Fig. 2

Zonal wind oscillation (nSV-simulation). Zonal wind envelopes \(\Delta u\) for the 27-day oscillation from the nSV-simulation without short-term solar forcing. The upper two panels show \(\Delta u\) over latitude for summer (a) and winter (b) periods. Strong activity of the 27-day oscillation occurred at mid-latitudes in the UTLS region and in the mesosphere. The lower panels show \(\Delta u\) at 42°N in the mesosphere and in the stratosphere for summer (c) and winter (d) periods. The spectra of mesospheric summer and UTLS winter show clear 27-day signals in the nSV-simulation without short-term solar forcing. The magenta dashed lines refer to the solar 27-day periodicity and its 13.5-day harmonic. Please mind the two different colour codes for the panels