Climate Modelling Group
School of Earth and Ocean Sciences


Progress Report (09/01/97 - 28/02/98) and Final Technical Report
 
Andrew Weaver
School of Earth and Ocean Sciences
University of Victoria
 
NOAA Scripts-Lamont Consortium on the Ocean's Role in Climate
 
Subcontract: "Ocean/Climate Modeling and Prediction on the Decadal Timescale"
 
 

1. Analysis of the role of eddies in transporting heat and freshwater poleward - Completed 30/06/1997

A coupled ocean-atmosphere model was used to study the influence of horizontal resolution and parameterized eddy processes on the poleward heat transport in the climate system. A series of experiments ranging from 4° to 0.25° resolution were performed. Our results contradicted earlier studies which showed that the heat transport associated with time varying circulations counteracts increases in the time mean so that the total remained unchanged as resolution was increased. To interpret our results, the oceanic heat transport was decomposed into its baroclinic overturning (related to the meridional overturning and Ekman transports), barotropic gyre (in the horizontal plane) and baroclinic gyre (associated with the core of the western boundary current) components. The increase in heat transport occurred in the steady currents. In particular the baroclinic gyre transport increased by a factor of 5 from the coarsest to the finest resolution case, equaling the baroclinic overturning transport at mid-high latitudes. To further assess the results, a parallel series of experiments under restoring conditions were performed to elucidate the differences between heat transport in coupled vs uncoupled models, and models driven by temperature and salinity or equivalent buoyancy. Although heat transport was more strongly constrained in the restoring experiments, results were similar to those in the coupled model. These results point to the importance of higher resolution in the oceanic component of current coupled climate models and stress the need to adequately represent the heat transport associated with the Warm Core region of the Gulf Stream (the baroclinic gyre transport) in order to adequately represent oceanic poleward heat transport. Published in Fanning and Weaver (1997b).

2. Simulation of the Younger Dryas event - Completed 30/06/1997

The temporal and geographical roles of meltwater discharge (from the Laurentide ice sheet) on North Atlantic Deep Water (NADW) production were investigated using a global coupled model. Model results suggested that preconditioning by meltwater discharge (to the Mississippi) prior to the Younger Dryas (YD) was capable of pushing NADW beyond the limit of its sustainability. The diversion of meltwater to the St. Lawrence then served to inhibit NADW production. The modeled change in surface air temperature (SAT) generally agreed with the global pattern and magnitude of change seen in paleoclimatic reconstructions of the YD and was linked to changes in NADW formation. The thermohaline circulation provided an interhemispheric teleconnection with the Southern Ocean, while changes in the atmospheric heat transport provided a mechanism for interbasin teleconnection. The inclusion of a wind stress/speed feedback was found to contribute to the resumption of NADW production, as suggested by previous studies. Contrary to these studies the coupled model indicated that an advective spin-up timescale was required for resumption of NADW production and hence the termination of the modeled YD-like climate event (as opposed to a decadal-century timescale). Published in Fanning and Weaver (1997a).

3. Climatic effects of opening and closing oceanic gateways- Completed 31/10/1997

The paleoclimatic effects of the closure of the Isthmus of Panama ~3 million years ago were investigated using our coupled model. Consistent with earlier ocean-only modelling studies, it was shown that prior to the closing of the Isthmus of Panama, NADW formation did not occur. Associated with the absence of NADW formation was a reduction in both the Atlantic and global oceanic heat transports. This reduction in oceanic heat transport was largely compensated for by an increase in the atmospheric heat transport, with the result that only small changes in total planetary heat transport occurred. Model results suggested that the present-day climate of the North Atlantic is significantly warmer than before the closure of the Isthmus. In addition, the regions surrounding the Pacific Ocean and South Atlantic are generally cooler while the Indian Ocean is generally warmer in the present-day climate simulation. Published in Murdock et al (1997).

While this particular component of the research is now complete, funding has been received under the NSERC Climate System History and Dynamics project to carry on important follow-up research. M. Yoshimori, a PhD student, is focusing on the interaction of the cryosphere, atmosphere and ocean. We are presently incorporating the continental ice sheet model, developed by Marshall and Clarke at UBC, into our coupled atmosphere-ocean-sea ice model. The ice sheet/climate model will be used to: 1)- examine the transition from the Last Glacial Maximum (see Weaver et al, 1998b) to the Holocene. This involves the integration of the coupled system from 21KBP to 6KBP under changing orbital forcing. 2)- examine whether or not the onset of northern hemisphere glaciation can be attributed to the closure of the Isthmus of Panama ~ 3million years ago.

