removal of east-west steric height gradients which cause the thermohaline circulation to intensify and weaken over a decadal timescale. This variability is in turn associated with the propagation to the eastern boundary of warm, saline anomalies, generated in a localized region of net evaporation in the mid-ocean, between the sub-polar and sub-tropical gyres. The separated western boundary current provides the source of warm, saline water required to initiate the anomaly development. Horizontal advection sets the oscillation timescale which is given by the length of time it takes a particle to be advected from the mid-ocean region, between the subpolar and subtropical gyres, to the eastern boundary and then, as subsurface flow, towards the polar boundary. Decadal internal oceanic variability still persists or may even be excited when a stochastic component is added to the freshwater forcing (Weaver et al., 1993; Weisse et al., 1994). It has also recently been shown that decadal internal oceanic variability can also exist in ocean models driven only by thermal forcing (Weaver et al., 1994; Greatbatch and Zhang, 1995; Winton, 1995).

The existence of such model results makes it difficult to interpret causes and effects of decadal variability in current coupled climate models which employ flux-corrections (see Weaver and Hughes, 1995). This follows since if the flux correction is large in magnitude, one might expect that the oceanic variability is determined by this structure, with the higher frequency air-sea flux variability providing a stochastic forcing which simply excites it.

In this proposal significant advances to our understanding of decadal-interdecadal variability will be achieved through the use of coupled models of varying complexity which do not employ flux corrections. This will expand upon the early uncoupled OGCM results. The use of an EMBM will allow for simple thermodynamic feedbacks. This EMBM will be extended to a simplified dynamical model by the end of WOCE-3. Furthermore, the use of OGCMs of varying resolution will allow for both an analysis of the effects of horizontal boundary layers (as discussed in Winton, 1995) and the role of eddies in decadal-interdecadal climate variability.

7.7.7.2. Centennial-timescale variability

A fundamental period for oceanic model variability also occurs on the overturning timescale (Mikolajewicz and Maier-Reimer, 1990; Winton and Sarachik, 1993; Weaver et al., 1993). The presence of a positive salinity anomaly in the low latitude surface regions tends to slow the meridional overturning slightly, since thermal effects tend to accelerate the thermohaline circulation and haline effects act to brake it. The weakened thermohaline circulation is then more affected by the specified flux on salinity which acts to intensify the positive anomaly at low latitudes and induce a negative salt anomaly at high latitudes. When the low latitude salinity anomaly reaches the high latitudes, convection and an intensified thermohaline circulation ensues. The whole process begins anew when the saline anomaly resurfaces at low latitudes. Thus the oscillation has a slow phase, which is associated with the saline anomaly being at low surface latitudes, and a rapid phase in which the salinity anomaly is at high latitudes or in the deep ocean.

Whether or not such centennial timescale variability exists in the coupled ocean-ice-atmosphere system, and the extent to which it is modified by allowing for feedbacks within the coupled system, is still an open question.

7.7.7.3. Millennial-timescale variability

While variability on this timescale is beyond the central theme of this proposal, we will be able to address an open and important question regarding the potential existence of the so-called flushes (discussed below). These catastrophic climate swings have previously only been observed in uncoupled OGCMs. If they are found to survive in coupled models then their mechanism serves as a potential explanation for variability found in past, and perhaps future, climates.

Ocean GCMS forced using mixed boundary conditions in which high latitude freshening is strong are often susceptible to polar halocline catastrophes. Associated with the polar halocline catastrophe is a collapsed thermohaline circulation state which is not stable since low latitude diffusion acts to make the deep waters warm and saline with horizontal diffusion acting to homogenize these waters laterally. Eventually, at high latitudes the deep waters become sufficiently warm so that the water column becomes statically unstable and rapid convection sets in. The result is a flush (Marotzke 1989, 1990; Weaver and Sarachik, 1991b; Wright and Stocker, 1991) in which a violent overturning occurs whereby the ocean loses all the heat it had taken thousands of years to store in a matter of a few decades. At the end of the flush, high latitude freshening eventually suppresses convection and overturning and the thermohaline circulation once more collapses. The collapse/flush sequence repeats itself with the timescale between flushing events being set by diffusion.

The existence of flushes is linked to both the relative importance of freshwater flux over thermal forcing, and the strength of the wind forcing compared to the high latitude freshening. As the stratification is usually homogeneous in near-surface layers at high latitudes, the Ekman-driven overturning cells contribute very little to the meridional heat and salt transports. This situation changes once the polar halocline catastrophe has occurred as the surface layer becomes very fresh and the equatorward Ekman transport of fresh water is compensated for by a poleward return transport of more saline water, resulting in a net poleward salt transport. Moreover, the poleward salt transport due to the horizontal subtropical gyre increases substantially during the collapsed phase of the thermohaline overturning.

