Climate Modelling Group
School of Earth and Ocean Sciences

to Scripps Institution of Oceanography

Subcontractor: University of Victoria

SEMI-ANNUAL REPORT PERIOD: April 1, 1996 through October 31, 1996

Agency: National Oceanic and Atmospheric Administration, Office of Global Programs

Project Title: The Lamont/Scripps Consortium for Climate Research - Dynamical Modeling of Climate Change.

NOAA Award No: NA47GP0188

Principal Investigator: Andrew Weaver

Project Period: May 1, 1994 through April 30, 1997

Budget Period: April 1, 1996 through October 31, 1996

Performance Report Completed: October 8, 1996

Below I summarize the progress on the research funded through the NOAA Lamont/Scripps Consortium for Climate Research.

1 Decadal Variability in a coupled Energy-Moisture Balance Model (EMBM) -Ocean General Circulation Model (OGCM) - Thermodynamic Ice Model (TIM)

Ed Wiebe, an MSc. student, is currently utilizing a version of our coupled EMBM-TIM-OGCM. The model employs the same horizontal resolution, geometry, and forcing as that described by Fanning and Weaver (1996a) although it is now coupled to the GFDL MOM2 model (Pacanowski, 1995). The only appreciable difference between the models is the implementation of the flux corrected tracer algorithm (Gerdes et al., 1991; Weaver and Eby, 1996) and the explicit convection scheme of Rahmstorf (see Pacanowski, 1995). Of particular importance is the generation of spontaneous decadal-scale variability (period 26 years) centered in the North Atlantic. Oscillations in the meridional overturning streamfunction span about 10 Sv in magnitude with accompanying temperature anomalies of almost 5oC.

We are currently analysing the mechanism for the decadal oscillation, and continuing the model integration time to ascertain whether centennial scale variability (centred in the Southern Ocean) is a robust feature of the coupled climate system, or merely a transient phenomena.

In addition to this analysis Ed Wiebe has also made extensive modifications to a collection of existing IDL routines used to visualize the output from this coupled model. Extensions include the capability to calculate mean fields and plot anomalies. In addition, movies of time-dependent phenomena can be created and saved in a compact form for later viewing. This software is extendible and adaptable and may be of use to other researchers in the field of ocean modelling.

2 Simulation of the Younger Dryas event

This version of the EMBM-TIM-OGCM has been used to investigate the transition between the last glaciation and the present Holocene (the Younger Dryas - hereafter the YD). The traditional viewpoint is that the YD was triggered by the diversion of meltwater (due to the retreating Laurentide Ice Sheet) from the Gulf of Mexico to the St. Lawrence (Broecker et al., 1988). In an attempt to clarify the temporal and geographical roles of meltwater discharges on triggering the YD, estimates for volumes of runoff from the Laurentide ice sheet (Teller, 1990) were applied for the 1500 year period encompassing the YD cold episode. Model results indicate the traditional Laurentide meltwater diversion theory is insufficient to induce a YD climate signature, although preconditioning by the pre-YD meltwater discharge in conjunction with the diversion is.

The model predicted YD climate shift is global in nature, and is intimately linked to North Atlantic deepwater (NADW) formation. The global thermohaline circulation provides an interhemispheric teleconnection with the Southern Ocean, while changes in the atmospheric heat transport (reacting to a global redistribution of oceanic heat transport) provides a mechanism for interbasin teleconnection. Changes in model surface air temperature generally agree with the pattern and magnitude of YD temperature change deduced from paleoclimatic reconstructions based on existing paleothermometers.

Our results indicate that supplying both pre and post-YD meltwater discharges results in a total collapse of the North Atlantic conveyor. In the absence of additional model feedbacks, this state appears to be relatively stable, and equivalent to the Southern Sinking state identified by Manabe and Stouffer (1988). Reestablishment, if it were to occur, would appear to be a diffusive process as in previous ocean-only model studies under a polar halocline catastrophe (e.g., Marotzke, 1989; Weaver and Sarachik, 1991). If instead we allow for the effects of this climate state to feedback onto the surface winds, reestablishment occurs on a faster time scale. This is due to an increased surface salinification through latent heat loss and Ekman transport of the salinity anomaly out of the region of deepwater formation. This result is consistent with previous studies of freshwater perturbations on the North Atlantic Conveyor (e.g., Schiller et al., 1996; Mikolajewicz, 1996), however, unlike these studies the model settles into a new equilibrium state with reduced NADW formation as in Rahmstorf (1994). We also note that unlike these same studies, the time scale for reestablishment of NADW, and hence termination of the YD signal is an advective spinup time scale (order 1000 years) as opposed to decadal-century. The reason for this discrepancy is unclear, but may be associated with the used of fixed salt flux fields employed by those models.

