The low-latitude Indo-Pacific climate system is shaped by orbital forcing and tectonic changes in ocean-gateway geometry, yet their combined influence on regional hydroclimate and inter-basin exchange remains poorly understood. Using a fully coupled climate model, we compare responses to maximum and minimum boreal summer insolation under pre-industrial and Miocene boundary conditions, focusing on the tropical hydrological cycle and Indonesian Throughflow (ITF).
The two orbital configurations produce broadly opposing climate responses, but the overall temperature response is weaker under the warm Miocene background state. An important regional exception occurs over tropical North Africa, where high boreal summer insolation induces stronger cooling in the Miocene because intensified monsoon rainfall and associated moisture feedbacks outweigh direct radiative warming. This spatially heterogeneous response demonstrates that background climate strongly modulates the expression of orbital forcing.
ITF sensitivity is further regulated by ocean-gateway geometry. Under pre-industrial conditions, high boreal summer insolation strengthens the summer ITF from 16.9 to 18.0 Sv while weakening winter transport by 3.6 Sv, with little change in annual-mean transport. This seasonally opposing response is driven by changes in the Pacific–Indian sea-surface-height gradient associated with wind anomalies, ITCZ migration, and freshwater-induced salinity contrasts. Under Miocene conditions, however, the orbital response is substantially dampened. A wider and deeper Indonesian gateway, particularly the New Guinea–Maluku passage, increases the South Pacific contribution to approximately 91% and nearly doubles bidirectional exchange from 12.4 to 23.9 Sv. This enhanced and more continuous inter-basin exchange buffers orbital-scale ITF variability.
Together, these results demonstrate that background climate state and gateway geometry jointly govern how orbital forcing is transmitted through the low-latitude ocean–atmosphere system, highlighting the importance of tectonic boundary conditions for interpreting orbital variability in Indo-Pacific paleoceanographic records.

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