High-latitude surface freshening may be counteracted by the wind-driven salt transport to make the high-latitude surface waters sufficiently saline, so that deep convection resumes and the thermohaline circulation reestablishes itself. If the surface freshening cannot be compensated for by the wind-driven salt transport, the thermohaline circulation remains in the collapsed state until a flush sets in.

When a stochastic term is added to the mean freshwater flux forcing field the frequency of flushing events increases while their intensity decreases, with increasing magnitude of the stochastic term. This follows since the probability of a sufficiently large evaporation anomaly increases so the ocean need not warm as much before a flush occurs.

As mentioned above, these flushes have never been found in coupled climate models. This may be due to the fact that feedbacks in the coupled system prevent their occurrence. On the other hand, no coupled model has ever been run long enough to quantitatively analyze this. Indeed, the coupled OGCM-AGCM of Manabe and Stouffer (1994) reveals that under 4xCO2 forcing, the thermohaline circulation collapsed and remained collapsed for 500 years. This situation with no ventilation of the deep waters cannot exist indefinitely as a stable equilibrium and so deep water must eventually form. Whether deep water will form through a flush or via a more gentle re-establishment is unknown. Through the use of our hierarchy of coupled models we will be able to address this question.

7.7.7.4. Surface boundary conditions

Much research has been conducted to develop improvements of the so-called restoring boundary conditions and mixed boundary conditions which have often been used by the ocean modelling community. In restoring boundary conditions, surface temperatures and salinities in an ocean model are relaxed, with a specified timescale, to some climatological values whereas under mixed boundary conditions, a specified flux on salinity is used in conjunction with the restoring condition on temperature.

Sea surface temperature anomalies, regardless of their scale, are strongly damped under restoring boundary conditions. In reality, the damping time should depend on the scale of the anomaly since latent and sensible heat loss are inefficient mechanisms for the removal of large-scale sea surface temperature anomalies. This follows since heat lost over one part of the ocean must be advected by the atmospheric winds away from the anomaly for it to be effectively removed. Therefore, as pointed out by Bretherton (1982), in the limit of a global scale anomaly, radiational damping (long timescale) is the only mechanism for its removal. These ideas have been utilized in the development of a number of improved parameterizations of the thermal surface boundary condition (e.g., Zhang et al., 1993; Mikolajewicz and Maier-Reimer, 1994; Rahmstorf and Willebrand, 1994; Seager et al., 1994).

Recent developments have also taken place in the improvement of the boundary condition on salinity in ocean models. Hughes and Weaver (1995) developed a new salinity boundary condition which incorporates the dependence of evaporation on sea surface temperature. The internal variability of the thermohaline circulation documented in sections 7.7.7.1-7.7.7.3 was found to still exist under this new boundary condition although it was modified slightly.

A cautionary note should be added here regarding the variability found in OGCMs under mixed boundary conditions. It is evident from the new surface boundary condition parameterizations discussed in this subsection that much of the variability is either damped or modified when more realistic surface boundary conditions are used. One should therefore view their results with some caution until they are verified with fully-coupled ocean-ice-atmosphere models.

Through the coupling of the OGCM to both an EMBM and a simplified dynamical AGCM we will be able to quantitatively examine whether or not feedbacks in the coupled system dampen, enhance or excite such variability.

7.7.7.5. Sub-grid-scale parameterizations

Recent advances have been made in the parameterization of sub-grid scale mixing associated with mesoscale eddies (Gent and McWilliams, 1990; Gent et al., 1995; Danabasoglu et al. 1994) in coarse resolution OGCMs. Danabasoglu et al. (1994) have illustrated promising initial results from experiments using their parameterization for mesoscale eddy-induced mixing. When their parameterization was incorporated into a global ocean model the thermocline became sharper, the deep ocean became colder, the meridional transport of heat and salt became greater and the overturning in the North Atlantic expanded meridionally. All of these features are improvements on the results obtained in global models using traditional horizontal/vertical mixing schemes. Furthermore, in the Southern Ocean, the Deacon Cell vanished through an eddy-induced cancellation of the mean flow advection of tracers. The result was a reduction in the heat and salt transport by the ocean across the Antarctic Circumpolar Current.

Robitaille and Weaver (1995) examined three sub-grid scale mixing parameterizations (lateral/vertical; isopycnal/diapycnal; Gent and McWilliams, 1990) using a global ocean model in an attempt to determine which yielded the best ocean climate. Observations and model Freon 11 distributions, in both the North and South Atlantic, were used in the model validation. While the isopycnal/ diapycnal mixing scheme did improve the deep ocean potential temperature and salinity distributions, when compared to results from the traditional lateral/vertical mixing scheme, the Freon 11 distribution was significantly worse due to too much mixing in the southern ocean. The Gent and McWilliams (1990) parameterization, on the other hand, significantly improved the deep ocean potential temperature, salinity and Freon 11 distributions when compared to both of the other schemes. The main improvement came from a reduction of Freon uptake in the southern ocean where the "bolus" transport canceled the mean advection of tracers and hence caused the Deacon Cell to disappear. These results suggest that the asymmetric response found in CO2 increase experiments, whereby the climate over the Southern Ocean does not warm as much as in the northern hemisphere, may be an artifact of the particular sub-grid scale mixing schemes used.

