Simulation of Seasonal and Interhemispheric Variations in the Stratospheric Circulation

This paper describes the stratosphere as simulated by the time integration of a global model of the atmosphere as developed at the Geophysical Fluid Dynamics Laboratory of NOAA. It is shown that the model is capable of simulating a number of the features of the seasonal variation in the stratosphere. For example, it qualitatively reproduces the seasonal reversals of zonal wind direction in the mid-stratosphere between westerlies in winter and the zonal easterlies prevailing during the summer season. In the mid-latitude region of the lower model stratosphere, zonal mean temperature is highest in the winter when solar radiation is weak. At the cold equatorial tropopause of the model, the seasonal variation of temperature is also quite different from that which would be expected from the seasonal variation of solar radiation. These results are in qualitative agreement with the observed variation. Attempts are made to identify the factors which are responsible for the various aspects of the seasonal variation of the model stratosphere, based upon detailed budget analyses of angular momentum, heat and eddy kinetic energy. It is found that, with the exception of the high-latitude regions, the seasonal variation of temperature in the lower model stratosphere is essentially controlled by dynamical effects rather than by the seasonal variation of local heating due to solar radiation. The stratosphere as simulated by the global model has large interhemispheric asymmetries in the shape of the polar westerly vortex, the magnitudes and the distributions of eddy kinetic energy, and the meridional circulation in the winter hemisphere. Interhemispheric asymmetries in orography are apparently responsible for the interhemispheric differences in the quasi-stationary component of energy flux from the troposphere to the stratosphere of the model, and thus account for many of the asymmetries in the stratospheric circulation. In particular, the simulated stratospheric Aleutian anticyclone is shown to be related to the presence of the strong quasi-stationary tropospheric jet stream off the east coast of Asia. Some of the important shortcomings of the model in simulating the stratosphere include an exaggeration of the magnitudes of the various components of the eddy kinetic energy budget at the top computational level (10 mb) of the model and an overestimation of the intensity of the polar westerly vortex. Also, the model fails to reproduce the mid-winter "sudden stratospheric warming" phenomenon and the quad-biennial wind reversal in the equatorial stratosphere. It is suggested that the performance of the model at the top level suffers from the coarseness in the vertical finite-difference resolution and the lid boundary condition imposed at the top of the model atmosphere.