Modulation of ENSO on the MJO Forecast

Recent studies suggest that El Niño-Southern Oscillation (ENSO) can influence Madden-Julian Oscillation (MJO) forecasts, though this relationship remains inadequately examined, especially in boreal summer. Here, we address this gap using multiple dynamical models to systematically evaluate MJO forecast skill across seasons, initial phases, and amplitudes. Results show distinct seasonal dependencies. In boreal winter, MJO forecasts are more skillful during La Niña. This is primarily attributed to improved prediction of phase-2 MJO, which is more intense during La Niña. The phase-2 MJO propagates too fast and encounters an exaggerated Maritime Continent barrier during El Niño, reducing forecast accuracy. The superior prediction of initially strong MJO events during La Niña further boosts forecasts skill. Conversely, during boreal summer, El Niño yields better MJO forecasts than La Niña, stemming from models' ability to accurately capture the eastward-propagating, wavenumber-1 structure of the MJO in phases 3–4 under El Niño. During La Niña, however, westward-propagating disturbances dominate in these phases, which models consistently fail to reproduce, thus lowering forecast skill. Furthermore, in La Niña summers, forecast skill declines with increasing MJO amplitude, especially when initialized from phases 1–3. While models adeptly predict the stationary pattern of low-amplitude MJO, they struggle with the continuous eastward propagation of high-amplitude MJO, partly due to the exaggerated representation of dry, westward-moving signals from the central-eastern Pacific. This work enhances understanding of MJO–ENSO interaction dynamics and has practical implications for subseasonal forecasting. • Reveals a new seasonal pattern where La Niña enhances MJO forecast skill during boreal winter, while El Niño improves it during boreal summer. • An improved MJO forecast skill under La Niña winter is due to a better prediction of phase-2 MJO, while it stems from phases 3–4 MJO under El Niño summer. • MJO forecast skill is likely linked to Maritime Continent barrier effect, convective structure and dry convection over the Pacific.