Summary
This study aims to assess the ability of climate models to reproduce observed stratosphere-troposphere coupling. Coupling between the stratosphere and troposphere is examined via the Southern and Northern Annular Modes (SAM and NAM). These modes, associated with the shifting of atmospheric mass between the poles and mid-latitudes, are the principal modes of atmospheric extra-tropical variability in their respective hemispheres. The modes have various effects on regional climate. For example, a positive SAM, representing atmospheric mass shifting towards mid-latitudes, is associated with anomalously strong westerlies over the Southern Ocean and a southward shift of the storm track. Baldwin and Dunkerton [2001] and Thompson et al. [2005] show that strong or weak NAM or SAM events, in the stratosphere are usually followed by an annular-mode anomaly of the same sign in the troposphere that can last for around 60 days. Such results show that the stratosphere can play an important role in extended-range weather prediction. It is important for climate models to adequately reproduce the observed stratosphere-troposphere coupling in order to predict changes in regional climate based on changes in the stratosphere (such as the creation and recovery of the ozone hole). The models analyzed in this study include an atmosphere-ocean-chemistry coupled climate model with a well resolved stratosphere and comprehensive stratospheric chemistry (NIWA-UKCA) as well as models participating in the Coupled Model Intercomparison Project 5 (CMIP5) which handle changes in ozone with varying degrees of complexity. The accuracy of the stratosphere-troposphere coupling is assessed relative to the ERA-Interim reanalyses. Comparison of the different models as well as comparison of the NIWA-UKCA sensitivity simulations with and without changes in ozone depleting substance emissions, and with or without changes in greenhouse gases, allow us to investigate the influence of ozone and other greenhouse gases on changes in the annular modes. Stratospheric Harbingers of Anomalous Weather Regimes, Baldwin, M. P., Dunkerton, T. J., Science, 294, 581-584, 2001 Stratosphere-Troposphere Coupling in the Southern Hemisphere, Thompson, D. W. J., Baldwin, M. P., Solomon, S., J. Atmos. Sci., 62, 708-715, 2005
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