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

Abstract

The boreal summer Indian Ocean Basin Mode (IOBM) profoundly impacts the summer climate over South and East Asia, yet its significant decadal variability poses a major challenge for seasonal predictions. Traditional dynamical prediction models are typically initiated from 1980 onwards due to computational constraints, limiting the comprehensive study of decadal changes in IOBM predictability. To address these limitations, this study investigates decadal changes in boreal summer IOBM predictability (1948–2022) using the Model-based Analog Forecast (MAF) method, based on a library of 29 CMIP6 models. The hindcast results reveal a distinct regime shift in predictive skill. Change-point analysis identifies transitions in prediction skill at 1978 and 2000, based on which three equal-length 21-year periods (1958–1978, 1980–2000, and 2002–2022) are selected to represent the early low-skill, high-skill, and recent low-skill regimes, respectively. This decadal shift is primarily attributable to the decadal modulation of the El Niño-Southern Oscillation (ENSO)–IOBM teleconnection. During the high-skill period, 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 forcing fails to sustain this feedback, leading to weakened IOBM signals 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.