Comparison of Climate Feedbacks in Atmospheric versus Coupled General Circulation Models

Oregon State University, Corvallis OR

Investigators

Abstract

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). Climate sensitivity is generally studied using two different types of modules. Atmosphere-ocean general circulation models (AOGCMs, or coupled GCMs) include interactive ocean dynamics and detailed heat uptake, but they are computationally expensive to run. Atmospheric GCMs (AGCMs) with mixed-layer oceans cannot fully simulate the ocean's response to and influence on atmospheric changes. However, AGCMs require much less computer time and thus are often used to quantify and understand climate feedbacks and climate sensitivity. No methodical study has verified that climate feedbacks are the same in AOGCMs and AGCMs. Earlier studies, as well as preliminary results included here, suggest that these feedbacks may in fact be different. Intellectual Merit. This research is a systematic comparison of individual climate feedbacks (water vapor, lapse rate, surface albedo, and clouds) between coupled GCMs and atmospheric GCMs with mixed-layer oceans. This work will identify the feedbacks responsible for differences in climate sensitivities between the two types of models and isolate these feedbacks using targeted GCM simulations. Calculating these model feedbacks can be a computationally expensive task, so the new "radiative kernel" technique will be used. First, differences in feedbacks between available AOGCM and AGCM simulations from the Coupled Model Intercomparison Project's multi-model dataset will be quantified. These feedbacks will also be compared to estimates of feedbacks obtained using other techniques as well as satellite observations. Atmospheric and coupled GCM experiments will isolate the oceanic features responsible for the altered feedbacks for a subset of models. Finally, the radiative kernel technique will be extended for use with earth system models to determine how the addition of biogeochemical feedbacks modifies the modeled physical feedbacks. Broader Impacts. This work directly addresses the assumption that feedback behavior in AGCMs can be extended to coupled atmosphere-ocean GCMs. It will determine the situations where AGCMs can be used in place of their corresponding AOGCMs to estimate climate feedbacks and sensitivity. The work will also inform model development. By comparing simulated feedbacks to feedbacks estimated from satellite data, it will identify weakness in current models. In addition, the extension of the technique to earth system models will enable assessment of later model versions. For the models that are studied more thoroughly, the project will provide information about the oceanic processes that influence sensitivity to help set priorities for model development. The ultimate goal of this line of research is a reduction in the uncertainty of the climate sensitivity, so that projections of future climate change can be better constrained. This project includes the training of a graduate student. A significant fraction of the research will be performed by the student, under the direction of the Principal Investigator (PI). Furthermore, the PI will train interested scientists in the use of the kernel technique so that this promising method can become a standard tool for analyzing feedbacks in climate models.