These initial results have inspired us to continue to use Freon as a method for validating the climatology of our ocean models and in particular their sub-grid scale parameterizations. We will examine the effects of stability dependent diapycnal mixing which will be added to the Gent and McWilliams (1990) parameterization. In addition, the role of boundary layer versus interior mixing will be analyzed through the inclusion of spatially varying diapycnal mixing. Furthermore, surface lateral mixing will be added as a parameterization of diabatic processes in the surface mixed layer arising from the presence of surface fluxes and small scale processes acting in series with the mesoscale eddies.

7.7.7.6. Eddy heat and salt transport

One of the most fundamental, yet unanswered, questions regarding the oceanÕs role in climate is whether or not eddies are important in transporting heat and salt poleward. Numerous studies (e.g., Bryan, 1982; Cox, 1985; Bryan, 1986; Boening and Budich, 1992; Drijfhout, 1994) have suggested that eddies do not play a significant role in the transport of heat and salt poleward. They suggest that the heat and salt transport is dominated by the meridional overturning transport in the North Atlantic. Wang et al. (1995) and Semtner and Chervin (1992) have further suggested that the barotropic component of the ocean circulation may have a significant effect in the transport of heat and salt, especially in the Pacific Ocean.

All of the above works suffer from a major shortcoming since all the experiments were conducted using ocean-only models. In the limit of specified fluxes of heat and freshwater, the oceanic heat and salt transports are necessarily predetermined at equilibrium since the divergence of the transport gives the zonally-averaged flux. Thus, whether or not eddies are resolved will not change the total heat or salt transport at equilibrium.

All of the above studies also used restoring boundary conditions on temperature and salinity. While these boundary conditions do not impose an exact constraint on the oceanic poleward heat and salt transports at equilibrium, they do largely determine the thermocline structure and hence, I suggest, clamp the ability of the ocean models to freely regulate their heat and salt transport.

To quantitatively address this problem we shall use an idealized OGCM of the North Atlantic with resolution ranging from 4 degrees x 4 degrees to 1/4 degree x 1/4 degree coupled to an EMBM. The only constraint on the coupled system will then be the incoming solar radiation at the top of the atmosphere. As the ocean model resolution increases, the coupled system will be allowed to adjust, subject to the specified constraint on the incoming solar radiation. By partitioning the heat transport into time mean and time dependent terms we will be able to see whether or not transient eddies are indeed important for the transport of heat and salt poleward. In addition, by examining the time-mean component we will be able to quantify the importance of the barotropic versus overturning transport.

7.7.7.7. Global teleconnections

The North Atlantic Ocean is fundamentally linked with the rest of the world's ocean via communication through the Antarctic Circumpolar Current and the Southern Ocean. The extent to which processes occurring in the North Atlantic affect the circulation in other oceans and vice versa is still unknown.

A fundamental question regarding the North Atlantic and its relationship with the rest of the worldÕs oceans concerns the reason why the North Atlantic forms deep water yet the Pacific does not. Warren (1983) singles out the more stable stratification of the North Pacific (where surface waters are on average 32.8 psu and deep waters 34.6-34.7 psu compared to 34.9 and 34.9-35.0 psu in the North Atlantic) as the explanation and identifies a number of causes for this. For example, there is nearly twice as much evaporation over the North Atlantic as over the North Pacific; the water introduced into the North Atlantic from lower latitudes is more saline than its counterpart in the North Pacific, and the residence time of this water in the region of net precipitation at high latitudes is shorter. However, as Warren (1983) concedes, none of these factors is independent of the already existing thermohaline circulation in the North Atlantic. The salinities of the surface and bottom water masses are similar because one is being actively converted into the other; the higher evaporation is related to higher sea surface temperatures, which are due in part to the greater northward advection of warm subtropical water by the Gulf Stream Ń but the Gulf Stream itself is partly thermohaline-driven. Finally, the thermohaline contribution to the western boundary current and the active conversion of surface to deep waters accounts for the shorter residence time in the North Atlantic.