4. Understand processes of decadal variability in coupled OGCM-EMBM-TIM - In progress

An idealized coupled model was used to study the influence of horizontal resolution and parameterized eddy processes on the thermohaline circulation. A series of experiments ranging from 4° to 0.25° resolution were performed for both coupled and ocean-only models. Spontaneous internal variability (primarily on the decadal time-scale) was found to exist in the higher resolution cases. The decadal variability was described via an advective-convective mechanism which is thermally driven, and linked to the value of the horizontal diffusivity used in the model. Increasing the diffusivity in the high resolution cases was enough to destroy the variability, while decreasing the diffusivity in the moderately coarse resolution cases was capable of inducing decadal-scale variability. As the resolution was increased still further, baroclinic instability within the western boundary current added a stochastic component to the solution such that the variability was less regular and more chaotic. These results point to the importance of higher resolution in the ocean component of coupled models, revealing the existence of richer variability in models which require less parameterized diffusion. Published in Fanning and Weaver (1998).

Work has not finished in this area as we and our partners (CICS and CCCma), have recently become excited about understanding low frequency variability of the North Atlantic Oscillation (NAO). Our conjecture is that sea ice plays an important role in this variability. In order to examine this issue, several improvements have been incorporated into the sea ice component of our coupled model. These include a parameterization of sea ice dynamics and a subgrid-scale ice thickness distribution. Self-sustained oscillations involving sea-ice appear possible in the model, although more work is needed to examine the sensitivity of these oscillations to internal parameterizations.

The impact of ice export on climate variability is also being addressed by applying an anomalous wind stress forcing in the coupled model. The model reveals significant decadal variability when the wind stress field includes interannual variability at high latitudes (north of 60°) and over the North Atlantic Ocean. The interannually varying wind stress field was derived from NCEP reanalysis daily mean SLP data which is available for the last 40 years. The SLP data were separated into spatial modes using an EOF analysis and only the leading 20 modes were kept. C. Bitz and M. Holland (Research Associates) are focusing their efforts on this exciting research area.

5. CFC-11 and sub-grid-scale mixing parameterizations - In progress

In attempting to address this objective we first had to overcome some numerical problems associated with the use of isopycnal and isopycnal thickness diffusion parameterizations for subgrid scale mixing associated with mesoscale eddies. It was shown that when the mixing tensor was rotated, so that mixing was 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 monotonicity 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. Published in Weaver and Eby (1997).

An ocean circulation model was also developed for a Cartesian coordinate flat-bottomed beta-plane, based on the planetary geostrophic (PG) equations, in order to test different parameterizations of the momentum dissipation (Laplacian, biharmonic, Rayleigh and none) and associated boundary conditions. The surface temperature fields and poleward heat transports were quite similar for the equilibrium states obtained using different momentum dissipation parameterizations. However, a comparison of the velocity fields and bottom water properties showed large discrepancies. Traditional Laplacian friction produced a more satisfying interior circulation, in better agreement with geostrophy and the Sverdrup balance, but generated excessively large vertical transports along the lateral boundaries. Rayleigh friction with a no-normal-flow boundary condition induced a more efficient thermohaline circulation with better agreement between convection regions and areas of downward velocities, colder deep water, much weaker meridional overturning and vertical transports along lateral boundaries, but higher poleward heat transport. Results from this parameterization were not as satisfying as the Laplacian closure in terms of interior geostrophic and Sverdrup balance. Nevertheless, it is an interesting alternative to implement it along with the no-normal-flow boundary conditions, since free-slip and no-slip boundary conditions were also shown to lead to very similar circulations, regardless of the momentum dissipation scheme. Published in Huck et al (1998b).

Recent measurements have shown that oceanic mixing varies with location, and tends to be an order of magnitude larger at ocean margins than in the thermocline. As a follow-up analysis, O. Dravnieks (a PhD student) is examining the effects of spatially varying vertical mixing parameterization on the global ocean circulation.

