Seasonal cycle of the climatological stationary waves in the NCEP-NCAR reanalysis

The maintenance mechanisms of the climatological stationary waves and their seasonal cycle are investigated with a linear stationary wave model and the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis data from 1985 to 1993, The stationary wave model is linearized about the zonal-mean flow and subjected to the zonally asymmetric stationary wave forcings. It has rhomboidal wavenumber 30 truncation and 14 vertical sigma levels. The forcings for the linear model include diabetic heating, orography, stationary nonlinearity, and transient vorticity and heat Aux convergences. The NCEP-NCAR reanalysis provides a high quality global dataset for this study. When the linear model is subjected to all forcings, it reproduces reasonably well the climatological stationary wave seasonal cycle. The linear stationary wave theory is quantitatively valid at the upper-tropospheric levels for all months and the lower-tropospheric levels for the northern summer months (with pattern correlation greater than 0.8). At the middle- and lower-tropospheric levels for most of the months, the stationary wave theory is qualitatively valid (with pattern correlation greater than 0.5). The effect and relative importance of each individual forcing mechanism in maintaining the stationary waves and their seasonal-cycle are determined by the linear model. Within the linear model framework, the global diabatic heating is found to be the most dominant forcing mechanism for the climatological stationary waves throughout the seasonal cycle. Subsequently, the seasonal cycle of the stationary waves is largely caused by the seasonal fluctuations of the atmospheric heating field. By comparison, the linear effect of orography is of less importance in both the Tropics and the extratropics. The effect of stationary nonlinearity is to modify the spatial structure of the stationary waves, particularly over extratropical North America. Comparatively, transient forcing has little contribution. By separating the tropical and the extratropical hearings in the linear model, it is found that the local thermal forcing has the dominant contribution to the local stationary wave seasonal cycle. The relative contribution of the seasonally varying zonal-mean basic state and the seasonally varying forcing fields is also examined using the linear model. The seasonally varying zonal-mean basic state can account for the zonal-mean amplitude fluctuation of the stationary waves in the Tropics, as well as the seasonal change of the stationary wave spatial structure from September to May. It fails to capture the amplitude fluctuation of the Northern Hemisphere extratropical stationary waves and the northern summer stationary wave spatial structure. On the other hand, the effect of the seasonally varying forcing accounts largely for the zonal-mean amplitude fluctuation of the stationary waves in the Northern Hemisphere extratropics, as well as the transition to the northern summer stationary wave regime.