Many geographical clues also exist regarding the asymmetry of the thermohaline circulation in the two oceans. The first and most obvious one is that the North Atlantic extends farther north than does the Pacific, and has a deeper connection with the Arctic. Furthermore, the Pacific is twice as wide as the Atlantic. In connection with the Broecker et al. (1990) argument that the interbasin atmospheric water vapour transport drives the conveyor, the narrow Isthmus of Panama at low latitudes allows westward freshwater export via the trade winds (Weyl, 1968), while the Rocky Mountains block an opposite flow at higher latitudes. Schmitt et al. (1989) have proposed that the narrower width of the Atlantic compared to the Pacific would cause a greater fraction of its area to be susceptible to the incursions of cold dry continental air that favour evaporation. Within the ocean, the salty Mediterranean outflow assists in preconditioning intermediate water flowing into the Norwegian Sea to undergo deep convection, as does the exchange with South Indian waters off the Cape of Good Hope. Finally, Reid (1961) has hypothesized that the poleward extension of South America compared to South Africa might impede the transport of freshwater out of the Pacific by the ACC.

The global coupled EMBM-TIM-OGCM will be used to try and unravel which of the different mechanisms is the most crucial in determining the observed preference for deep water formation in the Atlantic instead of the Pacific.

As discussed in section 7.7.7.1, numerical models have revealed decadal-interdecadal variability of the North Atlantic thermohaline circulation. It is not clear, however, to what extent this variability affects the rest of the world oceans. In addition, it is not clear how changes in other basins would affect the existence of North Atlantic decadal-interdecadal variability. For example, Toggweiler and Samuels (1993, 1994) and McDermott (1995) suggest that the southern ocean winds may regulate the rate of NADW formation. Hughes and Weaver (1994), however, suggest that the winds over the Southern Ocean are only one of a number of ways of regulating NADW formation. In order to examine the interrelationships between the North Atlantic and global oceans, the global coupled EMBM-TIM-OGCM will once more be used in various configurations as outlined in section 7.7.8.2.

Furthermore, we wish to address the debate as to where the return flow of North Atlantic Deep Water (NADW) occurs. Coarse resolution models (e.g. Hirst and Godfrey, 1993) suggest that most of the return flow happens in the cold water route through Drake Passage. Eddy resolving models, which essentially prescribe the deep temperature and salinity structure of the ocean, suggest that most of the return flow happens in the warm water route via eddy generation in the Agulhas Current (Semtner and Chervin, 1988, 1992). Observations suggest that some combination of the two routes is appropriate (Broecker 1991). In this project we shall apply perturbations (both passive and active) to the North Atlantic thermohaline circulation to examine, the global equilibrium response, the transient adjustment phase as well as the paths through which the perturbations travel.

7.7.7.8. References

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Bretherton, F.P., 1982: Ocean climate modelling. Prog. Oceanogr., 11, 103-129.
Broecker, W.S. 1991. Oceanogr., The great ocean conveyor. 4, 79-89.
Broecker, W.S., T.H. Peng, J. Jouzel and G. Russell, 1990: The magnitude of global fresh-water transports of importance to ocean circulation. Clim. Dyn., 4, 73-79.
Bryan, K., 1982: Poleward heat transport by the ocean. Ann. Rev. Earth Planet. Sci., 10, 15-38.
Bryan, K., 1991: Poleward heat transport in the ocean. A review of a hierarchy of models of increasing resolution. Tellus, 43, 104-115.
Cox, M., 1985: An eddy-resolving model of the ventilated thermocline. J. Phys. Oceanogr., 15, 1312-1324.
Danabasoglu, G., J.C. McWilliams and P.R. Gent, 1994: The role of mesoscale tracer transports in the global ocean circulation. Science, 264, 1123-1126.
Drijfhout, S.S., 1994: Heat transport by mesoscale eddies in an ocean circulation model. J. Phys. Oceanogr., 24, 353-369.
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Hughes, T.M.C. and A.J. Weaver, 1995: Sea surface temperature-evaporation feedback and the oceanÕs thermohaline circulation. J. Phys. Oceanogr., submitted.
Latif M. and T.P. Barnett, 1994: Causes of decadal climate variability over the North Pacific and North America, Science, 266, 634-637.
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Schmitt, R.W., P.S. Bogden and C.E. Dorman, 1989: Evaporation minus precipitation and density fluxes for the North Atlantic. J. Phys. Oceanogr., 19, 1208-1221.
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7.7.8. Methodology

In this section more details are provided as to the methodology which will be used to address the questions posed in section 7.7.7.

7.7.8.1. The EMBM-TIM

As part of WOCE-2 Augustus Fanning, a PhD student working under my supervision, developed a diffusive heat transport energy balance model (EBM), that has been tested in both simplified and global domains. The EBM is loosely based upon the models of Budyko (1969), Sellers (1969), and North (1975). We have extended these models to allow coupling with the GFDL-MOM OGCM (Geophysical Fluid Dynamics Laboratory Modular Ocean Model - Pacanowski et al., 1993) by allowing latent, sensible and radiative heat transfers between the ocean and atmosphere. In an effort to completely couple the ocean-atmosphere system, a moisture balance equation has also been added to the EBM so that freshwater fluxes can be predicted for the ocean model.

