Uncertainty in Indian Ocean Dipole response to global warming: the role of internal variability
The Indian Ocean Dipole (IOD) is one of the leading modes of interannual sea surface temperature (SST) variability in the tropical Indian Ocean (TIO). The response of IOD to global warming is quite uncertain in climate model projections. In this study, the uncertainty in IOD change under global warming, especially that resulting from internal variability, is investigated based on the community earth system model large ensemble (CESM-LE). For the IOD amplitude change, the inter-member uncertainty in CESM-LE is about 50% of the intermodel uncertainty in the phase 5 of the coupled model intercomparison project (CMIP5) multimodel ensemble, indicating the important role of internal variability in IOD future projection. In CESM-LE, both the ensemble mean and spread in mean SST warming show a zonal positive IOD-like (pIOD-like) pattern in the TIO. This pIOD-like mean warming regulates ocean-atmospheric feedbacks of the interannual IOD mode, and weakens the skewness of the interannual variability. However, as the changes in oceanic and atmospheric feedbacks counteract each other, the inter-member variability in IOD amplitude change is not correlated with that of the mean state change. Instead, the ensemble spread in IOD amplitude change is correlated with that in ENSO amplitude change in CESM-LE, reflecting the close inter-basin relationship between the tropical Pacific and Indian Ocean in this model.
We would like to acknowledge helpful suggestions from two anonymous reviewers. We acknowledge the CESM-LE project for providing model outputs, which may be obtained from http://www.cesm.ucar.edu/projects/community-projects/LENS/data-sets.html. We also acknowledge the World Climate Research Programme’s Working Group on Coupled Modelling, which is responsible for CMIP, and we thank the climate modeling groups (listed in Table 1 of this paper) for producing and making available their model output. We thank Qinyu Liu and Shang-Ping Xie for helpful discussions. This work was supported by the National Basic Research Program of China (2012CB955600 and 2015CB954300), the National Natural Science Foundation of China (41476003), NSFC-Shandong Joint Fund for Marine Science Research Centers (U1406401), and the China Meteorological Public Welfare Scientific Research Project (GYHY201306027).
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