SOLAR-CYCLE CHANGES TO PLANETARY WAVE-PROPAGATION AND THEIR INFLUENCEON THE MIDDLE ATMOSPHERE CIRCULATION

Recent observations suggest that there may be a causal relationship be tween solar activity and the strength of the winter Northern Hemispher e circulation in the stratosphere. A three-dimensional model of the at mosphere between 10-140 km was developed to assess the influence of so lar minimum and solar maximum conditions on the propagation of planeta ry waves and the subsequent changes to the circulation of the stratosp here. Ultraviolet heating in the middle atmosphere was kept constant i n order to emphasise the importance of non-linear dynamical coupling. A realistic thermosphere was achieved by relaxing the upper layers to the MSIS-90 empirical temperature model. In the summer hemisphere, str ong radiative damping prevents significant dynamical coupling from tak ing place. Within the dynamically controlled winter hemisphere, small perturbations are reinforced over long periods of time, resulting in s ystematic changes to the stratospheric circulation. The winter vortex was significantly weakened during solar maximum and western phase of t he quasi-biennial oscillation, in accordance with reported 30 mb geopo tential height and total ozone measurements.