While we have not explicitly addressed the question of the role of the Fennoscandian Ice Sheet's demise on the YD, our results suggest at most it would have prolonged the YD episode. This raises a final point, is the advective spinup time scale found here representative of the time scale for the YD termination? Considering the d18O record at Summit (Dansgaard et al., 1993), the bulk of warming signaling the transition from the YD to the Holocene occurred over a 200 year period (although full termination took much longer). So, it appears that further treatment of model feedbacks (e.g., cloud effects) and perhaps radiative forcing (due to increasing levels of CO2 are needed to investigate the Younger Dryas termination further. These results have recently been submitted for publication (Fanning and Weaver, 1996c).

3 Paleoclimatic response of the closure of the Isthmus of Panama

The role of meridional heat transport by the oceans is one of the most important current research topics in climate, and many difficulties exist in measuring it (Bryden, 1993). There exist, however, natural "experiments" in earth's history, whereby changes in ocean gateways have caused changes to the ocean heat transport (as well as other important changes). There exist increasing amounts of data on observations of paleoclimate. Ocean models have been run in the past (Mikolajewicz, Maier-Reimer, et al., 1990, 1993) to investigate paleocirculation, but a fully coupled climate model is required for investigation of important climate questions such as the changing roles of ocean and atmospheric heat transport under different gateway cases, and possible links to glaciation and initiation of glacial cycles

The effects on ocean circulation and world climate of changes in ocean gateways is being investigated. In particular, attention is being focussed on the role of meridional heat transport by the ocean with different arrangements of the Isthmus of Panama and Drake Passage gateways, the two most recent major changes in gateways (about 3 and 30 Million years ago, respectively), and the relation to glacial events in the paleoclimatic record.

The experiments are being performed using the coupled EMBM-OGCM-TIM. This model is ideal for this project as it does not require flux adjustments, thus imposing as little modern day observational bias as possible.

Research is ongoing, including running the coupled model under the open Isthmus of Panama configuration. Results thus far agree with previous (ocean only) model studies (Mikolajewicz, Maier-Reimer, et al., 1990, 1993). Further data obtainable from the coupled model, which is hoped to provide insight into the changed heat transport and relation to climate and glaciation, has been analysed. Initials results have been published in Murdock et al. (1996). The Drake Passage runs are almost complete, and analysis of their results will begin upon completion of the Isthus of Panama analysis.

4 Decadal Variability in the GFDL Coupled Model

The numerical simulations of Delworth et al. (1993), using the fully coupled ocean-atmosphere model developed by the Geophysical Fluid Dynamics Laboratory (GFDL) in Princeton, NJ, showed interdecadal variability of the thermohaline circulation in the North Atlantic. However, it is still unclear if this variability is a coupled ocean-atmosphere or an ocean-only phenomenon.

In order to clarify this problem, the GFDL coupled model, previously run on a Cray at GFDL, has been adapted to our IBM machines. The oceanic part of this model has been spun-up by Dr. S. Valcke (an NSERC postdoctoral fellow) to equilibrium in the same configuration as in the run of Delworth et al. (1993). This ocean model is now being run under fixed-flux boundary conditions, the fluxes being the sum of the atmospheric fluxes (diagnosed from the atmospheric model alone at equilibrium) and the flux adjustment terms (artificial terms used in coupled models to correct the fluxes going into the ocean in order to remove systematic climate drifts). If the same decadal variability as in the fully coupled experiments is observed, or if it appears when a stochastic forcing is added to the fixed-flux boundary conditions, we will conclude that the variability is due to the internal ocean dynamics.

