The effect of atmospheric nudging on the stratospheric residual circulation in chemistry–climate models
Authors: Chrysanthou Andreas , Maycock Amanda C. , Chipperfield Martyn P. , Dhomse Sandip , Garny Hella , Kinnison Douglas , Akiyoshi Hideharu , Deushi Makoto , Garcia Rolando R. , Jöckel Patrick , Kirner Oliver , Pitari Giovanni , Plummer David A. , Revell Laura , Rozanov Eugene , Stenke Andrea , Tanaka Taichu Y. , Visioni Daniele , Yamashita Yousuke 
Published: September 2019

Abstract. We perform the first multi-model intercomparison of the impact of nudged
meteorology on the stratospheric residual circulation using hindcast
simulations from the Chemistry–Climate Model Initiative (CCMI). We examine
simulations over the period 1980–2009 from seven models in which the
meteorological fields are nudged towards a reanalysis dataset and compare
these with their equivalent free-running simulations and the reanalyses
themselves. We show that for the current implementations, nudging
meteorology does not constrain the mean strength of the stratospheric
residual circulation and that the inter-model spread is similar, or even larger,
than in the free-running simulations. The nudged models generally show
slightly stronger upwelling in the tropical lower stratosphere compared to
the free-running versions and exhibit marked differences compared to the
directly estimated residual circulation from the reanalysis dataset they are
nudged towards. Downward control calculations applied to the nudged
simulations reveal substantial differences between the climatological lower-stratospheric tropical upward mass flux (TUMF) computed from the modelled
wave forcing and that calculated directly from the residual circulation.
This explicitly shows that nudging decouples the wave forcing and the
residual circulation so that the divergence of the angular momentum flux
due to the mean motion is not balanced by eddy motions, as would typically
be expected in the time mean. Overall, nudging meteorological fields leads
to increased inter-model spread for most of the measures of the mean
climatological stratospheric residual circulation assessed in this study. In
contrast, the nudged simulations show a high degree of consistency in the
inter-annual variability in the TUMF in the lower stratosphere, which is
primarily related to the contribution to variability from the resolved wave
forcing. The more consistent inter-annual variability in TUMF in the nudged
models also compares more closely with the variability found in the
reanalyses, particularly in boreal winter. We apply a multiple linear
regression (MLR) model to separate the drivers of inter-annual and long-term
variations in the simulated TUMF; this explains up to ∼75 %
of the variance in TUMF in the nudged simulations. The MLR model reveals a
statistically significant positive trend in TUMF for most models over the
period 1980–2009. The TUMF trend magnitude is generally larger in the nudged
models compared to their free-running counterparts, but the intermodel range
of trends doubles from around a factor of 2 to a factor of 4 due to nudging.
Furthermore, the nudged models generally do not match the TUMF trends in the
reanalysis they are nudged towards for trends over different periods in the
interval 1980–2009. Hence, we conclude that nudging does not strongly
constrain long-term trends simulated by the chemistry–climate model (CCM) in the
residual circulation. Our findings show that while nudged simulations may,
by construction, produce accurate temperatures and realistic representations
of fast horizontal transport, this is not typically the case for the slower
zonal mean vertical transport in the stratosphere. Consequently, caution is
required when using nudged simulations to interpret the behaviour of
stratospheric tracers that are affected by the residual circulation.