Comparison of SAMIL and BCC_AGCM Simulations of the Polar Vortex Oscillation in the Northern Hemisphere Winter

By using AMIP simulation results for 1950-1999 from SAMIL and BCC_AGCM, the performance of the models in simulating the winter stratospheric circulation and polar vortex oscillation is assessed by comparing their results with the NCEP data. Both models can reproduce the general patterns of the winter climatology and the leading oscillation mode of the winter stratospheric circulation. However, the meridional temperature gradients between the tropics and the extratropics simulated by two models, as well as the polar vortex and polar jet, are much stronger on average than those in the NCEP data. Harmonic analysis of the winter climatology geopotential height from 100 hPa to 20 hPa indicates that planetary waves in both models are significantly weaker than in the NCEP data. A one-month seasonal drift exists in the seasonal evolution of the 10 hPa polar jet in both models compared with the NCEP data, and the simulated subtropical westerly jet at 200 hPa is weaker. The first empirical orthogonal function modes of the zonal-mean zonal wind anomalies from both models reflect oscillations in the intensity of the stratospheric polar vortex, whereas the occurrence of polar vortex oscillation events is more frequent in BCC_AGCM than in SAMIL. The dominant period of the polar vortex oscillation events is 5.5 months in SAMIL, 4.8 months in BCC_AGCM, and 4.6 months in the NCEP data. Furthermore, polar vortex oscillation events occur mainly in February-March in SAMIL and in February-April in BCC_AGCM, whereas they occur from December to March in the NCEP data.