Climate Modelling Group
School of Earth and Ocean Sciences


NOAA Scripps Lamont Consortium on the Ocean's Role in Climate

Revised Workstatement for 1996

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ABSTRACT

Proposal Title: Ocean/Climate Modelling and Prediction on the Decadal Timescale

Investigator: Andrew Weaver

Address:
School of Earth and Ocean Sciences
University of Victoria
PO Box 1700
Victoria, BC, V8W 2Y2
Canada

Total Proposed Cost for Budget Period: $30,000

Budget Period: March 1, 1996 - October 31, 1996


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 energy/moisture balance model (EMBM) to an ocean general circulation model (OGCM). Since the timescale for variability of the cryosphere lies between that for the atmosphere and ocean, a thermodynamic ice model (TIM) will be used.

In addition the GFDL/NOAA coupled model will be used to investigate questions concerning the existence of climate variability in the coupled climate system and how it varies as the mean climatic state changes. This coupled model will allow for a better representation of atmospheric transport processes and surface winds.

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.
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Statement of Work

Summary of Research Projects


Below I begin by briefly summarizing the 2 research projects which I propose to undertake together with my students and research associates. A more detailed discussion of each individual project will follow. These summaries are for the project period (March 1996 - October 1996).

1) understand processes of decadal-interdecadal variability and poleward heat transport in the North Atlantic using a coupled OGCM-EMBM-TIM. The OGCM will have variable resolution ranging from 4deg. x 4deg. to 1/4deg. x 1/4deg.

2) use the GFDL coupled model and the coupled EMBM-OGCM-TIM to investigate processes of decadal-interdecadal variability in the coupled climate system and its dependence on the mean climatic state.


More Detailed Discussion of the Research Projects

1) Decadal-interdecadal variability and poleward heat transport in the North Atlantic

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, 1991; 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 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-adjustments (see Weaver and Hughes, 1995). This follows since if the flux adjustment 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 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 adjustments. This will expand upon the early uncoupled OGCM results. The use of an EMBM will allow for simple thermodynamic feedbacks. 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.

One of the most fundamental, yet unanswered, questions regarding the ocean's 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, 1991; 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 4deg. x 4deg. to 1/4deg. x 1/4deg. 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.

2) The dependence of climate variability in the GFDL coupled model on the mean climatic state

Both A. Weaver and T. Hughes recently visited GFDL in Princeton and acquired the GFDL coupled model. We have recently got this model working on our local workstation cluster. We plan to use it (in close collaboration with Ron Stouffer and Suki Manabe) to investigate the processes involved in decadal-interdecadal climate variability and in particular, to examine the sensitivity of this variability to the mean climatic state. To begin with we will run the ocean component of the GFDL coupled model with specified fluxes of heat and salt (obtained from the ocean spin-up) together with noise to see whether the decadal-interdecadal variability, seen in Delworth et al. (1993), is an internal mode of ocean variability which is a red response to white noise forcing. In addition, we propose to test the ideas of Weaver and Hughes (1995) to see whether or not we can reduce the necessary flux adjustments by simply adjusting the zonal mean fluxes of heat and salt (and hence the implied oceanic heat and salt transports).


References

Böning, C.W. and R.C. Budich, 1992: Eddy dynamics in a primitive equation model: sensitivity to horizontal resolution and friction. J. Phys. Oceanogr., 22, 361-381.

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.

Delworth, T., S. Manabe, and R.J. Stouffer, 1993: Interdecadal variability of the thermohaline circulation in a coupled ocean-atmosphere model. J. Climate, 6, 1993-2011.

Drijfhout, S.S., 1994: Heat transport by mesoscale eddies in an ocean circulation model. J. Phys. Oceanogr., 24, 353-369.

Greatbatch, R.J., and S. Zhang, 1995: An interdecadal oscillation in an idealized ocean forced by constant heat flux. J. Climate, 8, 81-91.

Latif M. and T.P. Barnett, 1994: Causes of decadal climate variability over the North Pacific and North America, Science, 266, 634-637.

Semtner A.J. and R.M. Chervin, 1992: Ocean general circulation from a global eddy-resolving model, J. Geophys. Res., 97, 5493-5550.

Trenberth, K.E., and J.W. Hurrell, 1994: Decadal coupled atmosphere-ocean variations in the North Pacific Ocean. Can. J. Fish. Aquat. Sci., 51, in press.

Wang, X., P.H. Stone and J. Marotzke, 1995: Poleward heat transport in a barotropic ocean model. J. Phys. Oceanogr., 25, 256-265.

Weaver, A.J., and E.S. Sarachik, 1991: Evidence for decadal variability in an ocean general circulation model: An advective mechanism. Atmos.-Ocean, 29, 197-231.

Weaver, A.J., and T.M.C Hughes, 1995: on the incompatibility of ocean and atmosphere models and the need for flux adjustments. Clim. Dyn., in press.

Weaver, A.J., E.S. Sarachik and J. Marotzke, 1991: Freshwater flux forcing of decadal and interdecadal oceanic variability. Nature, 353, 836-838.

Weaver, A.J., J. Marotzke, P.F. Cummins and E.S. Sarachik, 1993: Stability and variability of the thermohaline circulation. J. Phys. Oceanogr., 23, 39-60.

Weaver, A.J., Aura, S.M., and P.G. Myers, 1994: Interdecadal variability in a coarse resolution North Atlantic model. J. Geophys. Res., 99, 12,423-12,441.

Weisse, R., U. Mikolajewicz, E. Maier-Reimer, 1994: Decadal variability of the North Atlantic in an ocean general circulation model. J. Geophys. Res., 99, 12411-12421.

Winton, M., 1995: On the role of horizontal boundaries in parameter sensitivity and decadal-scale variability of coarse-resolution ocean general circulation models. J. Phys. Oceanogr., submitted.


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