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. Specified dynamics (SD) schemes relax the circulation in climate models toward a reference meteorology to simulate historical variability. Thesesimulations are widely used to isolate the dynamical contributions to variability and trends in trace gas species. However, it is not clear iftrends in the stratospheric overturning circulation are properly reproduced by SD schemes. This study assesses numerous SD schemes and modelingchoices in the Community Earth System Model (CESM) Whole Atmosphere Chemistry Climate Model (WACCM) to determine a set of best practices forreproducing interannual variability and trends in tropical stratospheric upwelling estimated by reanalyses. Nudging toward the reanalysismeteorology as is typically done in SD simulations does not accurately reproduce lower-stratospheric upwelling trends present in the underlyingreanalysis. In contrast, nudging to anomalies from the climatological winds or anomalies from the zonal-mean winds and temperatures betterreproduces trends in lower-stratospheric upwelling, possibly because these schemes do not disrupt WACCM's climatology. None of the schemessubstantially alter the structure of upwelling trends – instead, they make the trends more or less AMIP-like. An SD scheme's performance insimulating the acceleration of the shallow branch of the mean meridional circulation from 1980 to 2017 hinges on its ability to simulate the downwardshift of subtropical lower-stratospheric wave momentum forcing. Key to this is not nudging the zonal-mean temperature field. Gravity wavemomentum forcing, which drives a substantial fraction of the upwelling in WACCM, cannot be constrained by nudging and presents an upper limit on theperformance of these schemes.

geoscientific model development

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))