12 November 2015

Understanding the Indian Ocean Response to Double CO2 Forcing in a Coupled Model

Summary

Worldwide atmospheric concentrations of heat-trapping greenhouse gases have been rising at a relatively uniform rate, but tropical oceans respond to higher carbon dioxide (CO2) levels in different ways. In the tropical Pacific, the sea-surface temperature responds with enhanced equatorial warming compared to the subtropics. However, the tropical Indian Ocean exhibits reduced warming in its eastern equatorial area and increased warming in the western area. Three scientists, including a Department of Energy researcher from the Pacific Northwest National Laboratory, set out to isolate and analyze the formation of this Indian Ocean dipole (IOD) phenomenon using a partial coupling technique in a global climate model by overriding the wind speed or wind stress from certain climate scenarios. The design of the overriding experiments enabled them to isolate atmospheric feedback phenomena such as Bjerknes and WES (wind-evaporation-sea surface temperature) and identify the ocean warming response to double CO2 concentrations without interference. As baselines for the experiments, they used the Community Climate System Model version 3 (CCSM3) to perform control runs either with present-day data or with double CO2. By isolating the effects of the Bjerknes and WES feedbacks, which involve the southeasterly winds along Sumatra, they found an IOD-like pattern, which was most pronounced in July-October. Both feedbacks are clearly essential to the dipolar warming pattern because, when they are removed from the model, leaving only the “static” responses to increasing CO2, the pattern disappears. The scientists also noted that, while these results can be generalized qualitatively to most climate models, the quantitative contributions from these two feedbacks should be treated with caution.

Contact
L. Ruby Leung
University of California at San Diego (UCSD)
Publications
Liu, W, J Lu, and S Xie.  2015.  "Understanding the Indian Ocean response to double CO2."  Ocean Dynamics 65(7), doi:10.1007/s10236-015-0854-6.
Acknowledgments

The authors would like to acknowledge helpful suggestions from Dr. Xiaotong Zheng. WL is supported by NSF AGS-1249145. JL was supported by the Office of Science of the US Department of Energy as part of the Regional and Global Climate Modeling program.