The Causation and Sensitivity of the Northern Winter Planetary Waves

Abstract

We have made high resolution calculations of the wintertime stationary atmospheric response to planetary scale topographic and thermal forcings. A numerical model has been used that solves the spherical primitive equations linearized about observed zonal wind and temperature fields. The model?s lid is placed high enough so that spurious reflections do not affect the results. Considerable attention has been paid to numerical accuracy. The model?s response to realistic forcing is in general agreement with observational analysis of van Loon et al. We find that in middle latitudes the response to topographic forcing strongly dominates the response to thermal forcing. In the midlatitude troposphere, the topographic response is insensitive to changes in the zonal wind. The interannual variability of the tropospheric waves is due more to the heating response. The 500 mb amplitude of the interannual variability due to the sum of both forcings is, as indicated by our model, and as measured at the Himalayan low, less than 40 meters. In the stratosphere, both the variance and mean are dominated by the topographic response. The sensitivity of the topographic response is much greater in the stratosphere than in the troposphere. Refractive index theory is found to adequately explain the variability of the response. In general, sensitivity of the planetary waves to changes in the zonal wind is found to be much smaller than other recent calculations (Lin, Rong-hui and Gambo) have indicated. We believe that the high degree of sensitivity found by other models is spurious and due primarily to insufficient numerical resolution.