The resultant EMBM has been run in a global 2 deg x 2 deg domain with fixed sea surface temperatures (Fanning and Weaver, 1995). Under climatological oceanic conditions, the surface air temperatures, specific humidities and surface fluxes are comparable to direct estimates. As an extension to the climatological forcing case, a simple perturbation experiment was considered in which the 1955-59 pentad was compared to the 1970-74 pentad by driving the model under the respective sea surface temperatures. The model exhibits global, as well as basin-mean temperature changes in the latter pentad comparable to direct estimates (Jones, 1988).

A version of the fully coupled ocean-atmosphere model (EMBM coupled to the GFDL-MOM) has been run in a single-hemisphere (60 deg x 60 deg) basin, driven by zonally uniform wind stress and solar insolation forcing. A series of several experiments of varying horizontal resolution (ranging from 4 degrees x 4 degrees to 1/4 degree x 1/4 degree) and viscosity will be conducted to assess the effect on the components of the net poleward heat transport. We will analyze the relative contributions of the mean and time variant components of the heat transport. These include the effects of the barotropic gyre transport (in the horizontal plane), the meridional overturning transport (in the zonal plane), the baroclinic gyre transport, as well as the eddy and diffusive heat transport components.

In another project, a two-hemisphere model representative of the Atlantic basin will be developed. This model will incorporate a thermodynamic ice model (Semtner, 1976) (which includes heat insulation as well as brine rejection) into the coupled ocean-atmosphere model. It will be used to determine whether multiple equilibria exist in an idealized coupled ocean-atmosphere model (that does not use flux corrections - see below) and whether or not the asymmetry between the northern and southern hemispheres, due to the existence of the Antarctic Circumpolar Current (ACC), is important. Two sets of experiments will be conducted - one with an ACC and one without. Perturbations will be applied to both the northern and southern hemispheres and the hydrological cycle in an attempt to destabilize the equilibrium climate. By comparing the results from the twin experiments, a quantitative understanding of the role of the ACC in determining the stability properties of the North Atlantic thermohaline circulation will be obtained.

It is not clear whether or not the coupled atmosphere-ocean system can actually allow multiple equilibrium states when full coupling of both the heat and freshwater are allowed. This follows since a strong constraint is placed on the heat transport of the coupled system through the incoming solar radiation at the top of the atmosphere.

Ocean-only studies under mixed boundary conditions (Bryan, 1986; Marotzke and Willebrand, 1991; Hughes and Weaver, 1994) do reveal the existence of multiple equilibria. However, in these studies the freshwater flux forcing is specified and the atmosphere is allowed to have infinite heat capacity. The coupled two-dimensional zonally-averaged ocean model-EBM of Stocker et al. (1992) also revealed multiple equilibria but once more the freshwater flux forcing was essentially specified. While the fully coupled AGCM-OGCM of Manabe and Stouffer (1988) displayed multiple equilibria of the North Atlantic overturning, flux corrections an order of magnitude larger than climatological mean forcing fields had to be used.

As discussed by Weaver and Hughes (1995), the use of flux corrections places a strong constraint on the implied oceanic heat and salt transports. Therefore, multiple equilibria studies using flux corrections do not allow the ocean to freely adjust. Our fully coupled OGCM-TIM-EMBM will avoid the need for flux corrections and will allow free coupling of both heat and freshwater between the ocean and the atmosphere. Once we have analyzed the results from the two-hemisphere studies we will extend our OGCM to two-basin geometry (as in Hughes and Weaver, 1994).

Initial results from the single-hemisphere coupled OGCM-TIM-EMBM have revealed spontaneous decadal variability in the coupled system. These results will be analyzed during WOCE-3 and the effects of increasing horizontal resolution (i.e., the role of eddies) will also be studied. Initial benchmarks with our IBM workstation cluster have shown that these experiments are feasible with existing machines.

In addition, by integrating the coarse resolution version of the coupled model for many thousands of years, and by applying freshwater perturbations (as in Bryan, 1986) to the high latitude salinity budget or by applying stochastic forcing to the system (as in Weaver et al., 1993), we will be able to investigate the processes of centennial and millennial timescale variability discussed in sections 7.7.7.2 and 7.7.7.3, respectively.

7.7.8.2. The global OGCM

During WOCE-2, a global ocean model was developed in collaboration with Warren Lee for coupling of the CCC. Two versions of this model now exist: The first version is a high resolution model (1.8 deg x 1.8 deg x 29 levels) and is being coupled to the CCC AGCM to investigate the climatic response to increasing atmospheric greenhouse gases and aerosols. The second version of the model is at slightly coarser resolution (3.6 deg x 1.8 deg x 19 levels) and is currently being used to understand the structure, stability and variability of the global ocean thermohaline circulation. The description of this coarse resolution model may be found in Weaver and Hughes (1995).

