CSHD 5

Principal Investigator: Andrew Weaver

Co-investigator: Dr. D. Wright

Preamble:

Funding for CSHD work was received by the Office of Research Administration, University of Victoria at the beginning of October, 1996. As such the progress reported in sections 1.1 and 1.2 below is progress which has largely arisen from funding through the NOAA Scripps Lamont Consortium on the Ocean's Role in Climate. This NOAA funding source has now been replaced by the CSHD project. Of the $60,000 received in October, $20,000 was transferred to Dalhousie University to support Dr. D. Wright's work in this project. His progress is highlighted in Section 1.3 below.

1. Achievements

1.1 Simulation of the Younger Dryas event

An Energy-Moisture Balance Atmospheric Model (EMBM) has been developed and coupled to an Ocean General Circulation Model (OGCM) and a Thermodynamic Ice Model (Fanning and Weaver, 1996). This EMBM-TIM-OGCM has been used to investigate the transition between the last glaciation and the present Holocene and in particular the Younger Dryas event (hereafter the YD). The traditional viewpoint is that the YD was triggered by the diversion of meltwater (due to the retreating Laurentide Ice Sheet) from the Gulf of Mexico to the St. Lawrence (Broecker et al., 1988). In an attempt to clarify the temporal and geographical roles of meltwater discharges on triggering the YD, estimates for volumes of runoff from the Laurentide ice sheet (Teller, 1990) were applied for the 1500 year period encompassing the YD cold episode. Model results indicate that the traditional Laurentide meltwater diversion theory is insufficient to induce a YD climate signature, although preconditioning by the pre-YD meltwater discharge in conjunction with the diversion is.

The model predicted YD climate shift is global in nature, and is intimately linked to North Atlantic deepwater (NADW) formation. The global thermohaline circulation provides an interhemispheric teleconnection with the Southern Ocean, while changes in the atmospheric heat transport (reacting to a global redistribution of oceanic heat transport) provides a mechanism for interbasin teleconnection. Changes in model surface air temperature (Fig. 1a) generally agree with the pattern and magnitude of YD temperature change deduced from paleoclimatic reconstructions based on existing paleothermometers.

Our results (Fanning and Weaver, 1997) indicate that supplying both pre and post-YD meltwater discharges results in a total collapse of the North Atlantic conveyor. In the absence of additional model feedbacks, this state appears to be relatively stable, and equivalent to the Southern Sinking state identified by Manabe and Stouffer (1988). Reestablishment, if it were to occur, would appear to be a diffusive process as in previous ocean-only model studies under a polar halocline catastrophe (e.g., Marotzke, 1989; Weaver and Sarachik, 1991). If instead we allow for the effects of this climate state to feedback onto the surface winds, reestablishment occurs on a faster time scale. This is due to an increased surface salinification through latent heat loss and Ekman transport of the salinity anomaly out of the region of deepwater formation. This result is consistent with previous studies of freshwater perturbations on the North Atlantic Conveyor (e.g., Schiller et al., 1996; Mikolajewicz, 1996), however, unlike these studies the model settles into a new equilibrium state with reduced NADW formation as in Rahmstorf (1994). We also note that unlike these same studies, the time scale for reestablishment of NADW, and hence termination of the YD signal is an advective spinup time scale (order 1000 years) as opposed to decadal-century. The reason for this discrepancy is unclear, but may be associated with the used of fixed salt flux fields employed by earlier studies.

While we have not explicitly addressed the question of the role of the Fennoscandian Ice Sheet's demise on the YD, our results suggest at most it would have prolonged the YD episode. This raises a final point, is the advective spinup time scale found here representative of the time scale for the YD termination? Considering the d¹⁸O record at Summit (Dansgaard et al., 1993), the bulk of warming signaling during the transition from the YD to the Holocene occurred over a 200 year period (although full termination took much longer). So, it appears that further treatment of model feedbacks (e.g., cloud effects) and perhaps radiative forcing (due to increasing levels of CO₂ are needed to investigate the Younger Dryas termination further.

1.2 Paleoclimatic response of the closure of the Isthmus of Panama

The paleoclimatic effects of the closure of the Isthmus of Panama ~3 million years ago have also been investigated using the coupled OGCM-TIM-EMBM of Fanning and Weaver (1996). Consistent with earlier ocean-only modelling studies (Maier-Reimer et al., 1990; Mikolajewicz et al., 1993), it was shown that prior to the closing of the Isthmus of Panama, the Atlantic behaved more similar to the present-day Pacific Ocean with a conspicuous absence of deep water formation. Associated with the absence of North Atlantic deep water formation was a significant 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 total atmospheric heat transport, with the result that only small changes in planetary heat transport occurred. Model results suggest that the present-day climate of the North Atlantic is significantly warmer, together with a general cooler trend in the southern hemisphere and the region surrounding the Pacific Ocean, than before the closure of the Isthmus (Fig. 1b). Finally, possible relationships to glaciation and initiation of glacial cycles in the Northern Hemisphere, were discussed. These results have recently been accepted for publication (Murdock et al., 1997).

