The effect of atmospheric nudging on the stratospheric residual circulation in chemistry–climate models

• Published in: Atmospheric chemistry and physics 2019-09, Vol.19 (17), p.11559-11586
• Main 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
• Format: Article
• Language: English
• Subjects: Analysis; Angular momentum; Angular momentum flux; Annual variations; Atmospheric circulation; Atmospheric models; Chemistry; Circulation; Climate; Climate cycles; Climate models; Climatology; Computer simulation; Datasets; Divergence; Environmental assessment; Fields; Interannual variability; Intercomparison; Long-term changes; Lower stratosphere; Mass flux; Meteorology; Middle atmosphere; Momentum; Momentum flux; Momentum transfer; Ocean circulation; Organic chemistry; Regression analysis; Regression models; Residual circulation; Running; Simulation; Statistical analysis; Stratosphere; Stratospheric circulation; Tracers; Transport; Trends; Tropical climate; Upwelling; Variability; Vertical advection
• ISSN: 1680-7324; 1680-7316; 1680-7324
• DOI: 10.5194/acp-19-11559-2019

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