Climate Modelling Group
School of Earth and Ocean Sciences
Climate Research Network
Collaborative Research Agreement at the University of Victoria
on Behalf of the Canadian Institute for Climate Studies and Environment Canada
(#7 CICS-Variability)

Progress Report:
April 1, 1997

1. Principal Investigator
Andrew Weaver

2. Institution
School of Earth & Ocean Sciences
University of Victoria
PO Box 1700
Victoria, BC, V8W 2Y2

3. Executive Summary

In this project climate models are being be developed to study natural climate variability on the decadal-to-century timescale. 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 (such as the one used by the Canadian Centre for Climate Modelling and Analysis) for the purpose of climate prediction and in assisting policy makers in making informed decisions regarding climate change policy. This follows since before it is possible to detect a global warming signal in the observational network it is important to gain insight into the variability in which this signal is masked.

4. Scientific Report

This progress report highlights the work conducted during the 1996 fiscal year. It is available on the world wide web at:

4.1 Thermohaline Variability: The Effects of Horizontal Resolution and Diffusion

An idealized coupled ocean-atmosphere model was utilized without flux adjustments to study the influence of horizontal resolution and parameterized eddy processes on the thermohaline circulation (Fanning and Weaver, 1997). A series of experiments ranging from 4o x 4o to 0.25o x 0.25o resolution, with appropriate horizontal viscosities and diffusivities in each case were performed for both coupled and ocean-only models. Spontaneous decadal-intradecadal variability (whose period varies slightly between cases) was found to exist in the higher resolution cases (with the exception of one of the restoring experiments). The oscillation is described as an advective-convective mechanism which is thermally driven, and linked to the value of the horizontal diffusivity utilized in the model. Increasing the diffusivity in our high resolution cases is enough to destroy the variability, while decreasing the diffusivity in the moderately coarse resolution cases is capable of inducing the variability. As the resolution is increased still further, baroclinic instability within the western boundary current adds a more stochastic component to the solution such that the variability is 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 decadal-intradecadal scale variability in models which require less parameterized diffusion.

4.2 Decadal Variability of the Thermohaline Circulation in Ocean Models

Intrinsic modes of decadal variability driven by constant surface buoyancy fluxes were analysed using a box-geometry ocean model (Huck et al., 1997). 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 diffiusivities, the convective adjustment parameterization and the horizontal and vertical model resolution. The oscillation stands out as a robust geostrophic feature whose amplitude is mainly controlled by the horizontal diffusivity. Since the beta-effect had no influence, Rossby waves played no role in the mechanism.

Various experiments with different geometry and forcing are conducted to test the importance of numerical boundary waves in sustaining the oscillation. The results suggest that only the western boundary is crucial (see also the work conducted by Greatbatch's group at Memorial University). The analysis of the variability patterns differentiates two types of oscillatory behavior: temperature anomalies traveling westward in an eastward jet (northern part of the basin) and geostrophically 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 geostrophically this change and build the opposite temperature anomaly in the east, which finally reverse the merdional overturning anomaly (and thus the anomalous western boundary current transport). The analysis of the transition from steady to oscillatory states suggests, in agreement with a one layer and a half model, that the variability is triggered in the regions of strongest cooling. Finally, we proposed a simple analytic box-model analogy that captures the observed phase-shift between meridional overturning and north-south density gradient anomalies.

4.3. Interdecadal Variability in the GFDL Coupled Model

The ocean component of the GFDL coupled model was used to investigate whether or not the interdecadal variability found in the GFDL coupled model is an ocean-only mode or a mode of the full coupled system (Valcke and Weaver, 1997). In particular, it has been previously suggested that the variability in the full coupled model is either: 1) an ocean-only mode which is excited by atmospheric noise; 2) an internal ocean mode driven by fixed atmospheric fluxes (flux adjustment plus annual mean) which is 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 show that none of these three hypotheses holds and therefore conclude that the interdecadal variability found in the GFDL coupled model is a mode of the full coupled system.

We are now conducting a 1000 year run with the GFDL coupled model to further investigate its internal variability. We suspect (see section 4.4) that the variability involves sea ice/atmosphere/thermohaline circulation coupling and so our initial attention will be focused in this area.

4.4 Decadal Variability of the Coupled Air-Sea-Ice Climate System

Our global coupled energy-moisture balance atmsophere/thermodynamic ice/ ocean general circulation model (without flux adjustments) exhibits persistent decadal variability (period 26 years) centered in the North Atlantic with century timescale variability occurring in the Southern Ocean. Initial analysis suggests that a mechanism involving sea-ice/thermohaline circulation interaction exists, similar to that of found by Yang and Neelin (1995 -- Geophys. Res. Let., 20, 217-220.). An issue which needs to be addressed is to what extent this variability changes as the mean climate state itself changes (i.e., in warmer or colder mean climates).

This area of sea ice/sea/air interaction is largely unexplored yet of crucial importance to climate variability. As such I have hired two postdoctoral research associates (Dr. Cecilia Bitz, University of Washington and Dr. Marika Holland, University of Colorado) who will undertake research into the role of the Arctic and sea ice in climate variability. They will join my research group in August 1997.

5. 1996-1997 CICS Variability Papers Arising

1. 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.

2. Fanning, A.F. and A.J. Weaver, 1997: Thermohaline variability: The effects of horizontal resolution and diffusion. Journal of Climate, submitted.

3. Huck, T., A. Colin de Verdière and A.J. Weaver, 1997: Decadal variability of the thermohaline circulation in ocean models. Journal of Physical Oceanography, submitted.

4. Valcke, S. and A.J. Weaver, 1997. On the variability of the thermohaline circulation in the GFDL coupled model. Journal of Climate, submitted.

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