Stratospheric annular modes and stratosphere-troposphere coupling in CMIP5 models

Our study assesses how well CMIP5 models represent the annular modes of variability in the stratospheric polar vortex, including their vertical and temporal behavior and attendant stratosphere-troposphere dynamical coupling. Models are categorized in terms of their representation of the stratosphere (i.e., high top vs. low top models). Stratospheric annular modes are identified using the methods of Black and McDaniel (2009), which isolates the stratospheric northern annular mode (SNAM; associated with variations in the polar vortex strength) and polar annular mode (PAM; associated with polar vortex latitudinal position), respectively. Stratosphere-troposphere coupling is assessed via the study of the composite structural evolution, tropospheric power spectrum analysis and Eliassen-Palm fluxes. Tropospheric manifestations are assessed in terms of sea level pressure patterns, jet stream and storm track measures and regional surface air temperature.

To first order PAM and SNAM structures are both successfully replicated by CMIP5 models. SNAM variability is typically less persistent and more Gaussian in the CMIP5 models than in observations. We find that, although high top models provide a more accurate representation of the stratospheric zonal wind anomalies, there is relatively little distinction between high and low top models in the magnitude and structure of the associated zonal wind anomalies at tropospheric levels. This suggests a greater tropospheric sensitivity to stratospheric variability for low top models. Composite and dynamical analyses are presented to further illustrate the differences among high top and low top models. This includes an analysis of regional manifestations in surface air temperature and storm track behavior.

Black, R.X. and B.A. McDaniel, 2009: Sub-monthly polar vortex variability and stratosphere-troposphere coupling in the Arctic. Journal of Climate, 22, 5886-5901.