1.3 Low-order coupled model studies

Drs. Wright and Stocker have completed their manuscript (Stocker and Wright, 1996) on the influence of rapid changes in ocean circulation (such as believed to have occurred during the Younger Dryas event) on atmospheric radiocarbon.

Dr. Wright has written up a paper (Wright, 1996) on a very efficient equation of state for ocean water. This work was motivated by the fact that calculations of density using the full UNESCO equation of state can easily use up 50% of the CPU in low order models of the ocean. Many previous studies have used a linearized equation of state, but results presented by J. Tong (Dalhousie University, MSc thesis, 1995) show that this can lead to incorrect results. The new equation of state is accurate, very simply implemented and uses only about 10% as much CPU as the full UNESCO equation of state. It has been used in several of their previous studies and will be used in all of their future paleoclimate research.

Drs. Wright, Stocker and Mercer have written up a manuscript (Wright et al., 1997) on the various closure schemes used in zonally averaged models. The primary purpose of this manuscript is to clearly present the assumptions that are implicitly made in using various closure schemes. It is shown that the Rayleigh damping form of dissipation used by Wright and Stocker reasonably represents the important momentum and vorticity dissipation in the western boundary, and that this term is very crudely represented in the Fickian diffusion models. On the other hand, as indicated by the work of Sakai and Peltier, if high resolution is required, horizontal viscosity is required to suppress a numerical instability. An efficient scheme which includes both effects is presented. The third closure used in zonally averaged ocean models is that introduced by Wright et al (1995). This model is based on zonally averaging the vorticity equations and is shown to be clearly superior to either of the popular models based on zonally averaging the momentum equations.

Drs. Wright, Brickman and Hyde have collaborated on a study of the influence of the ocean on very long timescale climate variability. In this study, the ocean response is shown to result in an amplification of the direct eccentricity forcing (periods of about 100,000 and 400,000 years), and simple physical explanations are given. Surprising results on the relative phases of the responses at 40,000 years and 100,000 years are found in this study and a very simple model is developed to explain why these peculiar results occur. Sorting these effects out is an important step in the interpretation of paleoclimate records. A first draft of a paper on this work has been written and is expected to be submitted to Paleoceanography within the next month or two.

1.4 References not in attached list

Broecker, W.S., M. Andree, W. Wolfli, H. Oeschger, G. Bonani, J. Kennett, and D. Peteet, The chronology of the last deglaciation: Implications to the cause of the Younger Dryas event. Paleoceanogr., 3, 1-19, 1988.

Maier-Reimer, E., U. Mikolajewicz, T.J. Crowley, 1990, Ocean general circulation model sensitivity experiment with an open central american Isthmus, Paleoceanogr., 5, 349-366.

Manabe, S., and R.J. Stouffer, 1988: Two stable equilibria of a coupled ocean-atmosphere model. J. Climate, 1, 841-866.

Marotzke, J., 1989: Instabilities and multiple steady states of the thermohaline circulation, in Oceanic Circulation Models: Combining Data and Dynamics, NATO ASI Ser., edited by D.L.T. Anderson and J. Willebrand, 501-511 pp., Kluwer Acad.

Mikolajewicz, U., 1996: A meltwater induced collapse of the `conveyor belt' thermohaline circulation and its influence on the distribution of delta¹⁴ C and delta¹⁸O in the oceans, Max Planck Inst. for Meteorol., Rep. 189, Hamburg, Germany.

Mikolajewicz, U., E. Maier-Reimer, T.J. Crowley, and K.-Y. Kim, 1993, Effect of Drake and Panamanian gateways on the circulation of an ocean model, Paleoceanogr., 8, 409-426.

Rahmstorf, S, 1994: Rapid climate transitions in a coupled ocean-atmosphere model. Nature, 372, 82-85.

Schiller, A., U. Mikolajewicz, and R. Voss, 1996: The stability of the thermohaline circulation in a coupled ocean-atmosphere general circulation model, Max Planck Inst. for Meteorol., Rep. 188, Hamburg, Germany.

