The impact of ocean gateways on ENSO variability in the Miocene
Anna S. Von Der Heydt, Henk A. Dijkstra, 2011. "The impact of ocean gateways on ENSO variability in the Miocene", The SE Asian Gateway: History and Tectonics of the Australia-Asia Collision, R. Hall, M. A. Cottam, M. E. J. Wilson
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The existence of El Niño/Southern Oscillation (ENSO) variability in past climates is still debated. Based on evidence from geological records indicating a different long-term mean climate in the tropical Pacific, a permanent El Niño state has been hyothesized to exist prior to the Plio-Pleistocene transition. However, model studies of past climate and geological records suggest that ENSO variability has existed on Earth as far back as in the Eocene and the Miocene. In the early-to-middle Miocene, climate was not only warmer than today, but oceanic gateways such as the Indonesian Passage and the Central American Seaway established deep connections between the main ocean basins. Here, we analyse the effect of increased levels of atmospheric greenhouse gases and open tropical gateways on the amplitude, period and pattern of ENSO variability using results of fully coupled climate model simulations. While our model shows only small changes in ENSO variability under increased greenhouse gas levels, it suggests a significantly stronger and less frequent ENSO due to altered oceanic gateways. In particular, a deeper and more open Indonesian Passage does not prevent a Western Pacific warm pool from developing, but it allows the warm pool to shift into the Indian Ocean.
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Collision between Australia and SE Asia began in the Early Miocene and reduced the former wide ocean between them to a complex passage which connects the Pacific and Indian Oceans. Today, the Indonesian Throughflow passes through this gateway and plays an important role in global thermohaline flow. The surrounding region contains the maximum global diversity for many marine and terrestrial organisms. Reconstruction of this geologically complex region is essential for understanding its role in oceanic and atmospheric circulation, climate impacts, and the origin of its biodiversity.
The papers in this volume discuss the Palaeozoic to Cenozoic geological background to Australia and SE Asia collision. They provide the background for accounts of the modern Indonesian Throughflow and oceanographic changes since the Neogene, and consider aspects of the region’s climate history.