Projected Shifts in Monsoon Onset Timing Under High-Emission Climate Scenarios

Abstract

The timing of monsoon onset governs the planting calendar for hundreds of millions of smallholder farmers across
South Asia, yet how onset timing itself is projected to shift under high-emission warming, as distinct from projected
changes in total seasonal rainfall, has received comparatively little systematic attention across the current
generation of climate models. Here we analysed onset-date projections for the South Asian summer monsoon from
27 CMIP6 models under the SSP5-8.5 high-emission scenario, applying a standardised rainfall-threshold onset
detection algorithm consistently across historical (1985-2014) and future (2071-2100) simulation periods for each
model, and evaluated model skill against satellite-derived precipitation onset dates over the historical period before
pooling the ensemble. The multi-model ensemble projected a mean delay in monsoon onset of 6.4 days by the end
of the century relative to the historical baseline, with substantial inter-model spread (interquartile range 2.1 to 11.3
days) that was only weakly correlated with each model's projected change in total seasonal rainfall, indicating that
onset delay and seasonal rainfall change are governed by partially distinct physical mechanisms rather than being
two facets of the same response. Onset delay was most pronounced and most consistent across models over the
western and central Indian subcontinent, while onset dates over the eastern Bay of Bengal sector showed
comparatively little projected change and, in a minority of models, a modest advance. Models with a documented
warm sea surface temperature bias in the equatorial Indian Ocean projected systematically larger onset delays than
models without this bias, suggesting that inter-model spread in onset-date projections is partly attributable to a
specific, identifiable and model-evaluable source of ocean-atmosphere coupling error rather than being irreducible
structural uncertainty. These results indicate that agricultural planning frameworks keyed to historical monsoon
onset dates will need region-specific revision, particularly over the western and central subcontinent, and that
narrowing onset-delay projection uncertainty is tractable through targeted evaluation and correction of equatorial
Indian Ocean sea surface temperature bias in the contributing models.