Anthropogenic greenhouse forcing severs 400-year Indian-Pacific ocean coupling beyond 19th-century volcanic precedents

The WHOI team reconstructed sea-surface temperature and rainfall variability in both basins back to 1600 using precisely dated paleoclimate archives, then compared the observed covariance with CESM and other last-millennium simulations. Volcanic forcing in the early nineteenth century produced a statistically significant but temporary drop in coupling strength that recovered within decades once aerosols cleared; the post-1980 weakening exceeds that excursion by more than two standard deviations and shows no sign of rebound. Instrumental records alone could not distinguish a natural regime shift from anthropogenic change. The paleo-model synthesis supplies that baseline and demonstrates that the current breakdown coincides with the emergence of a stronger Indian Ocean Basin mode driven by uniform tropical warming rather than Pacific ENSO teleconnections. Continued weakening will degrade seasonal monsoon and cyclone forecasts that still rely on Pacific precursors. If emissions follow SSP2-4.5, models project coupling correlation falling below 0.2 by 2040, forcing operational centers to replace Pacific-based predictors with Indian Ocean-specific variables. Policy-relevant early-warning systems and attribution studies for extreme rainfall events across South Asia and East Africa will therefore require rapid integration of these revised ocean-atmosphere relationships.