Dr. S. Zhang is also working with the GFDL model. He has spent a good deal of time trying to make the atmospheric component of the model run more efficiently on our local workstation cluster. Dr. Zhang is attempting to obtain a version of the GFDL coupled model which does not require flux adjustments. He has identified a number of problems and is currently seeking methods to overcome them. These problems include: a) an ocean model surface flux which is significantly weaker than that produced by the atmospheric model. This is responsible for a large part of the flux adjustment; b) a very strong local salinity adjustment which is related to the melting of ice; c) unphysically strong restoring in the ocean with same strength in temperature and salinity during the oceanic spin up. If all of the problems are solved, then the oceanic component of the coupled model will probably not be the source of climate drift.

In the atmospheric component of the coupled model Dr. Zhang has reduced the frequency of synoptic eddies while retaining all the model physics. He implemented a number of other acceleration techniques in an attempt to speed the atmospheric model up. Specifically, this is achieved by using the original time step in the dynamical code and using a much longer time step for all other processes. This effectively assumes that the response time of the atmosphere is much shorter than the ocean, and that only the mean of synoptic system is important for the energy balance and that its variance only generates noise, at least on timescales longer than a decade.

5 Decadal variability in OGCMs with various subgrid-scale boundary layer parameterizations

Thierry Huck, a PhD student, is using locally-developed planetary geostrophic ocean models under various sub-grid-scale boundary layer parameterizations to study the mechanism of decadal variability found in ocean only models (Greatbatch and Zhang 1995). Under flux boundary condition on the surface density, a parameter sensitivity analysis has been carried out. The horizontal tracer diffusivity has a critical damping effect, while the vertical diffusivity (which determines the strength of the thermohaline circulation) enhances the oscillatory behaviour. The inclusion of a parameterization of convection (excluding the effect of vertical velocities) and the beta-effect are found not to be necessary in sustaining the variability, and so exclude the role of Rossby waves in the mechanism (Winton 1996). The influence of the lateral boundaries along which convection takes place (weakening the stratification so that Kelvin waves may propagate with decadal time scales -- Greatbatch and Peterson, 1996) has been rejected in two ways: 1) By moving northward the polar boundary (by several tens of degrees), with no atmospheric forcing in the extended area (a buffer zone where the stratification remains strong). In this case the oscillatory behaviour is weakly modified. 2) With a symmetric forcing in an f-plane model (that is twice as wide as in the control experiments), so that a 'tropical' thermocline is present along both zonal boundaries of the basin. In this case the oscillation is more profoundly affected but even stronger. None of these major changes weaken the variability, which is maximum in the region of Gulf Stream separation and its eastward extension.

Comparisons between the primitive equation GFDL-MOM code and the planetary geostrophic models at various horizontal resolution suggest that the higher the resolution, the more likely the occurrence of decadal variability (even without changing the horizontal diffusivity along with the resolution). In addition, below 100 km resolution, the non-linear terms and the time-derivative in the horizontal momentum equations play a driving role in producing the variability. In the vertical, a discretization as crude as two levels can sustain the oscillation.

Since the oscillations have a strong signature in the zonally-averaged fields, a two-dimensional mechanism is being investigated. This would be consistent with the findings of decadal variability within the idealized coupled ocean-atmosphere of Sarvanan and McWilliams (1995). With non-steady surface forcing in a zonally-averaged ocean model, we expect the ocean to generate decadal time-scales associated with the overturning period.

We are presently writing up this work in Huck et al. (1996c).

6 Climate Variability as a function of mean climatic state

Over the next two years I hope to continue improving our understanding of the mechanisms of decadal-interdecadal climate variability through the development of increasingly more sophisticated coupled models. The coupled EMBM-TIM-OGCM represents the simplest form of our coupled modelling studies. We shall continue to use it to explore simple thermodynamic feedbacks and gain insight into what results we might expect and which experiments we should undertake with the more complicated GFDL and CCC coupled models. In addition, the GFDL and EMBM-OGCM-TIM coupled model will be used to investigate questions concerning the existence of variability in the coupled climate system and how it varies as the mean climatic state changes (i.e, does the decadal-interdecadal climate variability found in the coupled model change as CO2 is increased in the atmosphere. Since the GFDL coupled model is far more computationally efficient than the CCC coupled model, it is hoped that the insight we gain from it will allow us to better streamline the future experiments that will be performed with the more sophisticated CCC coupled model.

