Upper ocean temperature change caused by the closing of the Indonesian Seaway from the late Miocene to early Pliocene

The Indo-Pacific warm pool plays a crucial role in regulating heat and water vapor exchange between low and high latitudes. Since the late Miocene epoch, the tectonic evolution of the Indonesian seaway, particularly its gradual closure, has controlled the development of the Indo-Pacific warm pool, leading to altered current patterns between the western Pacific and eastern Indian Oceans. Reconstructed palaeoceanographic records along with numerical simulation experiments have revealed that during the Pliocene period, there was a shift in water source for Indonesian throughflow from high temperature and high salinity South Equatorial Pacific waters to low temperature and low salinity North Equatorial Pacific waters. The closure of the Indonesian Seaway may have shifted the atmospheric convective center from the east Indian Ocean to the West Pacific Ocean, leading to the gradual strengthening of the Western Pacific Warm Pool while reducing surface temperatures and subsurface salinity in the eastern Indian Ocean. The synchronous evolution between the Indonesian Seaway closure and the throughflow not only impacts arid climates in northwest Australia and East Africa but also reduces heat transport towards higher latitudes in the Northern Hemisphere. Previous studies have indicated that changes in meridional heat transport caused by the closure of the Indonesian seaway may contribute to the formation of the Arctic ice sheet; however, further study of the influence of this process and the degree of influence is still weak.

Here, we analyzed the Mg/Ca ratio of surface and subsurface foraminifera shells of ODP (Ocean Drilling Program) sites 807 and 762 in the western Equatorial Pacific and Eastern Indian Ocean, and reconstructed changes in Sea Surface Temperature (SST) and Thermocline Water Temperature (TWT) between 6-3.8Ma. It was observed that ODP site 807 experienced a rise in surface water temperature from 5.2 to 4.9Ma, while ODP site 762 witnessed a drop in surface seawater temperature during this period. Additionally, both sites exhibited a deepening thermocline between 5-4.5Ma. These findings indicate that there was a contraction of the Indonesian seaway during 5.2-4.9Ma, leading to warm water accumulation within the Western Pacific Warm Pool, which subsequently increased surface water temperature in this region while decreasing it in the eastern Indian Ocean, thereby strengthening the Western Pacific Warm Pool. We performed a group of numerical simulation sensitivity experiments on the opening and closing of the Indonesian Seaway. The results showed that when the Indonesian Seaway is closed, the sea surface temperature of the Pacific Ocean and the Indian Ocean will both increase. However, for the subsurface layer, the temperature of the subsurface water in the Pacific Ocean increased, while that of the Indian Ocean decreased. At the same time, the West Pacific Warm Pool strengthening caused by the closure of the Indonesian Seaway was observed clearly.

Keywords： Indonesian Seaway, Indonesian Throughflow, Indo-Pacific Warm Pool, Mg/Ca ratio.