In order to address the scientific questions posed in section 7.7.7, we will use the coarse resolution version of this model in both coupled (EMBM-TIM) and uncoupled forms. We will investigate the response of the global oceans to perturbations in the North Atlantic by introducing passive tracers in the North Atlantic and examining the paths of the perturbations. A subset of these experiments (i.e., those that yield interesting scientific results) will be repeated using the higher resolution version of the GCM. Furthermore, the knowledge gained from the more idealized-geometry experiments discussed in the last section will be used to investigate mechanisms and processes involved in North Atlantic decadal-interdecadal variability and their global teleconnections.

As discussed above, the EMBM has been extended to a global domain (Fanning and Weaver, 1995). This model will be coupled to the global OGCM to address the question as to why the Atlantic forms deep water instead of the Pacific. We will use the global OGCM in various configurations to investigate the importance of the different mechanisms outlined in section 7.7.7.7. For example, basin geometries will be modified; the hydrological cycle will be perturbed in various regions; the winds will be varied. Hughes and Weaver (1994) suggest that the North Atlantic overturning is proportional to the depth-integrated steric height gradient from the tip of South America to the latitude of deep water formation. They further suggest that changes in the aforementioned external parameters simply act to change this depth-integrated steric height gradient and hence the rate of NADW formation. This hypothesis will be reexamined in the coupled system through the analysis of the steric fields.

7.7.8.3. Ocean model validation using Freon

As discussed in section 7.7.7.5 we will continue to use Freon to validate the climatology and sub-grid scale parameterizations of both our global and North Atlantic models. The theoretical analysis leading to the improvements of the Gent and McWilliams (1990) scheme will be developed in collaboration with Drs. Amit Tandon, Chris Garrett and Jim McWilliams. In particular, we shall advance this sub-grid scale parameterization through the inclusion of the processes discussed in the last paragraph of section 7.7.7.5.

At the surface of our ocean models, the flux of Freon across the air-sea interface is expressed as

Q=lambda(Cesw - C)

where lambda is the gas transfer velocity (taken from Liss and Merlivat, 1986), modified for use with Freon 11 (as in Wanninkhof, 1992); Cesw is the concentration of Freon 11 at equilibrium for a given water temperature and salinity (taken from Warner and Weiss, 1985); C is the concentration of Freon 11 at the top level of the model.

In ice-covered regions, the gas transfer velocity for the Freon 11 is reduced using the formula:

lambdai = lambda [ 1 - (F/10) ]

where F is the fraction of sea-ice cover in tenths (0 = no ice; 10 = complete ice cover). So far we have only considered annual mean forcing so that an annual mean ice cover climatology was obtained from the monthly ice concentrations of Gloersen et al (1992) and used to determine the appropriate gas transfer velocity.

The initial validation process of our global OGCM was conducted by Robitaille and Weaver (1995). Inspired by the success of this analysis we will analyze the climatologies of the OGCMs using each of the different sub-grid scale parameterizations outlined in section 7.7.7.5. By quantitatively assessing the ability of the OGCMs to simulate present Freon distributions the strengths and weakness of each parameterization will be realized.

7.7.8.4. The simple dynamical coupled AGCM-TIM-OGCM

As discussed in section 7.7.8.2, a global ocean model has been developed and coupled to the CCC AGCM. In order to quantitatively examine the decadal-century variability of the climate system and the dependence of this variability on the mean climatic state, the coarse resolution global OGCM, as well as idealized geometry OGCMs, will be coupled to a simple stripped down version of the CCC AGCM.

The CCC second generation AGCM (McFarlane et al., 1992) has recently been optimized so that it runs only a few times slower than the fine resolution global OGCM. In order to speed up the AGCM without losing too much essential physics, the diurnal cycle will be removed (saving enormous CPU time as the radiation code will not have to be executed as often). Consistent with the approximation, the atmospheric boundary layer code will be simplified. In addition, the resolution will be reduced in both the horizontal and vertical.

This project will be carried out by a research associate, Dr. S. Zhang, in close collaboration with researchers in the CCC.

