Cocos (Keeling) corals reveal 200 years of multidecadal modulation of southeast Indian Ocean hydrology by Indonesian throughflow

The only low latitude pathway of heat and salt from the Pacific Ocean to the Indian Ocean,known as Indonesian Throughflow (ITF), has been suggested to modulate Global Mean SurfaceTemperature (GMST) warming through redistribution of surface Pacific Ocean heat. ITF observations are onlyavailable since ~1990s, and thus, its multidecadal variability on longer time scales has remained elusive.Here we present a 200 year bimonthly record of geochemical parameters (δ18O-Sr/Ca) measured on Cocos(Keeling) corals tracking sea surface temperature (SST; Sr/Ca) and sea surface salinity (SSS; seawater-δ18O═δ18Osw) in the southeastern tropical Indian Ocean (SETIO). Our results show that SETIO SSS and δ18Oswwere impacted by ITF transport over the past 60 years, and therefore, reconstructions of Cocos δ18Osw holdinformation on past ITF variability on longer time spans. Over the past 200 years ITF leakage into SETIO isdominated by the interannual climate modes of the Pacific Ocean (El Niño—Southern Oscillation) andIndian Ocean (Indian Ocean Dipole). Pacific decadal climate variability (represented by the Pacific DecadalOscillation) significantly impacted ITF strength over the past 200 years determining the spatiotemporal SSTand SSS advection into the Indian Ocean on multidecadal time scales. A comparison of our SETIO δ18Oswrecord to GMST shows that ITF transport varied in synchrony with global warming rate, being predominantlyhigh/low during GMST warming slowdown/acceleration, respectively. This hints toward an important rolefor the ITF in global warming rate modulation