6. sub-grid-scale mixing parameterizations and decadal variability - Completed 31/10/1997

Intrinsic modes of decadal variability were analysed using the PG ocean model. A complete parameter sensitivity analysis of the oscillatory behavior was carried out with respect to the spherical Cartesian geometry, the beta-effect, the Coriolis parameter, the parameterization of momentum dissipation and associated boundary conditions and viscosities, the vertical and horizontal diffusivities, the convective adjustment parameterization and the horizontal and vertical model resolution. The oscillation stood out as a robust feature whose amplitude was mainly controlled by the horizontal diffusivity. The analysis of the variability patterns differentiated two types of oscillatory behavior: temperature anomalies traveling westward in an eastward jet (northern part of the basin) inducing an opposite anomaly in their wake; temperature anomalies in the north-west corner which respond to the western boundary current transport changes, but reinforce this change and build the opposite temperature anomaly in the east, which finally reverse the meridional overturning anomaly (and thus the anomalous western boundary current transport). The analysis of the transition from steady to oscillatory states suggested, in agreement with a 1 1/2 layer model, that the variability was triggered in the regions of strongest cooling. Finally, we developed a simple box-model analogy that captured the observed phase-shift between meridional overturning and north-south density gradient anomalies. Published in Huck et al (1998a).

Flux adjustments are often used in coupled models to correct for missing parameterized or resolved physics. We examined the effect of using flux adjustments on the climatic response of our coupled model to an imposed radiative forcing. A linear reduction to the planetary longwave flux of 4 W/m2 was applied over a 70 year period and held constant thereafter. Similar model responses were found during the initial 70 year period for global-scale diagnostics of hemispheric SAT due to the nearly linear SAT response to the radiative forcing. Significant regional scale differences did however exist. As the perturbation away from the present climate grew, basin-scale diagnostics began to diverge between flux adjusted and non-flux adjusted models. Once the imposed radiative forcing was held constant, differences in global mean SAT of up to 0.5°C were found, with large regional-scale differences in SAT and overturning rates within the North Atlantic and Southern Ocean. Additional experiments with the flux adjusted model suggested the coupling shock could be reduced by running out the control integration before the radiative forcing was applied. Our results suggested that perturbation experiments should not be undertaken until after the coupled model control experiment is continued for several hundred years in order to minimize the coupling shock. In addition, care should be exercised in the interpretation of regional-scale results (over the ocean) and global-scale diagnostics for large perturbations from the present climate, in coupled models which employ flux adjustments. Published in Fanning and Weaver (1997c).

7. Analysis of decadal variability in the GFDL coupled model - In progress

The ocean component of the GFDL coupled model was used to investigate whether or not earlier reported interdecadal variability was an ocean-only mode or a mode of the full coupled system. In particular, it was previously suggested that the variability in the full coupled model was either: 1) an ocean-only mode which was excited by atmospheric noise; 2) an internal ocean mode driven by fixed atmospheric fluxes which was made less regular through forcing from atmospheric noise; 3) a consequence of the use of flux adjustments. Through a series of experiments conducted under fixed flux boundary conditions we showed that none of these three hypotheses held and therefore concluded that the interdecadal variability found earlier was a mode of the full coupled system. Published in Weaver and Valcke (1998).

S. Zhang (Research Associate) also created a more efficient version of the GFDL coupled model in order to study low frequency variability using available computing resources. The increased efficiency was achieved by counting each atmospheric time step as being valid for six timesteps, thereby slowing down the synoptic scale motion. All of the basic physics in the original model was retained. This method used the fact that atmospheric response time is much shorter than that of the ocean. In addition, the climate memory of the coupled system is commonly believed to be resident in the ocean so the detailed history of atmospheric synoptic scale motion is not necessary. The atmosphere and ocean of this new version of the model are coupled monthly or roughly every slowed down synoptic cycle. Our results suggest that this method is an attractive alternative to other methods of achieving higher efficiency (such as poorer resolution and/or reduced physics). This model was then integrated many times for many hundreds of years to undertake a sensitivity analysis of the low frequency variability found in the coupled system.

El Niño/Southern Oscillation (ENSO) variability appeared in the model and is shown to be stronger when a lower vertical diffusivity is used in the ocean (reducing the tropical thermocline depth), or when the heat flux anomalies in atmosphere are reduced before passing them to the ocean (a method of reducing flux adjustments). Similar to the unmodified version of the GFDL model, interdecadal variability in North Atlantic was found, albeit slightly weaker. In addition, we found weak signals of a decadal oscillation in North Pacific with thermal anomalies rotating clockwise around the gyre, especially in those experiments which used a higher vertical diffusivity. The Pacific oscillation is supported by observational evidence and the results from the German (MPI) climate model. Currently being written up for publication.