7.7.8.5. References not in section 7.7.7.8

Bryan, F., 1986: High-latitude salinity effects and interhemispheric thermohaline circulations. Nature, 323, 301-304.
Budyko, M.I., 1969: The effect of solar radiation variations on the climate of the earth. Tellus, 21, 611-619.
Fanning, A.F., and A.J. Weaver, 1995: An atmospheric energy moisture-balance model for use in climate studies. J. Geophys. Res., submitted.
Gloersen, P., W.J. Campbell, D.J. Cavalieri, J.C. Comiso, C.L. Parkinson, and H.J. Zwally, 1992: Arctic and Antarctic Sea Ice, 1978-1987: Satellite passive-microwave observations and analysis. NASA Report SP-511, 290pp.
Hughes, T.M.C. and A.J. Weaver, 1994: Multiple equilibria of an asymmetric two-basin ocean model. J. Phys. Oceanogr., 24, 619-637.
Jones, P.D., 1988: Hemispheric surface air temperature variations: Recent trends and an update to 1987. J. Climate, 1, 654-660.
Manabe, S. and R.J. Stouffer, 1988: Two stable equilibria of a coupled ocean-atmosphere model. J. Climate, 1, 841-866.
Marotzke, J., and J. Willebrand, 1991: Multiple equilibria of the global thermohaline circulation. J. Phys. Oceanogr., 21, 1372-1385.
McFarlane, N.A., G.J. Boer, J.-P. Blanchet and M. Lazare, 1992: The Canadian Climate Centre second-generation general circulation model and its equilibrium climate. J. Climate, 5, 1013-1044.
Pacanowski, R., K. Dixon and A. Rosati, 1993: The GFDL Modular Ocean Model Users Guide, GFDL Ocean Group Technical Report #2, 46pp.
Sellers, W.D., 1969: A global climatic model based on the energy balance of the earth-atmosphere system. J. Appl. Meteorol., 8, 392-400.
Semtner, A.J., 1976: A model for the thermodynamic growth of sea ice in numerical investigations of climate. J. Phys. Oceanogr., 6, 379-389.
Stocker, T.F., D.G. Wright and L.A. Mysak, 1992: A zonally averaged, coupled ocean-atmosphere model for paleoclimate studies. J. Climate, 5, 773-797.
Wanninkhof, R., 1992: Relationship between wind speed and gas exchange over the ocean. J. Geophys. Res., 97, 7373-7382.
Warner, M.J., and R.F. Weiss, 1985: Solubilities of chlorofluorocarbons 11 and 12 in water and seawater. Deep-Sea Res., 32, 1485-1497.

7.7.9. Subproject budget

See attached page 4 of NSERC form 101 for details of the funding request for each year.

Justification for Proposed Budget

1) Salaries and benefits

a) Graduate Students

Full support for two PhD and 1 MSc students at the current NSERC rate. This project will involve 6 graduate students (PhD: Fanning, Robitaille, and two new students; MSc: Murdock and one new student). Fanning, Robitaille and Murdock should all graduate early into the term of this project. The new students will be accepted during 1995-1996. Fanning and Murdock will be largely supported off other sources. Robitaille will be fully supported from WOCE for year 1 as will a new MSc and a new PhD student. In year 2 Robitaille will have graduated and a new PhD student will join the project. The three new students will be fully funded from this WOCE grant throughout the period of the project.

b) Postdoctoral Fellows

Full support for a postdoctoral fellow who will be hired immediately upon the departure of Dr. Tertia Hughes. This postdoctoral fellow will be involved in using the coupled EMBM-OGCM-TIM to investigate processes involved in North Atlantic decadal-interdecadal climate variability and their teleconnections with the rest of the world ocean.

c) Technical/Professional Assistants

Partial support ($10,000) for a computer systems manager/programmer (R. Outerbridge) who is in charge of maintaining my cluster of 11 workstations and their peripheral devices. I no longer have the time to look after all of these machines and it is imperative that I hire a competent systems manager. Other system managers/operators (N. & M. Bakalov, D. Robitaille) will be supported from other sources.

d) Other (Research Associate)

Full support for a research associate (Dr. S. Zhang) who will be involved in the development of the simplified dynamical AGCM which will be coupled to the OGCM and the TIM.

2) Equipment

a) Purchase or Rental

Purchase of 9 GB of disk space in each of years 2 and 3 to store the voluminous GCM output.

b) Operation and maintenance costs

Partial support for the maintenance contract for my IBM workstations which was slightly over $10,000 for the 1994-95 fiscal year. I have found this maintenance contract to be imperative as I have a lot of hardware/peripherals. Every now and again a component breaks down and it is too expensive to replace them. At $1000 per day, on site repairs are not affordable without a maintenance contract.

3) Materials and Supplies

Toner cartridges, computer paper, magnetic tapes, computer manuals, drafting costs, video cassettes for computer movies, photocopying charges, communication charges (telephone, fax, courier) etc.

4) Travel

a) Conferences

Travel for students, postdoctoral fellow, and research associate and A. Weaver to annual national and international conferences (e.g., CMOS, AGU, AMS). Five trips a year at $1,000 each to be split amongst the researchers.

5) Dissemination Costs

Publication and reprint charges. The total ($10,000) is significantly less than I have had to pay out during the 1994-95 fiscal year (~$20,000). If one wishes to publish in the best and most widely read journals, these charges are inevitable.

