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:
http://wikyonos.seos.uvic.ca/projects/CCC-Variability-Progress7.html
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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