Coupled feedback between the tropics and subtropics of the Indian Ocean with emphasis on the coupled interaction between IOD and SIOD

Indian Ocean Dipole (IOD) is an air-sea coupled variability in the Tropical Indian Ocean (TIO), which has strong impact on the climate variability over the Indian Ocean rim countries. Though many IODs co-occurred with El Niño/Southern Oscillation (ENSO) in the past, IODs do evolve independently suggesting the possible role of internal dynamics of the Indian Ocean. In this study, the Subtropical IOD (SIOD) is reported as a trigger or a link for the formation of such pure IODs. The study further highlights the existence of a cyclic feedback between IOD and SIOD through tropical subtropical interaction when ENSO forcing is absent/weak. Through this feedback cycle, the positive (negative) SIODs favour positive (negative) IODs and positive (negative) IODs favour negative (positive) SIODs. The internal feedback between IOD and SIOD is found to be responsible for the observed biennial tendency in IOD and SIOD. The mechanisms associated with the feedback cycle and the causative factors are demonstrated in detail. The positive SIOD induced warming over the southwest subtropical Indian Ocean supports a meridional cell favouring the equatorward expansion of the subtropical anticyclone thereby initiating the equatorial easterlies during the pure IOD years. On the other hand, the southeastward extension of western warming associated with the positive phase of IOD initiates warm anomalies in the northeastern subtropics. The upper level divergence, high absolute vorticity gradient and the associated barotropic Rossby wave source in the southern subtropics then generate an equivalent barotropic Rossby wave pattern in the midlatitudes. The cyclonic circulation associated with this Rossby wave train supports the generation of negative SIOD through wind evaporation SST feedback.