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Modern and glacial tropical snowlines controlled by sea surface temperature and atmospheric mixing


During the Last Glacial Maximum, tropical sea surface temperatures were 1 to 3 °C cooler than present1,2,3,4, but the altitude of the snowlines of tropical glaciers5,6 was lower than would be expected in light of these sea surface temperatures. Indeed, both glacial and twentieth-century snowlines seem to require lapse rates that are steeper than a moist adiabat7,8. Here we use estimates of Last Glacial Maximum sea surface temperature in the Indo-Pacific warm pool based on the clumped isotope palaeotemperature proxy in planktonic foraminifera and coccoliths, along with radiative–convective calculations of vertical atmospheric thermal structure, to assess the controls on tropical glacier snowlines. Using extensive new data sets for the region, we demonstrate that mean environmental lapse rates are steeper than moist adiabatic during the recent and glacial. We reconstruct glacial sea surface temperatures 4 to 5 °C cooler than modern. We include modern and glacial sea surface temperatures in calculations of atmospheric convection that account for mixing between rising air and ambient air, and derive tropical glacier snowlines with altitudes consistent with twentieth-century and Last Glacial Maximum reconstructions. Sea surface temperature changes 3 °C are excluded unless glacial relative humidity values were outside the range associated with deep convection in the modern. We conclude that the entrainment of ambient air into rising air masses significantly alters the vertical temperature structure of the troposphere in modern and ancient regions of deep convection. Furthermore, if all glacial tropical temperatures were cooler than previously estimated, it would imply a higher equilibrium climate sensitivity than included in present models9,10.

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Figure 1: Sites studied.
Figure 2: LGM–recent changes in warm pool.
Figure 3: Temperature profiles.
Figure 4: Comparison of simulated changes in regional SSTs with global-scale changes for past and future.


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A.K.T. would like to thank L. Thompson, R. Alley, A. Carlson, F. Anslow, D. Cicerone, D. Lea, N. Meckler, T. Schneider, S. Bordoni, K. Emanuel, J. Adkins, T. Crowley, T. Merlis and H. Spero for discussions; S. Crowhurst, H. Elderfield, A. LeGrande, G. Schmidt and L. Yeung for discussion of this work and comments on an early draft of this manuscript; J. Booth and L. Booth for invaluable assistance with sample preparation; J. Eiler for access to his laboratory and discussion of the work; and T. de Garidel-Thoron for provision of published data from MD97-2138. A.K.T. acknowledges J-Y. Peterschmitt and J. Meyerson for assistance extracting climate model outputs and drafting figures, and the international modelling groups that participated in PMIP2 and CMIP5 for providing their model output for analysis. We thank C. Holloway for the entrainment calculation code. Support was provided to A.K.T. by NSF (CAREER award, EAR-0949191, ARC-1215551), DOE (DE-FG02-13ER16402), the Hellman Foundation, NERC, and the UCLA Division of Physical Sciences; to R.A.E. by NSF (ARC-1215551); and to J.D.N. by NSF (AGS-1102838), which supported S.S. Sounding data for the warm pool are from the DOE Atmospheric Radiation Measurement Climate Research Facility. Support for the NCEP2 (Twentieth Century Reanalysis Project) data set is provided by DOE and NOAA, and for the Reynolds Ocean Temperature Reanalysis Dataset is provided by NOAA. Sediment samples were obtained from the Ocean Drilling Program and CEREGE.

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A.K.T. designed the project and experiments, managed the project, measured most of the samples, guided the modern data analysis, adiabat calculations, initiated the collaboration with modellers, interpreted the results and wrote the manuscript with feedback from all authors. R.A.E. assisted with project design and measured some of the samples. D.P. and S.S. analysed the modern sounding data and conducted the radiative–convective equilibrium calculations under the supervision of J.D.N., J.L.M. and A.K.T. L.B. provided samples from MD97-2138.

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Correspondence to Aradhna K. Tripati.

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Tripati, A., Sahany, S., Pittman, D. et al. Modern and glacial tropical snowlines controlled by sea surface temperature and atmospheric mixing. Nature Geosci 7, 205–209 (2014).

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