THE RESPONSE OF THE MIDDLE ATMOSPHERE TO LONG-TERM AND SHORT-TERM SOLAR VARIABILITY - A 2-DIMENSIONAL MODEL

A two-dimensional chemical-dynamical-radiative model of the middle atmosphere is used to investigate the potential changes of temperature, ozone, and other chemical constituents in response to variations in the solar ultraviolet flux, associated with the solar rotation (27 days) and the solar cycle (11 years). The model reproduces satisfactorily the response (amplitude and phase) to the 27-day forcing of ozone and temperature in the stratosphere but does not properly explain the ozone and temperature responses of opposite sign observed near 70 km altitude. The change in the ozone column abundance associated with the 27-day solar forcing is estimated to be less than 0.5%. Variations in middle and upper atmospheric ozone concentrations and temperatures induced by solar variations on the 11-year time scale are not negligible compared to changes produced as a result of human activities over the same period of time. The calculated change in the ozone column abundance from solar minimum to solar maximum conditions is of the order of 1.1-1.3% in the tropics and increases with latitude, especially in winter, to reach up to 1.5-1.7% in the polar regions. There are large unexplained differences between the calculated and observed stratospheric responses. For example, in the photochemically controlled region of the upper stratosphere, the model seems to underestimate the ozone response by more than a factor 2. In addition, the negative temperature and ozone responses observed in the lower stratosphere cannot be reproduced by the model. Potential dynamical feedbacks exist, but reliable data sets covering a period longer than one solar cycle have to become available before this problem can adequately be addressed.