Increased heat transport of the Indonesian Throughflow linked to the spin-up of Southern-Hemisphere circulation gyres

Climate models project a weakening of the Indonesian Throughflow (ITF) under greenhouse warming. Here, using in-situ observations, we reveal an increase in the upper-700 m ITF heat transport into the Indian Ocean (IO) from 0.31 ± 0.05 PW during 1984–1997 to 0.57 ± 0.04 PW (1 PW = 1015 Watts) during 1998–2020. This strengthening, accompanied by a compensating weakening below, reflects baroclinic oceanic adjustments to an enhanced Pacific Walker Circulation and intensified southeasterly winds over the southern IO, associated with increased La Niña events and a positive phase of the Southern Annular Mode. The increased heat transport is primarily sourced from the Lifamatola Passage and secondly from the Makassar Strait. Furthermore, the ITF strengthening is linked to the spin-up of Southern-Hemisphere circulation gyres. These results highlight the dominance of internal variability in the ITF over recent decades, implying that human-induced changes have yet to emerge in this vital current. The Indonesian Throughflow’s heat transport into the Indian Ocean has increased as a result of shifting winds related to internal variability since 1998, revealed by observations and ocean model experiments.