Teller, J.T., 1990: Meltwater and precipitation runoff to the North Atlantic, Arctic, and Gulf of Mexico from the Laurentide Ice sheet and adjacent regions during the Younger Dryas, Paleoceanogr., 5, 897-905.

Weaver, A.J., and E.S. Sarachik, 1991: The role of mixed boundary conditions in numerical models of the ocean's climate, J. Phys. Oceanogr., 21, 1470-1493.

Wright, D.G., Vreugdenhil, C.B. and T.M.C. Hughes, 1995: Vorticity dynamics and zonally averaged ocean circulation models, 25, 2141-2154.

2. Proposed Activities over the Next Year

I would like to further our understanding of the mechanisms of climate variability through the use of increasingly more sophisticated coupled models. The coupled energy-moisture balance atmosphere/ocean/ice model represents the simplest form of my proposed coupled modelling studies. I hope to use it to explore simple thermodynamic feedbacks and gain insight into what results we might expect and which experiments we should undertake with the more complicated Geophysical Fluid Dynamics Laboratory (GFDL) coupled model. The GFDL coupled model is more sophisticated than the aforementioned coupled model as it includes full atmospheric dynamics and physics. In addition, the GFDL coupled model will be used to investigate questions concerning the existence of variability in the coupled climate system and how it varies as the mean climatic state changes (e.g., does the decadal-interdecadal climate variability found in the coupled model change as CO₂ is increased in the atmosphere? Is a colder, glacial climate inherently more unstable?).

Two important paradoxes exist in the paleoclimatic literature. The first of these concerns how it was possible for the Ordovician climate (~440 million years ago) to support glaciers when the atmosphere had CO₂ levels 16 times higher than today. Similarly, during the Cretaceous, atmospheric CO₂ levels were 8 times the present yet recent evidence has suggested that it was much cooler than previously thought, with tropical temperatures similar to those of today and polar temperatures hovering around freezing. I hope to unravel these questions through the use of our newly-developed coupled model. This research will be funded by my NSERC Steacie Fellowship Supplement Grant but is relevant to the CSHD project.

In order to test whether or not the assumption of the use of present day sea surface temperatures is valid in the 6 kyr BP simulation of CSHD-8, we will use our coupled model to examine the oceanic response to 6 kyr BP orbital forcing and CO₂ levels. Furthermore, we will conduct a Last Glacial Maximum (LGM) experiment, analagous to that done in CSHD-8, to look at potential oceanic changes. The results of both experiments will likely lead to better sea surafce temperature estimates with which to drive the AGCM of CSHD-8.

Finally, Drs. G. Clarke and S. Marshall at the University of British Columbia (also CSHD participants) and I have initiated a collaboration which will entail coupling their newly developed ice sheet model into the coupled EMBM-TIM-OGCM of Fanning and Weaver (1996). A. Fanning who has submitted his PhD thesis and will move to MIT shortly has also expressed an interested in this collaboration. Upon coupling the ice sheet model to our EMBM-TIM-OGCM we propose to examine ice sheet/climate interactions during the last glaciation and in particular during the transition from the last glacial maximum to the present. This study will be complementary to that already undertaken with our coupled model (section 1.1) as sources of freshwater discharge will be computed internally. We also hope to examine the climatic response to Milankovitch Cycles as well as the possible existence and mechanisms of Heinrich events in the coupled system. In addition, we will try to use this climate system model to investigate the onset of northern hemisphere glaciation by examining the climate system response to the opening and closing of oceanic gateways (i.e., the Isthmus of Panama, Bering Strait and the Greenland-Iceland-Faroes rise).

3. Training of Highly Qualified Personel since 1994

The CSHD project provides full or partial funding for one postdoctoral research associate (Dr. S. Valcke), two MSc students (T. Murdock; P. Poussart) and one PhD student (A. Fanning). I have a large group of postdocs/research associates (see below) and have recently offered positions to three new graduate students and three new postdoctoral research associates (as S. Valcke recently accepted a permanent job in France).

At Dalhousie University, two Msc students (J. Tong and D. Mercer) have defended their theses and since graduated during the course of this project.