8. Analysis of decadal variability in warmer and colder climates using the GFDL coupled model - In progress

We have decided to approach this objective through the use of a hierarchy of models. To begin with a zonally-averaged atmospheric model is used which incorporates a parameterization of transient eddy activity. This model has been coupled by A. Brasket (a visiting PhD student) to an idealized Atlantic basin OGCM to investigate the influence of the atmospheric transports of heat and fresh water by transient storm activity on the strength and variability of the thermohaline circulation. We find that the strength of the thermohaline variability is determined by the dissipation rate of the ocean SST anomalies by the atmosphere. This has the interesting consequence of making the variability dependent on the mean state. Further work in this area will concentrate on ensuring that these results are general for the model and hold for a wide range of parameter space representative of the coupled climate system.

9. teleconnections in global OGCM- In progress

Research into meeting this objective has recently begun. In the first phase we wished to examine the ocean response (and its internal teleconnections arising from NADW perturbations) to global warming radiative perturbations. As such, E. Wiebe (an MSc student nearing completion) is undertaking a sensitivity analysis using the coupled model to investigate the effect of various ocean mixing schemes on the ocean's response to global warming. His experiments have revealed the fascinating result (supported by recent observations) that there is a significant intrusion of warmed Atlantic water into the sub-surface Arctic Ocean during the initial response to the radiative forcing. In the southern hemisphere, we find that the particular mixing scheme used plays a significant role in the strength of the response of the sea surface temperature (SST).

10. simple models of decadal variability - In progress

This objective is ongoing and initial analysis has been discussed under milestone 6. We are also attempting to develop a simple model for decadal variability in the Pacific. Since ENSO is a nonlinear coupled tropical atmosphere-ocean phenomenon, it is possible that decadal modulation of ENSO and its subsequent teleconnection to the North Pacific could explain the observed low frequency variability there. As pointed out by Gu and Philander a delayed negative feedback can be achieved through extratropical subduction of thermal anomalies (generated through the atmospheric teleconnection response to equatorial SST anomalies) which slowly propagate along isopycnals towards the equator where they reverse the sign of equatorial SSTs. We are in the process of developing a simple delayed oscillator model (involving the Battisti/Hirst model) to understand mechanisms for tropical/subtropical interactions and interdecadal variability. One of the parameters that is assumed to be constant in the Battisti and Hirst model is the pycnocline depth. By adding a meridional delay (through pycnocline subduction) to the equatorial pycnocline depth one can envision a mechanism for ENSO modulation.

Publications Supported by NOAA 1994-1998

1. Weaver, A.J. and T.M.C. Hughes, 1994: Rapid interglacial climate fluctuations driven by North Atlantic ocean circulation. Nature, 367, 447-450.

2. Hughes, T.M.C. and A.J. Weaver, 1994: Multiple equilibria of an asymmetric two-basin ocean model. Journal of Physical Oceanography, 24, 619-637.

3. Weaver, A.J., S.M. Aura and P.G. Myers, 1994: Interdecadal variability in a coarse resolution North Atlantic model. Journal of Geophysical Research, North Atlantic Deep Water Formation: Observation and Modeling Special Edition, 99, 12,423-12,441.

4. Boyle, E and A.J. Weaver, 1994: Conveying past climates. Nature, 372, 41-42.

5. Myers, P.G. and A.J. Weaver, 1995: A diagnostic barotropic finite element ocean circulation model. Journal of Atmospheric & Oceanic Technology, 12, 511-526.

6. Tang, B. and A.J. Weaver, 1995: Climate stability as deduced from an idealized coupled atmosphere-ocean model. Climate Dynamics, 11, 141-150.

7. Das, S.K. and A.J. Weaver, 1995: Semi-Lagrangian advection algorithms for ocean circulation models. Journal of Atmospheric & Oceanic Technology, 12, 935-950.

8. Weaver, A.J., 1995: Driving the ocean conveyor. Nature, 378, 135-136.

9. Robitaille, D.Y. and A.J. Weaver, 1995: Validation of sub-grid scale mixing schemes using CFCs in a global ocean model. Geophysical Research Letters, 22, 2917-2920.

