363: Western Pacific Warm Pool

The sites drilled as part of IODP Expedition 363 in the western Pacific warm pool region of the western equatorial Pacific ocean. Sites are denoted by yellow dots. Figure from IODP 363 Summary.

El-Niño is a phenomenon that occurs in the equatorial Pacific Ocean, causing a short-term shift in  global weather dynamics  around the world. El-Niño involves weakening of the trade winds, which usually push warm surface waters from the eastern equatorial Pacific to the western equatorial Pacific. Such weakening of the trade winds leads to warmer waters to occur in the eastern equatorial Pacific, when there is usually a ‘tongue’ of  cold water at the surface ocean in this region. The rate of change in ocean temperatures, which is the driver of much of the global weather effects associated with El-Niño, have changed alongside global climate dynamics. Every few years, the wind patterns across the equatorial Pacific change, sometimes also shifting to a La Niña phase. The La Niña phase includes strengthening of the trade winds, which leads to even more warm water piling up in the western equatorial Pacific, and cooler waters to appear in the eastern equatorial Pacific. Preliminary studies show that the amount of time between La-Niña and El-Niño conditions are shifting over geologic time. A separate process affected by ocean surface water temperatures in the southeastern Asia region are the Australian Monsoons. These monsoons come off the Indian Ocean and provide rainfall to a significant portion of the northwestern Australian continent. There are two seasons in monsoon climates, a rainy season where moisture laden air moves over land, and a dry season where dry air moves over the oceans. Figuring out how both global and local processes based in the southeast Pacific  are affected by warming oceanic conditions will play a large role in understanding climate change in the future. 

The El-Niño Southern Oscillation pattern in the Pacific Ocean versus the Pacific Ocean in a year under normal conditions.

The best way to study how the climate system interacts with El-Niño conditions is to look at the past. A prime example is the Middle Miocene, a time period of warming occurring 15 million years ago. The warming patterns of the Middle Miocene are similar to warming trends that are seen today, and thus represent a clear example of how carbon dioxide (a greenhouse gas) changes in the atmosphere can influence global sea surface trends. 

Expedition 363 took advantage of the geologic record of sediments in the western equatorial Pacific to reconstruct the La Niña and El Niño phases of the geologic past. The goal of Expedition 363 was to determine how different climate variables changed in the Western Pacific Warm Pool, the pile of warm water in the western equatorial Pacific. Of particular importance was to determine how these changes related to global climate change through the mid-Miocene to the mid-Pleistocene (~15 to 3 million years ago). More specifically, the expedition wanted to determine how  shorter scale climate variability across a millennium. Particularly the study looked at the balance between El-Niño and La- Niña conditions, which can be correlated to previously obtained data on climate and greenhouse gas emissions. Additionally this expedition focused on understanding the history and variables that affect the Australian Monsoon cycle. 

An illustration of La Niña Conditions over the eastern to western equatorial Pacific Ocean. During La Nina conditions, the trade winds strengthen, pushing warm surface waters from the eastern equatorial Pacific to the western equatorial Pacific. Figure from the National Oceanic and Atmospheric Administration.

Sediments recovered during Expedition 363 will allow for a better understanding of the Middle to Late Miocene Periods. Additionally it will allow for a better understanding of the Australian Monsoon system. Geochemical analyses  from this site shows that mixed layer surface ocean temperatures did not cool over time, while subocean temperatures cooled significantly suggesting a change to more overall La-Niña conditions over time. Additionally, maximum greenhouse gas emissions coupled with peak insolation for the Southern Hemisphere provided the shortest Australian Monsoon season. Thus future predictions in a world with increased carbon dioxide levels and warming, would suggest that given rising global temperature a shorter monsoon season would occur.


Rosenthal, A.E. Holbourn, D.K. Kulhanek, I.W. Aiello, T.L. Babila, G. Bayon, L. Beaufort, S.C. Bova, J.-H. Chun, H. Dang, A.J. Drury, T. Dunkley Jones, P.P.B. Eichler, A.G. Fernando, K. Gibson, R.G. Hatfield, D.L. Johnson, Y. Kumagai, T. Li, B.K. Linsley, N. Meinicke, G.S. Mountain, B.N. Opdyke, P.N. Pearson, C.R. Poole, A.C. Ravelo, T. Sagawa, A. Schmitt, J.B. Wurtzel, J. Xu, M. Yamamoto, and Y.G. Zhang. (n.d.). Expedition 363 summary. https://doi.org/10.14379/iodp.proc.363.101.2018

Pei, R., Kuhnt, W., Holbourn, A., Hingst, J., Koppe, M., Schultz, J., Kopetz, P., Zhang, P., & Andersen, N. (2021). Monitoring Australian Monsoon variability over the past four glacial cycles. Palaeogeography, Palaeoclimatology, Palaeoecology, 568, 110280. https://doi.org/10.1016/j.palaeo.2021.110280

Steinthorsdottir, M., Coxall, H. K., De Boer, A. M., Huber, M., Barbolini, N., Bradshaw, C. D., Burls, N. J., Feakins, S. J., Gasson, E., Henderiks, J., Holbourn, A. E., Kiel, S., Kohn, M. J., Knorr, G., Kürschner, W. M., Lear, C. H., Liebrand, D., Lunt, D. J., Mörs, T., … Strömberg, C. A. E. (2021). The miocene: The future of the past. Paleoceanography and Paleoclimatology, 36(4). https://doi.org/10.1029/2020PA004037

Microfossils that were contained within the sediments retrieved during Expedition 363. The chemistry of these fossils’ shells, or tests, can be used to reconstruct ancient ocean conditions. Image from the LIMS database.


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