Since April 1998, I have been the senior NSERC/MARTEC/MSC Industrial Research Chair in "Regional Ocean Modelling and Prediction". Dr. Jinyu Sheng is the junior chair. Our chair is sponsored jointly by the Meteorological Service of Canada (MSC) and MARTEC, a Halifax company.
The North Atlantic Oscillation (NAO)
The NAO is a measure of the strength of the westerly winds blowing across the North Atlantic Ocean. It is the most important mode of variability in the northern hemisphere atmospheric circulation, is closely related to the "Arctic Oscillation", and influences winter surface air temperature deep into the Eurasian continent and over North America (see Greatbatch, 2000, for a review). The NAO is fundamentally an internal mode of variability in the atmospheric circulation, the different phases of which appear to be self-sustained by the eddy transfer of momentum associated with synoptic storms. Topics that I am interested in are the relationship between (i) atmospheric blocking events and the NAO, (ii) quasistationary regimes of the atmospheric circulation and the NAO, and (iii) events such as stratospheric sudden warmings and the NAO. In addition, Dr. Alex Medvedev will work on the predictability of the NAO.
In recent decades, the NAO index has shown a strong upward trend, associated with stronger westerly winds than in the past, and closely related to the observed warming over the northern hemisphere land masses. Concurrent with the upward trend, there has also been an eastward shift in the centres of action of the NAO. My Ph.D. student, Jian Lu is studying these changes as part of his Ph.D. thesis work. One possibility is that the recent changes in the NAO are associated with anthropogenic climate change.
The NAO and the North Atlantic Ocean
Dr. Carsten Eden is working with me on this topic. It is well established that the NAO is an important forcing for the North Atlantic Ocean. For his Ph.D., Dr. Eden used models of the North Atlantic Ocean to identify different responses of the ocean. On the time scale of months to years, a boundary adjustment plays an important role, whereas on interdecadal time scales, changes in the large-scale circulation were shown to be important for the evolution of observed sea-surface temperature anomalies. The robustness of the boundary adjustment as the resolution of the model is increased remains a concern. A related issue is the time lag between the forcing from the atmosphere and changes in the strength of the thermohaline overturning circulation. If the NAO is being driven to a higher index state by anthropogenic forcing, how will this effect the circulation of the North Atlantic Ocean, and what is the time lag between the changing forcing and the ocean response?
Predictability in the coupled ocean/atmosphere system
Although there is no doubt that the atmosphere drives the North Atlantic Ocean, the influence of the ocean on the overlying atmospheric circulation is less clear. Dr. K. Andrew Peterson is working on a coupled ocean/atmosphere model developed with colleagues at McGill University. The model will be used to study the NAO within the coupled system, coupled ocean/atmosphere predictability, and the influence of southern hemisphere winds on the North Atlantic circulation.
With help from Dr. Alex Medvedev, we plan to couple the GEM model, used by MSC for atmospheric weather prediction, to models of the North Atlantic Ocean. One application is to study "extratropical transition" which occurs when hurricanes move north towards Atlantic Canada and transform into extratropical storms. These storms can be very dangerous. In both 1999 and 2000, the passage of a hurricane over the Grand Banks of Newfoundland led to large amplitude changes in sea level along the east coast of Newfoundland and serious damage in a number of harbours. The atmospheric dynamics of "extratropical transition" has similarities to that of "bomb" storms responsible for severe weather in Atlantic Canada in the winter months, a topic we also plan to study. In both cases, interaction with the ocean plays an important role. A Ph.D. student has been accepted to work with myself and Dr. Hal Ritchie on this topic.
Dr. Sheng Zhang is working with myself and Dr. Jinyu Sheng on modelling sea-ice over the Labrador Sea and the Newfoundland/Labrador shelf. Dr. Zhang has succeeded in implementing a fully coupled ice/ocean model.
Improving the performance of ocean circulation models
A problem with models of the North Atlantic is that the Gulf Stream is usually too far to the north, resulting in contamination of the model solution, and the incorrect representation of the air/sea fluxes. We are working on a novel technique to overcome this difficulty called the "semi-prognostic" method (Sheng, Greatbatch and Wright, 2001). The approach is to add forcing terms to the momentum equations of an ocean model using climatological density data, while keeping the heat, freshwater and other tracer budgets unconstrained. The approach is particularly suited to studies of the uptake of tracers by the ocean (e.g. carbon) and air/sea interaction studies such as described above. The method is currently being extended to the whole North Atlantic Ocean.