Details of Graduate Supervision (present):

Augustus Fanning (PhD): Coupled ocean-atmosphere modelling
started Sept. 1, 1993 (Atlantic Accord Career Development Award)

Trevor Murdock (MSc): Paleoclimatic changes associated with the opening of Drake Passage and the closure of the Isthmus of Panama
started Sept. 1, 1995 (UVic Graduate Fellowship)

Edward Wiebe (MSc): The role of the Southern Ocean in global climate change
started May 1, 1996 (UVic Graduate Fellowship)

Pascale Poussart (MSc): Paleoclimatic Modelling
Started September 1, 1996 (FCAR Scholarship)

Details of Graduate Supervision (past):

Trudy Wohlleben (MSc): Decadal climate variability in the subpolar North Atlantic
degree conferred: 1994 (AES Educational Leave)

Daniel Robitaille (MSc): On the use of CFCs in an ocean general circulation model
degree conferred: 1997

Thierry Reynaud (PhD): Dynamics of the northwestern Atlantic Ocean: a diagnostic study
degree conferred: 1994 (FCAR Scholarship)

Tertia Hughes (PhD): Uniqueness and variability of the ocean's thermohaline circulation
degree conferred: 1995 (NSERC Scholarship; Tri-Council Eco-Research Doctoral Fellowship)

Paul Myers (PhD): Finite element solutions of the barotropic vorticity equation: Applications to the North Atlantic and North Pacific
degree conferred: 1996 (Tri-Council Eco-Research Doctoral Fellowship)

Other Visiting Doctoral Students Working under my Supervision (past):

Thierry Huck (PhD): Visiting from IFREMER, Laboratory of Oceanography (Brest, France)
January, 1995- September, 1996

Geert Lenderink (PhD): Visiting from KNMI (Royal Netherlands Meteorological Institute)
November, 1994 - December, 1994

Postdoctoral/Research Associate Supervision (present):

Dr. Sheng Zhang: Coupled Atmosphere-Ocean-Ice Modelling
July 1, 1995 - to date

Mr. Michael Eby: Ocean-Climate Modelling
September 1, 1995 - to date

Postdoctoral/Research Associate Supervision (past):

Dr. Salil Das: Semi-Lagrangian Advection Algorithms in Ocean Models
August 1, 1992 - December 31, 1994

Dr. Benyang Tang: Simple Models of Air-Sea-Ice Climate Interactions
July 1, 1993 - March 31, 1995

Dr. Tertia Hughes: Ocean-Climate Modelling
January 1, 1995 - December 31, 1995

Dr. Amit Tandon: Decade-to-Century Climate Variability.
(UCAR fellowship for Decadal-Centennial Variability of the Oceanic Thermohaline Circulation -- CSMP Project)
January 1, 1995 - December 31, 1996

Dr. Sophie Valcke: Coupled Atmosphere-Ocean-Ice Modelling
(NSERC Postdoctoral Fellowship)
September 1, 1995 - February, 1997

Computing Research Associate/Assistant Supervision (present):

Daniel Robitaille: Computer Systems Manager
March 1, 1995 - to date

Computing Research Associate/Assistant Supervision (past):

Richard Outerbridge: Computer Systems/Software Manager
July 1, 1992 - December 31, 1995

Nicholas Bakalov: Assistant Computer Systems Operator
February 1, 1994 - August 31, 1995

Magdelina Bakalov: Assistant Computer Systems Operator
February 1, 1994 - November 30, 1995

Undergraduate Summer Students Supervised

Trevor Murdock: NSERC Summer Undergraduate Research Award holder
May 1, 1995 - August 31, 1995

Kevin Bartlett: UVic Physics Coop Student
May 1, 1996 - August 31, 1996

Undergraduate Student Honours thesis/Final Project Supervision (past)

John Campbell Scenario Manager at the Department of Fisheries and Oceans
Certificate Program in Computer Based Information Systems.
certificate awarded: 1996

Edward Wiebe (BSc):The circulation of the Greenland Sea as obtained from hydrographic and ice drift data
degree conferred: 1996

Secretarial/Accountant Supervision (present):

Lucy Aldridge Accountant
May 1, 1994 - to date

Wanda Lewis Secretary
January 1, 1995 - to date

4. Collaborations

I continue to have extensive collaborations with members of the Canadian Centre for Climate Modelling and Analysis with regards to the development of fully coupled Atmosphere/Ocean GCMs for undertaking climate change experiments. On January 28, 1997 I visited Drs. G. Clarke and S. Marshall with Mr. A Fanning to initiate a whole new avenue of investigation within the CSHD. Specifically, we will be incorporating their ice-sheet model into our coupled EMBM-TIM-OGCM to investigate feedbacks within the coupled system and the question concerning the onset of northern hemisphere glaciation (see section 2). The LGM and 6 kyr BP experiments discussed in section 2 will lead to collaboration with all other CSHD groups. Finally, I have begun to investigate mechanisms for collaboration with Dr. A. Bush who recently moved to the University of Alberta from Princeton University. We are both interested in issues regarding the paleoclimate of the Ordovician and the Cretaceous and expect to conduct model intercomparisons. Collaborations with Dr. I. Fung (Carbon Cycle) and C. Barnes (Paleoclimate of the Ordovician) at the University of Victoria are continuing.

