Australia: The Land Where Time Began

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Pliocene El Niņo-like Atmospheric Circulation in the Western US - Stable Isotope Evidence

According to the authors1 it is important to understand the way in which the hydrological cycle has responded to increased global temperatures in the past, in light of the increasing atmospheric CO2 concentrations of the present and into the future. The Pliocene is suggested by the authors1 to be an ideal window into a climate system that has equilibrated with the current atmospheric pCO2. There is evidence that during the Pliocene  the western United States was wetter than at the present, though this observation is at odds with current understanding of warming scenarios predicted for the future involving expansion and migration towards the poles of the subtropical dry zone. In this study the authors1 compared oxygen isotope profiles from the Pliocene of pedogenic carbonates across the western US to modern isotope anomalies in precipitation between phases of the El Niņo-Southern Oscillation (ENSO). Their results indicate that when they accounted for seasonality of the formation of carbonate, isotope changes that occurred through the Late Pliocene match the isotope anomalies that are associated with El Niņo years of the present. The results of this study also suggest that through the Late Pliocene isotopic shifts mirror changes through the Early Pleistocene, the authors1 suggesting this likely represents the migration to the south of the western storm track that resulted from the growth of the Laurentide Ice Sheet. The authors propose that the western storm track migrated to the north through the Late Pliocene, with the development of the cold tongue of the present in the east equatorial Pacific, returning to the south as growth of widespread glaciation in the Northern Hemisphere occurred, and they suggest this scenario is supported by terrestrial climate proxies throughout the US. Background El Niņo-like conditions in western North America during the warm Pliocene, that the authors1 propose, is supported by these data combined. This observation is suggested by the authors1 to have important implications for the future amount and distribution of precipitation in western North America if the Earth behaves a similar way with future warming.

Authors1 conclusions

In this study the authors1 compared the δ18O obtained from carbonates at multiple sites, that were dated to the Plio-Pleistocene, across the western US to δ18O changes in precipitation of the present between phases of the  El Niņo-Southern Oscillation with the aim of testing the hypothesis that reduced gradients of SST in the tropical Pacific result in El Niņo-like conditions, specifically in a Pacific jet that has been shifted to the south. The observed isotopic signals match differences between  El Niņo phases and neutral phases of ENSO of the present, and are consistent with the responses of the Pacific jet that have been predicted to the development of modern SST gradients through the Late Pliocene, in terms of direction, magnitude, and timing. It is suggested by convergence on the δ18O values of the mid-Pliocene during the mid-Pleistocene in pedogenic carbonates at localities covering the interval that circulation conditions that are similar occurred during the 2 time intervals. Mid-Pleistocene circulation, characterised by a deep Aleutian low and a Pacific jet that was shifted to the south, similar to  El Niņo circulation, is currently better constrained. According to the authors1, together, these are seen as strong evidence that conditions in the western US during the Pliocene, that have been reconstructed, were wetter than at present, were actually a product of the background  El Niņo-like temperature structure of the tropical Pacific, in spite of global temperatures that were higher.

It is has been suggested by the authors1 that this finding is also broadly consistent with initial results of PlioMIP ensemble data that were run with PRISM3 boundary conditions, indicating increased precipitation over the western US resulting from the zonal temperature gradient that was reduced across the tropical Pacific (Haywood et al., 2013). The authores1 suggest future research on the nature of topical teleconnections, that are encapsulated in these ensemble data, as well as the use of GCMs that are isotope-enabled, and run with PlioMIP boundary conditions, will allow data-model comparison at unprecedented levels.

The dynamical effects of a background  El Niņo-like tropical Pacific on Pliocene climate (e.g. Barreiro et al., 2006; Shukla et al., 2009; Brierly & Federov, 2010; Vizcaino et al., 2010; Goldner) have been demonstrated by idealised GCM experiments, though the mechanisms that led to this state are still being debated (Federov  et al., 2006, 2010). The authors1 also say identifying these mechanisms and determining how relevant they are to climate change at the present is very important in the management of freshwater resources in the western US, as well as other regions that are affected by atmospheric teleconnections from the tropical Pacific.

Sources & Further reading

  1. Winnick, M. J., Welker, J. M., and Chamberlain, C. P.: Stable isotopic evidence of El Niņo-like atmospheric circulation in the Pliocene western United States, Clim. Past, 9, 903-912, doi:10.5194/cp-9-903-2013, 2013
Author: M. H. Monroe
Last updated: 18/04/2013
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