Decadal change in seasonal prediction skills of the Indian Ocean Basin Mode during boreal summer

This study investigates decadal changes in boreal summer Indian Ocean Basin Mode (IOBM) predictability (1948–2022) using the Model-based Analog Forecast (MAF) method, based on a library of 20 CMIP6 models. A pronounced decadal shift is identified, with forecast skill markedly increasing after 1980. This shift is primarily attributable to the decadal modulation of the ENSO–IOBM teleconnection. During the high-skill period, prolonged El Niño events induce significant southwestern Indian Ocean (SWIO) warming. This, in turn, activates a robust wind-evaporation-SST (WES) feedback, which maintains the basin-wide warming into summer, thereby providing an enhanced signal component for IOBM predictions. In contrast, during the low-skill period, weaker ENSO events fail to sustain this feedback, leading to premature termination of IOBM events and consequently lower forecast skill. These findings demonstrate that boreal summer IOBM predictability is nonstationary and reveal that accurately representing the ENSO–IOBM teleconnection is essential for advancing forecast skill.