The Indonesian Seas have sustained robust changes in heat and freshwater in recent decades, partially attributed to trends in Indonesian Throughflow (ITF) transport induced by strengthened equatorial Pacific wind forcing as part of the negative Interdecadal Pacific Oscillation (IPO) phase since the 2000s. Yet, sparse observations across the region prior to the late 20^th century limit our understanding of long-term variability of drivers of Maritime Continent and eastern Indian Ocean variability related to changes in oceanic teleconnections via the ITF throughout the 20^th century.

Here we use a series of high-resolution ocean model experiments to investigate variability and change in upper-ocean temperature and salinity structure in the Indonesian Seas and into the eastern Indian Ocean across a range of timescales. Sensitivity experiments provide a mechanistic understanding of drivers and allow us to quantify the relative contributions from buoyancy and wind stress forcing to variability and trends in the temperature and salinity depth-structure across the region since 1960. Preliminary results using coral records as indicators of regional hydroclimate in main ITF pathways in Makassar and Lombok Strait reveal co-variability between proxy reconstructions of sea surface salinity and simulated subsurface heat content and thermocline variations in the model. Notably, the variability in these coral and model responses reveals sensitivity to phase changes in the IPO since the 1960s.

Our findings have important implications for the fate of heat and freshwater transport into the equatorial Indian Ocean and southward via the Leeuwin Current and how it might modulate the leading mode of Indian Ocean variability, the Indian Ocean Dipole, as well as marine heatwave characteristics in the Ningaloo Nino region off Western Australia.