The neutral October Effect in the lower Mesosphere simulated by different Models

Abstract

The October effect is long known as a sharp decrease in the amplitude of radio waves with Very Low Frequency (VLF) reflected in the D-region (60-90km). However, the mechanism of the October effect is unclear until today. Recent studies show that neutral atmosphere dynamics might cause the October effect. Simultaneously with the October effect in the ionized D-region, there is a warming in the lower mesosphere, which we call the neutral October effect and which cannot be observed in spring, resulting in a spring-fall asymmetry. This spring-fall asymmetry is reproduced by MERRA-2 in years after 2005 only when satellite observations are assimilated in the mesosphere. Other models like WACCM-X, ERA5 and GAIA also have difficulties reproducing this asymmetry. Only CMAM30 can reproduce the neutral October effect. A modelling study and various analysis techniques are used to investigate the mechanism of the neutral October effect in the neutral atmosphere. Based on our results we assume that the onset of the planetary wave activity and westward gravity wave drag after the quiet summer season induces a poleward and downward motion resulting in the observed warming in the lower mesosphere.