7 Oceanic poleward heat transport as a function of OGCM resolution

The idealized climate model (consisting of an energy-moisture balance atmosphere, thermodynamic ice, and an ocean general circulation model, hereafter referred to as the EMBM-TIM-OGCM -- see Fanning and Weaver, 1996a) previously developed by A. Fanning (a PhD. student) has been utilized to study the influence of horizontal resolution and parameterized eddy processes on the poleward heat transport in the climate system. The results have recently been submitted for publication in Journal of Climate (Fanning and Weaver, 1996b).

Model results suggest that as resolution is varied from 4o to 0.25o the oceanic heat transport steadily increases. Owing to the strong constraint imposed by the radiation balance at the top of the atmosphere, the planetary (ocean plus atmosphere) heat transport changes little throughout our resolution experiments. As a consequence, the atmospheric heat transport generally decreases to offset the increasing oceanic transport.

The increase in oceanic heat transport as resolution increases is in contrast to previous ocean-only model studies (e.g., Cox, 1985; Bryan, 1987; Böning and Budich, 1992; Drijfhout, 1994). This result is also evidenced in a parallel series of ocean-only experiments where forcing is diagnosed from our 4O coupled model's equilibrium state (e.g. Haney, 1971; Han, 1984). Although heat transport is generally higher in the coupled model, both models behave similarly, with the primary increases occurring in the baroclinic gyre component of the oceanic heat transport.

The conspicuous absence of an eddy transport compensation mechanism is in contrast to previous ocean-only model studies. Boning and Budich (1992) found eddy length-scales ranging from 50 to 175 km in their 1/6o model study. The highest resolution case studied here (0.25o) is adequate to resolve some of these features, and spectral analysis of the basin mean kinetic energy density reveals variability (above 95% significance) in the range weeks to a year. Such time scales are consistent with those found by Cox (1985,1987).

To investigate this contradiction further, an additional set of ocean-only experiments (more closely approximating the earlier studies) were performed. In particular we wished to test whether an inclusion of salinity forcing (and hence a breakdown of the non-acceleration theorem -- eg. McDougall, 1984; Cox, 1985; Bryan, 1991; Drijfhout, 1994) could explain the differences in our results. Results suggest this is not the case, however. Restoring to temperature alone (as in previous studies) results in higher heat transports than the thermal/haline case (due to haline effects on the baroclinic overturning transport). The latter two experiments are consistent with our previous cases, again increases in the baroclinic gyre transport result in an increasing oceanic heat transport.

The thermocline adjustment time scale due to a perturbation (e.g. induced upon switching resolution) should be that for a first mode baroclinic Rossby wave to cross the basin. Owing to the generally short integration time of these studies (generally 10 years or less at highest resolution) it is not clear whether the time-variant compensation noted is eddy generated or rather an aliased Rossby wave signal (see Cox, 1985,1987). The poleward oceanic heat transport can be scaled as TO ~ V delta(T) where delta(T) is the contrast between an average thermocline temperature and an average deep water temperature, and V is an average northward transport in the thermocline (with southward transport below). Although the thermocline may undergo adjustment on a baroclinic Rossby wave time scale, the surface to deep water contrast is set by an advective spin up time scale (order of hundreds of years). Therefore, earlier studies involving rather short integration times are not sufficient to remove the transients at deep levels (on long advective time scales), or allow full equilibration of the meridional overturning circulation.

Although the identification of an eddy compensation mechanism found in previous studies may be due to the rather short integration times employed, additional factors exist which may explain the differences we note. Cox, (1985); Boning and Budich, (1992); and Drijfhout, (1994) each employed an idealized continental shelf along the western boundary with a promitory at approximately 35oN. Sufficient nonlinearity, along with inertial overshoot could give rise to enhanced eddy activity. Additionally, previous studies utilized biharmonic closure schemes at highest resolution. Here we chose not to do so since a change in closure ultimately alters the 'control' of the experiment.

Spontaneous decadal-intradecadal scale variability is found to exist in our higher resolution experiments. The intradecadal scale variability (period 3-5 years) is linked to the nonlinear advection terms in the momentum equations. This variability is similar to that noted by Cox (1985,1987) who found a 4-4.5 year variation in his model. Such variability (period 3 years) was also noted by Boning and Budich (1992). Spontaneous decadal scale variability is also found in our present study and its existence is intimately linked to the value of the horizontal diffusivity we employ. Increasing the diffusivity in our high resolution cases (below 0.5o) is enough to destroy the variability, while decreasing the diffusivity in our moderately coarse resolution cases (above 1o) is enough to induce the variability.

