A new activity, "Coordinated Coupled Model Experiments", was approved at the Working Group on Coupled Models meeting at the Hadley Centre in February, 2002, and is being coordinated by CMIP. These are not intended to be standard CMIP integrations, but specific experiments conceived by an individual or group of individuals who wish to invite other modeling groups to participate on a voluntary basis. Thus, this activity is a collaboration among modeling groups rather than a centrally organized intercomparison project. It is expected that each group will mainly concentrate on analyzing results from its own model. However, collaborative exchange of results and conclusions, and shared participation in the writing of papers describing results, should help gain a better understanding of why the responses of the various models differ. We therefore welcome participation by the modeling group you represent.

Standard sensitivity tests for the response of the Atlantic meridional overturning to surface forcing

So far the following groups have committed to participate in this initial coordinated experiment, using AOGCMs or EMICs: CCCma (CGCM2), DKRZ (ECHAM5(T42)/C-HOPE), GFDL, Hadley Centre (HadCM3), Louvain (ECBILT-CLIO), MRI (MRI-CGCM2.2), NCAR, PIK (CLIMBER), UVic. Concerning timetable and deadlines, the organizers view this effort as a process and not an event. The groups above are already running or are planning on running shortly the experiments described below. If your group wants to participate in the first paper, then you should be in the analysis phase by early 2003 when the organizers plan to compare the results and begin the process of publishing a paper.

If you or someone in your group is interested in joining this project, please contact Jonathan Gregory (jonathan.gregory@metoffice.com), Ron Stouffer (rjs@gfdl.noaa.gov) and Andrew Weaver (weaver@uvic.ca), with a copy to Jerry Meehl (meehl@ncar.ucar.edu) and Curt Covey (covey1@llnl.gov) as soon as possible. ISpecific questions should be addressed to Jonathan, Ron or Andrew. Description of the experiment and updates on status will be posted on this web page. Suggested standard diagnostics to produce once the experiments are completed, as a starting point for discussion, are given below.

The experiments are:

1. Response to time-dependent climate change on the century timescale

   Starting from a control state, (a) CO2 increasing at 1% per year compounded for 140 years (up to 4xCO2), and (b) a corresponding control run. This is an extended CMIP2 experiment. Daily surface water fluxes will be saved from each run, and two further runs will be then done: (c) like (a), but with the daily water fluxes from (b) used instead of those generated by the model, and (d) like (b) with the daily water fluxes from (a). We refer to (c) and (d) as "partially coupled experiments". Note that the surface water fluxes include the runoff from land into the ocean i.e. P-E+R. This design was developed by Dixon et al., 1999, "The influence of transient surface fluxes on North Atlantic overturning in a coupled GCM climate change experiment", Geophys. Res. Lett., 26, 2749-2752, and by Mikolajewicz and Voss, 2000, "The role of the individual air-sea flux components in CO2-induced changes of the ocean's circulation and climate", Clim. Dyn, 16, 627-642. As shown by these papers, the GFDL and ECHAM3/LSG models differ in the relative importance of changes in surface water fluxes and surface heat fluxes (caused by raising CO2), and it will be useful to extend this analysis to other models. The groups have made different choices regarding whether the surface water flux in sea-ice regions is taken to be at the surface of the ice (i.e. the P-E) or at the underside of the ice (in which case it also includes ice melting and freezing).

   Diagnostics: A plot like Fig 1 of Dixon et al. or Fig 5 of Mikolajewicz and Voss.

   A further aim is to find a diagnostic of the density field which can be used as a predictor of the overturning strength, such as the steric height gradient identified by Hughes and Weaver, 1994, "Multiple equilibria of an asymmetric two-basin model", J. Phys. Oceanogr., 24, 619-637. If possible it is recommended to include diagnostics for the rate of change of temperature and salinity in the ocean due to advection, diffusion and vertical mixing, at least on decadal timescales. These can then be used to relate changes in the density field back to the surface flux changes, as by Thorpe et al., 2001, "Mechanisms determining the Atlantic thermohaline circulation response to greenhouse gas forcing in a non-flux-adjusted coupled climate model", J. Climate, 14, 3102--3116.

   Diagnostics: A plot like Fig 13 of Hughes and Weaver or Fig 3 of Thorpe et al. As a first step towards performing an analysis like that of Thorpe et al., it is necessary to find an appropriate diagnostic of the density field.

2. Sensitivity to surface water flux forcing

   This experiment aims to establish a benchmark for the sensitivity to an imposed surface freshwater flux. The design is to apply a surface flux of 0.1 Sv in total, uniformly distributed over the Atlantic between 50N and 70N, for a period of 100 years, starting from a control state. This additional flux is a net addition of freshwater to the ocean; it is not compensated for by removal elsewhere. The "Atlantic" may include parts of the Labrador Sea and North Sea within the latitude band; groups have made different decisions about this. After 100 years, the imposed water flux will be switched off, and the experiment continued to run, so that any recovery can also be investigated. It could be valuable to try this run starting from an LGM state also, if one is available.

   Diagnostics: Timeseries of maximum of the Atlantic overturning streamfunction, maps of surface air temperature differences and of differences in surface fluxes (heat, freshwater and windstress).

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