Three types of Indian Ocean Basin modes
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The persistence of the Indian Ocean Basin Mode (IOBM) from March to August is important for the prediction of Asian summer monsoon. Based on the observational data and the pre-industrial control run outputs of the Community Climate System Model, version 4 (CCSM4), the IOBM is categorized into three types: the first type can persist until August; the second type transforms from the positive (negative) IOBM into the negative (positive) Indian Ocean Dipole Mode (IODM), accompanied by the El Niño-to-La Niña (La Niña-to-El Niño) transition in the boreal summer; the third type transforms from the positive (negative) IOBM into the positive (negative) IODM in early summer. It is discovered that aside from the influence of anomalous Walker Circulation resulted from the phase transition of ENSO, the persistence of Australia high anomaly (AHA) over the southeastern tropical Indian Ocean (TIO) and the west of Australia from March to May is favorable for the positive (negative) IOBM transformation into the positive (negative) IODM in the boreal summer. The stronger equatorially asymmetric sea surface temperature anomalies (SSTAs) in the boreal spring are the main mechanism for the persistence of IOBM, because the asymmetric atmospheric responses to the stronger equatorially asymmetric SSTAs in the TIO confine the AHA to the east of Australia from May to August. This result indicates a possibility of predicting summer atmospheric circulation based on the equatorial symmetry of SSTAs in the TIO in spring.
KeywordsIndian Ocean Basin mode Indian Ocean Dipole mode Persistence Australia high anomaly ENSO
We acknowledge the World Climate Research Programme’s Working Group on Coupled Modeling. We thank the climate modeling groups for producing and making their model outputs available. We also thank the NCEP Reanalysis data provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA. We appreciate Prof. Yu’s technical editing. We thank the anonymous reviewers for their constructive comments and suggestions. This work is supported by the National Basic Research Program of China (2012CB955603), the Natural Science Foundation of China (41406001, 41476003 and 41490641), the China Meteorological Public Welfare Scientific Research Project (GYHY201306027 and GYHY201506001), and Shandong Joint Fund for Marine Science Research Centers (Grant No. U1406401).