The decadal oscillation we describe is a thermally driven advective-convective oscillation, characterized by the turning on and shutting off of convective activity in the northwestern corner of the model domain (cf. Weaver et al., 1994; Greatbatch and Zhang, 1995). The fact that decadal scale variability exists in an idealized coupled ocean-atmosphere model (which does not employ flux adjustments) is an intriguing result. While our model is highly idealized, the question naturally arises: is the variability found in more complete coupled models (e.g. Delworth et al., 1994) a feature of the coupled state, or determined by the flux adjustment employed as suggested by Weaver et al. (1994), and Greatbatch and Zhang (1995). These results point to the importance of higher resolution in the ocean component of coupled models, revealing the existence of richer decadal-intradecadal scale variability in models which require less parameterized diffusion.

8 Flux adjustments and their influence in coupled models

In another project, A. Fanning is currently investigating the role of flux adjustments on the transient and long-term behavior of induced climate change experiments. A version of the EMBM-TIM-OGCM has been configured for a four-basin, two-hemisphere, sector geometry model which includes a Mediterranean, Arctic, Pacific and Atlantic basin, joined at the southern extent by a cyclic circumpolar ocean. This model has been spun up to near equilibrium, and the resulting surface temperature and salinity fields were then used to spinup an ocean only model (using a restoring timescale of 50 days). At equilibrium, the resulting differences between the atmospheric fluxes (in equilibrium with the surface SST's) and those implied by the restoring boundary conditions yields a flux adjustment such that the atmospheric state of the coupled model and the oceanic state of the ocean-only model are compatible. We therefore couple these states to yield a flux adjusted model, this procedure is formally equivalent to one of the standard procedures used in coupling an atmospheric model in equilibrium with fixed SST's to an ocean model spun-up by restoring to SST and SSS (e.g., Weaver and Hughes, 1996). The flux adjusted and non-flux adjusted model are then subjected to a 4 W/m2 (linearly increasing over 75 years) net heating perturbation.

Although still preliminary, results suggest that the transient behaviour (over the first 75 years) of each model is similar, with results diverging after that point. Additional experiments to test the sensitivity of the flux corrected model's initial conditions are still being performed, and these results will be reported on at a later date.

We are also investigating the role of flux adjustments on interdecadal climate variability. The numerical simulations of Delworth et al. (1994), using the GFDL coupled model revealed interdecadal variability of the thermohaline circulation in the North Atlantic. It is not clear to what extent the variability in that study is preconditioned by the heat and salt flux adjustment fields required to prevent climate drift in the coupled model. It is also unclear whether or not this variability is linked to coupled ocean-atmosphere dynamics or to ocean dynamics alone. In order to do elucidate this, the GFDL coupled model has been adapted to our local IBM cluster. The oceanic part of this model is now being run under fixed-flux boundary conditions, made up of atmospheric fluxes (diagnosed from the atmospheric model at equilibrium) and the flux adjustment terms. If similar variability as in the fully coupled experiments is found, we can conclude that the variability is due to internal ocean dynamics alone.

9 Finite element modelling

Dr. Paul Myers, partially funded through the CICS Global Oceans Grant received his PhD and has moved to undertake postdoctoral research at the University of Edinburgh in Scotland. He was working on the development of a global finite element model with specific applications to the circulation of the North Pacific and North Atlantic Oceans. The North Atlantic work was reported in earlier Progress Reports. Here I only summarize the results of the Pacific work which has appeared as Myers and Weaver (1996).

A finite element diagnostic model was used to study the circulation of the North Pacific Ocean. With the inclusion of the JEBAR term, the model produced a realistic picture of the circulation. All major currents were reproduced with the calculated transports agreeing well with observations. The three dimensional velocity structure was diagnosed from the thermal wind equation, assuming a reference velocity at the bottom. This bottom reference velocity was calculated from the Ekman, thermohaline and total transport (from the finite element model) velocities. The diagnosed velocity fields were then compared with a number of observational sections.

