A comprehensive assessment of tropical stratospheric upwelling in the specified dynamics Community Earth System Model 1.2.2 – Whole Atmosphere Community Climate Model (CESM (WACCM))
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Published:2020-02-24
Issue:2
Volume:13
Page:717-734
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ISSN:1991-9603
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Container-title:Geoscientific Model Development
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language:en
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Short-container-title:Geosci. Model Dev.
Author:
Davis Nicholas A.ORCID, Davis Sean M.ORCID, Portmann Robert W.ORCID, Ray Eric, Rosenlof Karen H.ORCID, Yu PengfeiORCID
Abstract
Abstract. Specified dynamics (SD) schemes relax the circulation in climate models toward a reference meteorology to simulate historical variability. These
simulations are widely used to isolate the dynamical contributions to variability and trends in trace gas species. However, it is not clear if
trends in the stratospheric overturning circulation are properly reproduced by SD schemes. This study assesses numerous SD schemes and modeling
choices in the Community Earth System Model (CESM) Whole Atmosphere Chemistry Climate Model (WACCM) to determine a set of best practices for
reproducing interannual variability and trends in tropical stratospheric upwelling estimated by reanalyses. Nudging toward the reanalysis
meteorology as is typically done in SD simulations does not accurately reproduce lower-stratospheric upwelling trends present in the underlying
reanalysis. In contrast, nudging to anomalies from the climatological winds or anomalies from the zonal-mean winds and temperatures better
reproduces trends in lower-stratospheric upwelling, possibly because these schemes do not disrupt WACCM's climatology. None of the schemes
substantially alter the structure of upwelling trends – instead, they make the trends more or less AMIP-like. An SD scheme's performance in
simulating the acceleration of the shallow branch of the mean meridional circulation from 1980 to 2017 hinges on its ability to simulate the downward
shift of subtropical lower-stratospheric wave momentum forcing. Key to this is not nudging the zonal-mean temperature field. Gravity wave
momentum forcing, which drives a substantial fraction of the upwelling in WACCM, cannot be constrained by nudging and presents an upper limit on the
performance of these schemes.
Publisher
Copernicus GmbH
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