7.7.10. Subproject Milestones

Year 1

Date: -- Milestone description

Jul 01 -- Project Starts
Dec 31 -- Analysis of the role of eddies in transporting heat and freshwater polewards
Dec 31 -- Fully-coupled OGCM-TIM-EBM developed and the present-day climate validated
Jun 30 -- Multiple equilibria and decadal-interdecadal variability in the idealised geometry coupled system analyzed.
Year 2

Date: -- Milestone description

Dec 31 -- Simplified dynamical AGCM developed
Jun 30 -- Initial results from the coupled OGCM-TIM-AGCM
Jun 30 -- Multiple equilibria and decadal-interdecadal variability in the global OGCM- TIM-EBM analyzed.
Jun 30 -- Subgrid scale parameterizations validated using Freon
Jun 30 -- Completion of project involving the analysis of global teleconnections associated with perturbations in the North Atlantic.
Year 3

Date: -- Milestone description

Dec 31 -- Project concerning why deep water forms in the Atlantic and not the Pacific completed.

Jun 30 -- Analysis of decadal-interdecadal variability of decadal variability in the AGCM-TIM-OGCM

Students involved in this subproject:
Daniel Robitaille (PhD)
Tracer distributions in a global ocean model,
Started Aug. 1, 1993.
Funding: 100% WOCE
Expected graduation: 1997

Augustus Fanning (PhD)
Decadal-century variability in the coupled atmosphere-ocean system.
Started Sept. 1 1993.
Funding: 75% Atlantic Accord Career Development Award / 25% WOCE
Expected graduation: 1997

Trevor Murdock (MSc)
Paleoclimatic changes associated with the opening of Drake Passage and the closure of the Isthmus of Panama
To start Sept. 1, 1995.
Funding: 100% NOAA Scripps-Lamont Consortium Research Grant
Expected graduation: 1997

New Student (PhD)
To start Sept 1, 1995.
Funding: 100% WOCE

New Student (PhD)
To start Sept 1, 1996
Funding: 100% WOCE

New Student (MSc)
To start Sept 1, 1996
Funding: 100% WOCE

Postdoctoral Fellows/Research Associates involved in this subproject:

Dr. Amit Tandon
Decade-to-Century Climate Variability.
January 1, 1995 - to date
Funding: 91% UCAR fellowship - CSMP Project / 3% NOAA Scripps-Lamont Consortium Research Grant / 6% Chris Garrett and Inez Fung

Dr. Tertia Hughes
Global Ocean Modelling
January 1, 1995 - to date
Funding: 100% WOCE

Dr. Sheng Zhang
Coupled Atmosphere-Ocean-Ice Modelling
July 1, 1995 - to date
Funding: 100% WOCE

Dr. Sophie Valcke
The Ocean's Thermohaline Circulation
September 1, 1995 - to date
Funding: 83% NSERC Postdoctoral Fellowship / 17% NOAA Scripps-Lamont Consortium Research Grant

New PDF
To start Sept 1, 1995
Funding: 100% WOCE

Technical Assistants involved in this subproject:

Mr. Richard Outerbridge
Computer Systems/Software Manager
July 1, 1992 - to date
Funding: 20% WOCE / 80% Chris Garrett , Jim Bishop, Dave Farmer, Rolf Lueck and Inez Fung

Mr. Nicholas Bakalov
Computer/Systems Operator
February 1, 1994 - to date
Funding: 50% NOAA Scripps-Lamont Consortium Research Grant / 50% Chris Garrett , Jim Bishop, Dave Farmer, Rolf Lueck and Inez Fung

Ms. Magdelina Bakalov
Computer/Systems Operator
February 1, 1994 Š to date
Funding: 30% NSERC Operating / 70% Chris Garrett , Jim Bishop, Dave Farmer, Rolf Lueck and Inez Fung

Mr. Daniel Robitaille
Assistant Computer/Systems Manager
March 1, 1995 - to date
Funding: 100% NOAA Scripps-Lamont Consortium Research Grant

Lucy Aldridge
Accountant
May 1, 1994 - to date
Funding: 100% NOAA Scripps-Lamont Consortium Research Grant

Wanda Lewis
Secretary
January 1, 1995 - to date
Funding: 43% Canadian Climate Research Network - Global Oceans / 57% University of Victoria

List of Potential Reviewers:

1- Dr. David Anderson
Department of Atmospheric, Oceanic and Planetary Physics
Clarendon Laboratory
Oxford University
Oxford, OX1 3PU
United Kingdom

2- Dr. Matthew England
School of Mathematics
University of New South Wales
Sydney, NSW, 2052
Australia

3- Dr. Jurgen Willebrand
Institut fur Meereskunde an der Universitet Kiel
Dusternbrooker Weg, 20
D-2300 Kiel
Germany

4- Dr. Kirk Bryan
GFDL
NOAA/Princeton University
PO Box 308
Princeton, NJ, 08542
USA

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