How ENSO modifies the Boreal summer intraseasonal oscillation (BSISO) in the Asian monsoon region

The monsoon intraseasonal oscillation (ISO), marked by alternating active and break phases, plays a crucial role in modulating water resources and high-impact weather events in the tropics. The tropical ISO comprises of two distinct seasonal modes: the Madden-Julian Oscillation (MJO), which is active during boreal winter (December to February), and the boreal summer intraseasonal oscillation (BSISO), which dominates during boreal summer (May to October). While the dependence of the MJO on interannual variations associated with the El Niño-Southern Oscillation (ENSO) has received considerable attention, the corresponding influence of ENSO phases on the BSISO remains poorly understood. Mechanisms controlling the BSISO may be made more complex since it operates on a sheared mean state arising from the monsoon. In this study, we investigate the nonlinear interaction between ENSO and the BSISO, focusing on how the slowly varying, seasonally persistent ENSO signal modulates the background mean state through which the BSISO propagates. Using 43 years (1979–2021) of observational and reanalysis data during the summer monsoon period (June-September), we examine how the frequency, amplitude, phase speed, and spatial extent of BSISO-related convection vary between El Niño and La Niña years by performing simple compositing and statistical analysis. Results reveal the following notable features: (1) Overall, El Niño years support a greater number of active BSISO days than La Niña years. (2) El Niño years tend to produce zonally extended stronger BSISO convection anomalies over the west and central Pacific (during BSISO phase 6), whereas La Niña years form a more conducive environment for convective activity over the Indian Ocean basin (in phase 3). (3) The northward propagation of the BSISO is stronger during El Niño than La Niña, both over the Bay of Bengal and the western North Pacific. The findings are statistically robust based on Welch’s t-test and bootstrapping. To investigate the physical mechanisms, we analyse the meridional structures of key atmospheric variables and conduct vorticity budget analyses for each phase of BSISO under El Niño and La Niña conditions to assess how ENSO induced changes in the background mean state influence the vertical shear mechanism governing BSISO propagation. The findings in this study potentially pave the way for conditional forecasts of BSISO based on ENSO mean state.