10. Wohlleben, T.M.H. and A.J. Weaver, 1995: Interdecadal climate variability in the subpolar North Atlantic. Climate Dynamics, 11, 459-467.

11. Weaver, A.J. and T.M.C Hughes, 1996: On the incompatibility of ocean and atmosphere models and the need for flux adjustments. Climate Dynamics, 12, 141-170.

12. Hughes, T.M.C. and A.J. Weaver, 1996: Sea surface temperature - evaporation feedback and the ocean's thermohaline circulation. Journal of Physical Oceanography, 26, 644-654.

13. Myers, P.G., A.F. Fanning and A.J. Weaver, 1996: JEBAR, bottom pressure torque and Gulf Stream separation. Journal of Physical Oceanography, 26, 671-683.

14. Fanning, A.F. and A.J. Weaver, 1996: An atmospheric energy moisture-balance model: climatology, interpentadal climate change and coupling to an OGCM. Journal of Geophysical Research, 101, 15111-15128.

15. Myers, P.G. and A.J. Weaver, 1996: On the circulation of the North Pacific Ocean: Climatology, seasonal cycle and interpentadal variability, Progress in Oceanography, 38, 1-49.

16. Murdock, T.Q., A.J. Weaver and A.F. Fanning, 1997: Paleoclimatic response of the closing of the Isthmus of Panama in a coupled ocean-atmosphere model. Geophysical Research Letters, 24, 253-256.

17. Weaver, A.J. and M. Eby, 1997: On the numerical implementation of advection schemes for use in conjunction with various mixing parameterizations in the GFDL ocean model. Journal of Physical Oceanography, 27, 369-377.

18. Fanning, A.F. and A.J. Weaver, 1997a: Temporal-geographical meltwater influences on the North Atlantic conveyor: Implications for the Younger Dryas, Paleoceanography, 12, 307-320.

19. Fanning, A.F. and A.J. Weaver, 1997b: A horizontal resolution and parameter sensitivity study of heat transport in an idealized coupled climate model, Journal of Climate, 10, 2469-2478.

20. Fanning, A.F. and A.J. Weaver, 1997c: On the role of flux adjustments in an idealized coupled model. Climate Dynamics, 13, 691-701.

21. Fanning, A.F. and A.J. Weaver, 1998: Thermohaline variability: The effects of horizontal resolution and diffusion. Journal of Climate, 11, 709-715.

22. Weaver, A.J. and S. Valcke, 1998: On the variability of the thermohaline circulation in the GFDL coupled model. Journal of Climate, 11, 759-767.

23. Weaver, A.J., M. Eby, A.F. Fanning and E.C. Wiebe, 1998: Simulated influence of carbon dioxide, orbital forcing and ice sheets on the climate of the last glacial maximum. Nature, 394, 847-853.

24. Huck, T., A. Colin de Verdière and A.J. Weaver, 1998: Interdecadal variability of the thermohaline circulation in box-ocean models forced by fixed surface fluxes. Journal of Physical Oceanography, in press.

25. Huck, T., A.J. Weaver and A. Colin de Verdière, 1998: On the influence of lateral boundary layers on the thermohaline circulation in coarse-resolution ocean models. Journal of Marine Research, submitted.

26. Weaver, A.J., C.M. Bitz, A.F. Fanning and M.M. Holland, 1998: Thermohaline circulation: High latitude phenomena and the difference between the Pacific and Atlantic. Annual Review of Earth and Planetary Sciences, submitted.

27. Wiebe, E.C. and A.J. Weaver, 1998: On the sensitivity of global warming experiments to the parameterisation of sub-grid scale ocean mixing. Climate Dynamics, submitted.

28. Flato, G.M., G.J. Boer, N.A. McFarlane, D. Ramsden, M.C. Reader and A.J. Weaver, 1998: The Canadian Climate Centre for Climate Modelling and Analysis global coupled model and its climate. Climate Dynamics, submitted.

29. Weaver, A.J., 1998: Extratropical subduction and decadal modulation of El Niño. Geophysical Research Letters, submitted.

30. Duffy, P.B., M. Eby and A.J. Weaver, 1998: Effects of sinking of salt rejected during formation of sea ice on results of a global ocean-atmosphere-sea ice climate model. Geophysical Research Letters, submitted.

 


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