CCSM Ocean Model Working Group Report
25 June 2003
The agenda had to repeat much of the March working group meeting, so discussions have continued among the newly formed "executive" group. Significant progress was made on setting priorities for the near, intermediate and long term future of the ocean component of CCSM, but a consensus has not yet been reached on all issues. In addition, the meeting began the process of defining the science and development activities for each of these timescales. The "executive" has responded to a number of immediate action items.
1. Its comments on a first draft have been included in this working group report.
2. Suggested speakers at next years CCSM meeting in Santa Fe, NM, have been forwarded to the SSC.
3. The consensus is to hold the next working group meeting, mid to late January 2004, perhaps in conjunction or overlapping with a joint CCSM/GFDL Ocean Collaboration meeting. It is tentatively being scheduled for the week of January 13, with the next week (January 20) in reserve, so that there is minimal conflict with the Ocean Sciences Meeting the week of January 27.
4. Suggested names from the oceanographic community to join the SSC have been forwarded to the SSC.
5. There is almost no support for committing to another Special Issue of J. Climate dedicated to the next (IPCC) model release. At best this was deemed premature until model solutions were available.
A. WORKING GROUP BUSINESS
The agenda was largely an abbreviated version of the March working group meeting. This was necessitated by the small attendance in March due to the snowstorm, but left little time for discussion. The discussions continue among an "executive" group of about twenty participants interested in engaging in the high frequency (week-to-week) business of the working group. To facilitate these activities an email alias (ccsm-oceanexec) has been created. The first task of this group was to review the first draft of this report. Most comments have been incorporated, but differences remain on several issues, and these will need to be worked out at the next OMWG meeting. The executive has also provided input on the above Action Items that stem mainly from the Monday SSC and Tuesday joint SSC and CAB meetings.
B. WORKING GROUP PRIORITIES
- Near term (< 3 months)
First, an ocean model (nominal 1 degree horizontal resolution) suitable for use in IPCC scenarios needs to be finalized. The final configuration is to be run on both parallel and vector computer architectures. The recent "improvements" over the CCSM2 ocean (deeper mixing by KPP, new double diffusion, temporally (monthly) and spatially dependent solar absorption based on chlorophyll, and diurnal cycle of solar heat flux) have combined to turn the equatorial cold bias into a warm bias, which has increased equatorial precipitation to the point where much too fresh equatorial surface waters are formed. This has a particularly negative effect on the Equatorial Under Current, which then crosses the Pacific at about 100m.
Second, the momentum gained at the Breckenridge meeting, both in the working group meeting and in outside discussions for collaborative ocean modeling project efforts between CCSM and GFDL needs to be sustained. In particular, the development of an agreed common ocean forcing and experimental protocol needs to be concluded.
The ocean climate science associated with these near term priorities are the reproduction of the observed ocean variability over the past several decades. There is also an expressed desire to examine how robust model sensitivities are over a range of ocean models.
- Intermediate (within the next three years)
Further discussion leading to prioritizing these will be a focus of the next working group meeting.
The version of the ocean model put forward for IPCC needs to be more fully understood than some previous versions. Not unrelated is the sense that this model must remain essentially unchanged in its physics for the intermediate future. This would not preclude development of alternative model versions with the same physics. The solutions of the stand-alone ocean model and of a coupled ocean-sea ice configuration, which are required in addition to fully coupled results, have not yet been agreed upon.
Develop and maintain a low cost version (~ 3 degree horizontal resolution and 25-30 levels) of the model. This follows up the work presented by Steve Yeager suggesting that it may be possible to overcome the collapse of the Atlantic Meridional Overturning Circulation (AMOC) as shown in Thomas Stocker's plenary presentation. Thomas will be directly involved in this development and has set a requirement for a maintained AMOC greater than 15 Sv. He is planning to oversee distribution of the model to users without access to supercomputers. The ocean biogeochemists are also interested in such a model version, particularly equilibrium solutions and more realistic interannual variability in tropical SST.
Develop and maintain an ocean model version whose equilibrium time scale is commensurate with that of the atmosphere (~ 10 years). CCSM has been challenged "to raise the standard" of climate sensitivity experiments, and such a model offers the opportunity to do so by giving an alternative to "Slab Ocean Models." There is not yet agreement on which approach to follow in setting up such a model.
Initiate a series of fully coupled climate integrations designed to address specific ocean science questions. These will be collective integrations, distinguished from individual research efforts by requiring extensive collaboration within the working group, and/or contributions from other working groups in the configuration of component models other than the ocean. These experiments will require a substantial increment in the ocean working group's CSL production allocation.
Respond to the CCSM Science Plan that calls for the capability to embed high-resolution sectors within the coarser resolution CCSM ocean. These sectors are to be coupled to high-resolution atmospheres embedded within the CCSM atmosphere. The plan is largely driven by biogeochemistry, but an application along eastern ocean boundaries would address the regions with the largest and most persistent SST biases in CCSM.
Verify that POP2.0 is not answer changing relative to CCSM POP, and then complete the transition to POP2.0.
Continue to develop and explore solutions of global ocean models in the eddy-resolving regime. This work will lead to a refinement of our understanding of their sensitivity to subgrid scale closure assumptions and other model choices, minimize systematic biases, and support scientific research on the role of ocean variability in the climate system, all in preparation for coupled climate experiments with eddy resolving ocean components on a longer timetable.
- Long term (within 3 to 4 years).
Perform a meaningful (~100 year) fully coupled climate integration with an eddy resolving ocean component. This effort will build on the above exploration of the eddy-resolving regime.
Explore the possibilities of utilizing data assimilation techniques within CCSM.
W. Brechner Owens