Understanding the Boussinesq approximation and parameterizing transport by mesoscale eddies in ocean models
In April 1999, I attended a meeting in London, UK, with the aim of preparing for the GRACE satellite mission (Hughes et al., 2000, EOS). GRACE is expected to measure bottom pressure variability in the ocean at unprecedented accuracy, leading to a demand for ocean circulation models that relax the Boussinesq approximation. A method for modifying existing Boussinesq code to make it fully non-Boussinesq is described in Greatbatch et al. (2001). Some 9 years ago now, McDougall and Garrett (1982) raised concern that the Boussinesq tracer equation carried by models could be in error by up to 100% compared to the diapycnal mixing term associated with water mass conversion. The paper by McDougall, Greatbatch and Lu (2001; MGL) solves this "conundrum" and shows that by making a suitable interpretation of the velocity variable carried by Boussinesq ocean models, these models are actually a lot more accurate than had previously been thought. I am also interested in how mesoscale eddies transport tracers in the ocean and in ways to parameterize this transport. Dr. Peterson and I have a paper in press that analyses output from some idealised numerical experiments to test ideas on this important issue, and Greatbatch (2001) combines ideas about mesoscale eddy parameterization with the MGL solution to the Boussinesq conundrum.
Work on the NAO is being carried out through the Climate Variability Group led by Prof. Jacques Derome at McGill University. My group regularly interacts with Prof. Derome, Prof. Charles Lin and Dr. Hai Lin at McGill. A new project under the CLIVAR umbrella seeks ways to parameterize internal gravity waves in ocean circulation models and is a collaborative venture being led by Dr. David Straub at McGill University and Dr. Kevin Lamb at The University of Waterloo. I also continue to work with Dr. Tom Delworth (GFDL, Princeton University, USA) on North Atlantic climate variability.
Work on atmosphere/ocean prediction, "extratropical transition" and "bomb" storms, is being done in collaboration with Dr. Hal Ritchie, Dr. Gilbert Brunet and Jim Abraham of the MSC. Dr. Sheng and I are also collaborating with Doug Mercer from the MSC in Gander, NF, on the ocean response to hurricanes and other severe storms that track across the Grand Banks. Dr. Ritchie, Dr. Brunet and Prof. Derome are also involved in the work on quasistationary regimes and the influence of the stratosphere on the NAO. We are also collaborating with Dr. John Fyfe (CCCma, Victoria) and Dr. Lionel Pandolfo (UBC) through the CLIVAR project.
Work on the Boussinesq approximation has been done in a very productive collaboration with Dr. Youyu Lu (now at Scripps) and Dr. Trevor McDougall from CSIRO, Australia. I also collaborate with Dr. John Dukowicz and Dr. Balu Nadiga from Los Alamos National Laboratory in the USA on mesoscale eddy parameterization problems.
I interact extensively with Dr. Jinyu Sheng through our joint work on our Industrial Research Chair. I also continue to have links with my colleagues Dr. Keith Thompson and Dr. Dan Wright.
POSTDOCTORAL FELLOWS
Dr. Carsten Eden: Ph.D. from the Institut f|r Meereskunde an der Universitdt Kiel, Germany.
GRADUATE STUDENTS
Jian Lu: Jian is a Ph.D. student in the Atmospheric Science Program. His project is described under the heading "The North Atlantic Oscillation", above.
RESEARCH STAFF
Dr. Drew Peterson, Research Associate.
Dr. Alex Medvedev, Research Associate (joint with Dr. Ritchie).
Dr. Sheng Zhang, Research Associate (joint with Dr. Sheng).
Mike Casey, Research Assistant (computer specialist, joint with Dr. Sheng)
David Jackson, System Administrator (shared among a group of faculty)
Jackie Hurst, Research Assistant (shared among a group of faculty)
RESEARCH FUNDING
My research is funded by NSERC through their Research Grant and Industrial Research Chair programs; by our industrial sponsors, MSC and MARTEC; and by the Canadian Institute for Climate Studies (CICS). Funding from CICS is to be replaced by joint funding from NSERC and the Canadian Foundation for Climate and Atmospheric Sciences (CFCAS) to support the Canadian contribution to international CLIVAR (CLImate VARiability).