Seasonal Response of the Miocene Indonesian Throughflow to Orbital Forcing Buffered by an Open Seaway

Abstract The Indonesian Throughflow (ITF) is a vital link between the Pacific and Indian Oceans, shaping Indo‐Pacific climate dynamics. While its variability during the Quaternary is well documented, its response to orbital forcing under different tectonic configurations remains unclear. Using climate simulations with minimum and maximum boreal summer insolation, this study investigates how orbital forcing modulates the ITF seasonal cycle. The results show that orbital changes produce distinct two‐phase seasonal ITF responses despite similar annual mean transport (difference within 1.0 Sv) under pre‐industrial (PI) conditions. During the boreal summer insolation maximum, orbital‐induced insolation changes amplify the boreal summer ITF from 16.9 Sv in the PI control condition to 18.0 Sv, while reducing the winter ITF by 3.6 Sv. These responses arise from changes in Pacific‐Indian sea surface height gradients, modulated by wind anomalies and freshwater‐salinity effects on inter‐basin pressure gradients linked to ITCZ migration. In the Miocene context, this two‐phase seasonal ITF response to orbital forcing is notably dampened. A more open Indonesian gateway—particularly the deepened New Guinea–Maluku passage—permits greater South Pacific inflow (∼91% vs. ∼20% in PI) and nearly doubles bidirectional exchange (23.9 Sv vs. 12.4 Sv in PI), with strong seasonal reversal flows. This enhanced connectivity buffers the ITF orbital‐scaled variability by allowing more symmetric and continuous inter‐basin exchange. These findings underscore how orbital forcing and seaway geometry jointly regulate tropical ocean circulation, offering insights for interpreting paleoceanographic records and understanding the Miocene climate system's response to orbital forcing.