The effect of using different wind stress climatologies was also examined. Due to the dominance of the JEBAR term in the solution, the resulting circulations were all similar. Analysis of the seasonal cycle in the model supported the suggestion of Sakamoto and Yamagata (1995) that JEBAR rectification can explain the decreased amplitude of the seasonal cycle and the out of phase relationship between observations and the predictions of flat-bottomed Sverdrup theory.

Finally, density fields from 1955-1959 and 1970-1974 were used to examine aspects of interpentadal variability in the North Pacific Ocean.

10 On the role of various subgrid-scale boundary layer parameterizations in coarse resolution ocean models

Amongst the numerous sub-grid-scale parameterizations necessary in an ocean general circulation model, the influence of the momentum dissipation scheme and dynamical boundary conditions has been relatively ignored compared to tracer mixing. However, the ability of the ocean to transport heat poleward may be very sensitive to such closures, since they are the only way the large-scale circulation can depart from geostrophy and thus produce noticeable vertical velocities that feed the overturning. A thermohaline circulation model has been developed for a Cartesian coordinate flat-bottomed beta-plane, based on the planetary geostrophic equations, in order to compare different parameterizations of the momentum dissipation (Laplacian, biharmonic, Rayleigh and none) and associated boundary conditions (no-slip, free-slip and no-normal-flow). It is used at coarse-resolution for a mid-latitude basin with restoring boundary conditions for the surface density and no wind-stress.

Comparison with the GFDL MOM code confirms the negligible effects of vertical viscosity and total derivatives in the momentum equations. The surface temperature fields and poleward heat transports are quite similar for the steady-states obtained using the different viscosity schemes. However, large discrepancies in the bottom water properties and the velocity field show an order one effect of these closures on the mass transports. The traditional Laplacian friction produces a more satisfying interior circulation, in better agreement with geostrophy and Sverdrup balance, but generates excessively large vertical transports along the lateral boundaries (especially upwelling in the western boundary current - the Veronis effect - and downwelling in the north-east corner). The meridional overturning is thus enhanced but drives to depth surface waters that are not as cold as the ones in the deep convection regions.

Rayleigh friction with a no-normal-flow boundary condition (a vorticity closure is used whose primary effect is to reduce vertical velocities along the boundaries by allowing horizontal recirculation) induces a more efficient thermohaline circulation with better agreement between convection regions and areas of downwelling, colder deep water, much weaker meridional overturning and Veronis effect, but higher poleward heat transport. However, this parameterization lacks physical justifications and is not as satisfying as the Laplacian closure in terms of interior geostrophic and Sverdrup balance. The analysis of the correlations between the large scale diagnostics of these models points out the Veronis effect as the major contributor to warm deep water, diffuse thermocline, large overturning but weak poleward heat transport, in agreement with Böning et al. (1995). The role of dynamical boundary conditions is more important than the interior momentum dissipation in reducing this short-cut of the thermohaline loop.

This research has either been submitted (Huck et al., 1996a) or will be submitted shortly (Huck et al., 1996b, c) for publication.

11 Flux Corrected Transport Algorithms and Sub-grid-scale Mixing in an OGCM

Finally Weaver and Eby (1996) have implemented a flux-corrected transport advection algorithm (Gerdes et al., 1991) into the GFDL MOM2 and compared it with traditional second order centred difference advection schemes. This technology has been passed to the CCC and may be implemented in the next generation of global ocean models.

The results from ocean model experiments conducted with isopycnal and isopycnal thickness diffusion parameterizations for subgrid scale mixing associated with mesoscale eddies were examined from a numerical standpoint. It was shown that when the mixing tensor is rotated, so that mixing is primarily along isopycnals, numerical problems may occur and non-monotonic solutions which violate the second law of thermodynamics may arise when standard centred difference advection algorithms are used. These numerical problems can be reduced or eliminated if sufficient explicit (unphysical) background horizontal diffusion is added to the mixing scheme. A more appropriate solution is the use of more sophisticated numerical advection algorithms, such as the flux-corrected transport algorithm. This choice of advection scheme adds additional mixing only where it is needed to preserve monotonicty and so retains the physically-desirable aspects of the isopycnal and isopycnal thickness diffusion parameterizations, while removing the undesirable numerical noise. The price for this improvement is a computational increase.

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