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Tightly linked zonal and meridional sea surface temperature gradients over the past five million years

Abstract

The climate of the tropics and surrounding regions is defined by pronounced zonal (east–west) and meridional (equator to mid-latitudes) gradients in sea surface temperature. These gradients control zonal and meridional atmospheric circulations, and thus the Earth’s climate. Global cooling over the past five million years, since the early Pliocene epoch, was accompanied by the gradual strengthening of these temperature gradients. Here we use records from the Atlantic and Pacific oceans, including a new alkenone palaeotemperature record from the South Pacific, to reconstruct changes in zonal and meridional sea surface temperature gradients since the Pliocene, and assess their connection using a comprehensive climate model. We find that the reconstructed zonal and meridional temperature gradients vary coherently over this time frame, showing a one-to-one relationship between their changes. In our model simulations, we systematically reduce the meridional sea surface temperature gradient by modifying the latitudinal distribution of cloud albedo or atmospheric CO2 concentration. The simulated zonal temperature gradient in the equatorial Pacific adjusts proportionally. These experiments and idealized modelling indicate that the meridional temperature gradient controls upper-ocean stratification in the tropics, which in turn controls the zonal gradient along the equator, as well as heat export from the tropical oceans. We conclude that this tight linkage between the two sea surface temperature gradients posits a fundamental constraint on both past and future climates.

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Figure 1: Palaeodata sites superimposed on a modern SST map and the new South Pacific SST record.
Figure 2: Temperature trends.
Figure 3: Results of numerical experiments.
Figure 4: A comparison of the modern and reduced-gradient simulations.

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Acknowledgements

Financial support was provided by grants from the US Department of Energy Office of Science (DE-SC0007037), NSF (AGS-1405272), NSF (OCE-1304366), NOAA (NA14OAR4310277) and the David and Lucile Packard Foundation. The CESM project is supported by the National Science Foundation and the Department of Energy Office of Science. Support from the Yale University Faculty of Arts and Sciences High Performance Computing facility is acknowledged. We thank C. Brierley, S. Hu, C. Ravelo and G. Philander for discussions of this topic and B. Dobbins for help in setting up the CESM simulations. We are indebted to C. Riihimaki for help with making the SST map for Fig. 1 of the paper.

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A.V.F. and N.J.B. contributed equally to the writing and ideas of this manuscript. N.J.B. conducted numerical experiments with CESM and, together with A.V.F., analysed the experimental results. K.T.L. and L.C.P. generated the new SST record at ODP site 1125 and contributed to the writing of the paper.

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Correspondence to Alexey V. Fedorov.

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Fedorov, A., Burls, N., Lawrence, K. et al. Tightly linked zonal and meridional sea surface temperature gradients over the past five million years. Nature Geosci 8, 975–980 (2015). https://doi.org/10.1038/ngeo2577

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