Polar Vortex Variability in the CMIP5 Models

Variability in the wintertime stratospheric polar vortex is dominated by annular modes that have important influence on tropospheric climate and surface weather patterns. This variability is dominated by variations in the strength of the polar vortex and is represented by the stratospheric Northern Annular Mode (SNAM). The second mode, designated the Polar Annular Mode (PAM), represents variability in the North-South location of the vortex. The ability of state-of-the-art climate models to accurately represent these modes is crucial in accurately simulating variability in the stratosphere and stratosphere-troposphere interactions. To assess the veracity of stratospheric simulations in current models, a suite of runs from different members of the CMIP5 experiment are analyzed. The climatological winter stratosphere for each model is calculated and the strength and location of the polar vortex are compared with observations. Principal component analyses of daily stratospheric zonally averaged zonal winds from the historical scenario are then employed with the first principal component characterizing the SNAM and the second representing the PAM. For each model, the latitudinal structure of the spatial pattern and the relative variance explained for each mode are calculated and compared to observations. In addition, long-term trends in the principal component timeseries and preferred timescales of each mode and model are examined. Finally, models are binned into "high-top" and "low-top" categories based on the degree of resolution of the stratosphere. High-top and low-top averages are calculated to test for biases in the ability to accurately simulate observed stratospheric variability among models with different vertical resolutions.