Based on three centennial SST data,this paper investigated the interdecadal variation of the interannual transition from Indian Ocean Dipole (IOD) to Indian Ocean Basin Mode (IOB) by using statistical analysis and interannual signal removal methods.Results show that there is almost no such transition from 1940 to 1970,but it is very significant after 1970.It is found that the interdecadal variation of air-sea coupling between IOD and ENSO is the main reason for this transition.The occurrence and development of IOD and ENSO are independent from 1940 to 1970,but they are closely related after 1970.Through further diagnostic analysis of physical quantity fields,this paper reveals the main dynamic mechanism.Before 1970,the anomalous monsoon circulation over the tropical Indian Ocean could not be coupled with the anti-walker circulation over the tropical Pacific Ocean,and the IOD event cannot be connected with the tropical Pacific when it occurs.On the contrary,after 1970,the coupling effect between the two latitudinal circulation anomalies over the two tropical oceans is strong.When the positive (negative) IOD event occurs,it promotes the development of El Niño (La Niña) through air-sea interaction,and the Indian Ocean will receive the positive feedback from ENSO.Therefore,this “geared-like” coupling model can last until winter and the following spring,and the tropical Indian Ocean is continuously affected by the easterly (westerly) anomaly and guided by the low-level circulation.There is an inflow (outflow) of subsurface warm water in the western Indian Ocean,coupled with the small basin size of the Indian Ocean itself,so the western warm (cold) water area increases significantly,the difference between the east and west SST anomalies rapidly decreases,and it changes to the uniform warming (cold) of the basin,which leads to the occurrence of positive (negative) IOB events in the late winter and spring.