Change from Basin Mode to Dipole Mode in Tropical Indian Ocean

Based on the observational data, the Coupled Model Inter-comparison Project Phase 5 (CMIP5) model outputs (pre-industrial control run) of Community Climate System Model, version 4 (CCSM4) and the historical simulations of 17 CMIP5 models, two kinds of possible change from Indian Ocean Basin Mode (IOBM) to Indian Ocean Dipole Mode (IODM) are revealed during May-July. Some IOBM can maintain into late boreal summer or early boreal autumn. But some positive (negative) IOBM can switch to negative (positive) IODM by Walker Circulation anomaly as response to ENSO’s phase transforming during May-July. It also discovered that some positive (negative) phase of IOBM can also switch to positive (negative) phase of IODM, when sea surface pressure anomaly appears in Northwest of Australia during May-July. When positive IOBM phase, if the Australia Cold High extends northwestwards (South Asian Monsoon anomaly), there is southeasterly anomalies in southeast Equator Indian Ocean, which is benefit for the occurrence of the positive phase of IODM during May-July. Then we classify the IOBM into three types: the first type of IOBM sustains till Sep; the second type of IOBM can out-phase change into the IODM, the last one is in-phase change from IOBM into the IODM.