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).
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