An examination of the sensitivity of a GCM to a coupled and uncoupled mode

The Atmospheric Model Intercomparison Project (AMIP) is a project whereby, an Atmospheric General Circulation Model (AGCM) is forced with observed Sea Surface Temperatures (SSTs) and sea ice in the hopes that the model will mimic observed atmospheric behavior. Results from participant modeling groups in AMIP I and II show that these prescribed boundary conditions appear to be sufficient in supplying enough information to an AGCM to allow it to reasonably represent recent atmospheric behavior. Some concerns have been raised about the use of prescribing SSTs in a model integration such as, the specified SST's might alter the spatial and or the temporal variability of the simulation compared to a coupled run and presumably reality. This effect is expected to be most prominent in the mid to high latitudes. Our research herein directly addresses this issue by using the SSTs from the Coupled System Model (CSM) run which is a fully coupled Ocean-Atmosphere General Circulation Model (OAGCM) as boundary conditions to force the identical atmospheric component alone. Quantifiable differences between the two runs will be attributable in part to the use of prescribing SSTs (as a representation of the ocean) versus using a fully coupled (OAGCM) run.

A simple and yet important question that we can quantify and answer is "Does the ocean component in this model act to diminish or enhance atmospheric variability when compared to a prescribed SST run and observations?". This research will highlight variables and regions of greatest magnitude and variability changes.

This work was sponsored in part by the U.S. Department of Energy Environmental Sciences Division and performed by the Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48.