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.
Highlights
Stratospheric ozone loss has been halted by a concerted international effort to eliminate emissions of ozone-depleting substances under the Montreal Protocol (WMO, 2018)
We examine one of the models and nudging schemes employed in Ball et al (2018): the Community Earth System Model-Whole Atmosphere Community Climate Model (CESM (WACCM))
We found that UT nudging does not constrain the meridional circulation and found that VT nudging is too similar to UVT nudging to warrant further investigation
Summary
Stratospheric ozone loss has been halted by a concerted international effort to eliminate emissions of ozone-depleting substances under the Montreal Protocol (WMO, 2018). While ozone is recovering in the upper stratosphere, there has been some indication of a decline in the tropical lower stratosphere since the late 1990s when ozone-depleting substance emissions peaked (Ball et al, 2018). Ozone in this region is strongly mediated by the vertical advection of ozone-scarce tropospheric air by the residual circulation – the wave-driven, thermally indirect overturning circulation of the stratosphere (Butchart, 2014, and references therein) – as well as eddy mixing of ozone-rich air from the extratropical lower stratosphere (Abalos et al, 2013).
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