Dr. Thomas Stocker visited D. Wright at the Bedford Institute for the month of August, 1996. During this period Drs. Wright and Stocker completed revisions of the paper "Rapid changes in ocean circulation and atmospheric radiocarbon", continued work on the manuscript "Closures used in zonally averaged models" and initiated new work on the stability of the thermohaline circulation. He hopes to have Dr. Stocker visit for a similar period again this coming summer.

Drs. Wright and Brickman also collaborated with Dr. William Hyde on the manuscript "Filtering of Milankovitch forcing by the Thermohaline Circulation".

5. Publication list from January 1994

CSHD 5-1
Stocker, T.S., W.S. Broecker and D.G. Wright, 1994: Carbon Uptake experiments with a zonally-averaged global ocean circulation model. Tellus, 46B, 103-122.

CSHD 5-2
Weaver, A.J. and T.M.C. Hughes, 1994: Rapid interglacial climate fluctuations driven by North Atlantic ocean circulation. Nature, 367, 447-450.

CSHD 5-3
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.

CSHD 5-4
Weaver, A.J., S.M. Aura and P.G. Myers, 1994: Interdecadal variability in a coarse resolution North Atlantic model. Journal of Geophysical Research, 99, 12,423-12,441.

CSHD 5-5
Boyle, E and A.J. Weaver, 1994: Conveying past climates. Nature, 372, 41-42.

CSHD 5-6
Wright, D.G., C.B. Vreugdenhil and T.M.C. Hughes, 1995: Vorticity dynamics and zonally averaged circulation models. Journal of Physical Oceanography, 25, 2141-2154

CSHD 5-7
Tang, B. and A.J. Weaver, 1995: Climate stability as deduced from an idealized coupled atmosphere-ocean model. Climate Dynamics, 11, 141-150.

CSHD 5-8
Weaver, A.J., 1995: Driving the ocean conveyor. Nature, 378, 135-136.

CSHD 5-9
Wohlleben, T.M.H. and A.J. Weaver, 1995: Interdecadal climate variability in the subpolar North Atlantic. Climate Dynamics, 11, 459-467.

CSHD 5-10
Stocker, T.F. and D.G. Wright. 1996. Rapid changes in ocean circulation and atmospheric radiocarbon. Paleoceanography, 11, 773-796.

CSHD 5-11
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.

CSHD 5-12
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.

CSHD 5-13
Wright, D.G. 1997. An equation of state for use in ocean models: Eckart's formula revisited. Journal of Atmospheric and Oceanic Technology, 14, in press.

CSHD 5-14
Fanning, A.F. and A.J. Weaver, 1997: Temporal-geographical meltwater influences on the North Atlantic conveyor: Implications for the Younger Dryas, Paleoceanography, in press.

CSHD 5-15
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, in press.

CSHD 5-16
Wright, D.G., T.F. Stocker and D. Mercer, 1997. Closures used in zonally averaged models. Journal of Physical Oceanography, submitted.

CSHD 5-17
Weaver, A.J. and C. Green, 1997: Global climate change: Lessons from the past -- policy for the future. Ocean and Coastal Management, submitted.

CSHD 5-18
Fanning, A.F. and A.J. Weaver, 1997: A horizontal resolution and parameter sensitivity study of heat transport in an idealized coupled climate model, Journal of Climate, submitted.

CSHD 5-19
Fanning, A.F. and A.J. Weaver, 1997: On the role of flux adjustments in an idealized coupled model. Climate Dynamics, submitted.

CSHD 5-20
Fanning, A.F. and A.J. Weaver, 1997: Thermohaline variability: The effects of horizontal resolution and diffusion. Journal of Climate, submitted.

CSHD 5-21
Valcke, S. and A.J. Weaver, 1997. On the variability of the thermohaline circulation in the GFDL coupled model. Journal of Climate, submitted.

Figure Caption:

Figure 1: Surface air temperature (SAT) difference (^oC) from the coupled EMBM-OGCM-TIM paleoclimatic modelling experiments discussed in Sections 1.1 and 1.2. a) SAT from Younger Dryas experiment minus SAT from present day simulation. Dashed lines indicate a negative temperature difference which implies YD conditions cooler than today. b) SAT from present day experiment minus SAT from open Isthmus of Panama experiment. Solid lines indicate a positive temperature difference which implies the present day conditions are warmer than when the Isthmus was open.

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