Contrasting Impacts of the Indian Ocean Dipole and ENSO on the Tropospheric Biennial Oscillation

There is a biennial tendency for the interannual variabilities of the Asian summer monsoon (ASM) referred to as the tropospheric biennial oscillation (TBO). Previous studies have emphasized that land-atmosphere-ocean interactions in association with the El Nino/Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) trigger a phase shift of the TBO from one year to another. However, simple lead-lag correlations among the ASM, IOD and ENSO indices reveal that there is a relatively stronger signal of the phase shift of the ASM over two years than one year. Meanwhile, the IOD has been shown to contribute to the TBO by triggering the phase shift of the ENSO by the following winter, although correlations of the ENSO do not identify such a biennial tendency. Through composite analyses, IOD-ENSO coupled events are shown to induce the phase shift of the ENSO within a year, which results in a distinctive TBO pattern of the ASM in the following two years. In contrast, ENSO events without the IOD cannot induce the phase shift, and there is no TBO pattern in the following two years. Thus, IOD-ENSO coupled events and independent ENSO events have contrasting roles in inducing different (two-year and four-year) frequencies of the ASM interannual variability in subsequent years.