Modelling of long-term trends in the middle and upper atmosphere.

From the 1950s/1960s to the present, long-term trends in temperature, density and winds in the middle and upper atmosphere, and in the height of the peak of the ionospheric F2 layer (hmF2) and its critical frequency (foF2) have been observed. These trends are usually attributed to increases in CO2 concentration that have occurred over the same time span, which cause a cooling and contraction of the middle and upper atmosphere. However, modelling studies generally predict smaller trends in temperature, larger trends in density, and more globally uniform trends in hmF2 and foF2 due to changes in CO2 concentration than have been observed. When additional changes in ozone concentration are accounted for, modelling results are in better agreement with observations, but so far this had only been studied up to ~150-200 km. Here we used the Coupled Middle Atmosphere and Thermosphere model version 2 to study the combined effects of changes in CO2 and ozone concentration on the middle and upper atmosphere, including the ionosphere, from ~15-300 km. It was confirmed that changes in ozone concentration affect trends in temperature and density substantially until 200 km, and also effects above 200 km and on hmF2 were found. The results depended on the gravity wave parameterization used by the model, showing that dynamical factors can influence long-term trends. The Thermosphere-Ionosphere-Electrodynamics General Circulation Model was used to model the effects of changes in the Earth’s magnetic field on hmF2 and foF2. Substantial trends were found over South America and the Atlantic Ocean, while other parts of the world were little affected. Sensitivity analyses showed that the responses obtained with both models depend on geophysical conditions such as season and solar and geomagnetic activity. In general it can be concluded that long-term trends are probably caused by multiple coupled radiative and dynamical processes.