Revisiting the Relationship among Metrics of Tropical Expansion

D. W. Waugh Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland
School of Mathematics, University of New South Wales, Sydney, New South Wales, Australia

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K. M. Grise Department of Environmental Sciences, University of Virginia, Charlottesville, Virginia

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W. J. M. Seviour Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland

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S. M. Davis NOAA/Earth System Research Laboratory, Boulder, Colorado
Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, Colorado

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N. Davis NOAA/Earth System Research Laboratory, Boulder, Colorado
Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, Colorado

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O. Adam Hebrew University of Jerusalem, Jerusalem, Israel

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S.-W. Son School of Earth and Environmental Sciences, Seoul National University, Seoul, South Korea

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I. R. Simpson Climate and Global Dynamics Laboratory, National Center for Atmospheric Research, Boulder, Colorado

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P. W. Staten Department of Earth and Atmospheric Sciences, Indiana University Bloomington, Bloomington, Indiana

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A. C. Maycock School of Earth and Environment, University of Leeds, Leeds, United Kingdom

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C. C. Ummenhofer Department of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts

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T. Birner Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado

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A. Ming British Antarctic Survey, Cambridge, United Kingdom

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Abstract

There is mounting evidence that the width of the tropics has increased over the last few decades, but there are large differences in reported expansion rates. This is, likely, in part due to the wide variety of metrics that have been used to define the tropical width. Here we perform a systematic investigation into the relationship among nine metrics of the zonal-mean tropical width using preindustrial control and abrupt quadrupling of CO2 simulations from a suite of coupled climate models. It is shown that the latitudes of the edge of the Hadley cell, the midlatitude eddy-driven jet, the edge of the subtropical dry zones, and the Southern Hemisphere subtropical high covary interannually and exhibit similar long-term responses to a quadrupling of CO2. However, metrics based on the outgoing longwave radiation, the position of the subtropical jet, the break in the tropopause, and the Northern Hemisphere subtropical high have very weak covariations with the above metrics and/or respond differently to increases in CO2 and thus are not good indicators of the expansion of the Hadley cell or subtropical dry zone. The differing variability and responses to increases in CO2 among metrics highlights that care is needed when choosing metrics for studies of the width of the tropics and that it is important to make sure the metric used is appropriate for the specific phenomena and impacts being examined.

Supplemental information related to this paper is available at the Journals Online website: https://doi.org/10.1175/JCLI-D-18-0108.s1.

Current affiliation: Meteorological Institute, Ludwig-Maximilians-University, Munich, Germany.

© 2018 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Darryn Waugh, waugh@jhu.edu

Abstract

There is mounting evidence that the width of the tropics has increased over the last few decades, but there are large differences in reported expansion rates. This is, likely, in part due to the wide variety of metrics that have been used to define the tropical width. Here we perform a systematic investigation into the relationship among nine metrics of the zonal-mean tropical width using preindustrial control and abrupt quadrupling of CO2 simulations from a suite of coupled climate models. It is shown that the latitudes of the edge of the Hadley cell, the midlatitude eddy-driven jet, the edge of the subtropical dry zones, and the Southern Hemisphere subtropical high covary interannually and exhibit similar long-term responses to a quadrupling of CO2. However, metrics based on the outgoing longwave radiation, the position of the subtropical jet, the break in the tropopause, and the Northern Hemisphere subtropical high have very weak covariations with the above metrics and/or respond differently to increases in CO2 and thus are not good indicators of the expansion of the Hadley cell or subtropical dry zone. The differing variability and responses to increases in CO2 among metrics highlights that care is needed when choosing metrics for studies of the width of the tropics and that it is important to make sure the metric used is appropriate for the specific phenomena and impacts being examined.

Supplemental information related to this paper is available at the Journals Online website: https://doi.org/10.1175/JCLI-D-18-0108.s1.

Current affiliation: Meteorological Institute, Ludwig-Maximilians-University, Munich, Germany.

© 2018 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Darryn Waugh, waugh@jhu.edu

Supplementary Materials

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