What Causes the Tropical Atlantic SST Bias in Coupled General Circulation Models?

PI: Sang-Ki Lee

External PIs: Benjamin P. Kirtman (RSMAS/UM)

Despite our growing recognition for the important role of tropical Atlantic atmosphere-ocean processes on climate variability, almost all of the state-of-the-art atmosphere-ocean coupled climate models cannot reproduce the annual cycle of tropical Atlantic SSTs. Due to this shortcoming in the climate models, currently we do not have a skill to simulate the tropical Atlantic climate variability, especially in the regions of equatorial Atlantic and Atlantic warm pool .

The overall goal of this project funded by NSF is to identify processes and/or parameterizations in the coupled models that are responsible for generating tropical Atlantic SST biases. With this goal in mind, we will analyze the mixed layer heat budget of a major coupled model to identify the heat budget terms that contribute significantly to the tropical Atlantic SST bias. One of our primary interests is to examine the possible role of local ocean dynamics on the SST bias in three key regions, namely the southeastern Atlantic Ocean, the eastern and central equatorial Atlantic Ocean, and the tropical North Atlantic Ocean. We will also determine and quantify the external influences and to understand if and how local atmosphere-ocean feedback processes amplify the remote signals. These questions will be addressed by carefully designing numerical model experiments using the NCAR community climate system model version 3 (CCSM3).

Figure 1. The tropical Atlantic SST bias (simulated - observed) of IPCC models during boreal spring (upper panels) and summer (lower panels). The left column is from the ensemble average of IPCC models and the right column is from CCSM3. The total of twelve IPCC models are used to construct the ensemble mean. All IPCC model data are obtained from “the climate of 20th century” scenario. ERSST2 for the same period (1870-1999) is used for the observation.


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