Zonal prevailing wind
It has been investigated several times, whether the mesosphere/lower thermosphere winds show a connection with the 11-year solar cycle (e.g., Jacobi, 1998). It has been foun that, while a clear connection is not seen in winter, during summer the zopnal prevailing wind is negatively (this means that the mesospheric easterlies/the thermospheric westerlies) are stronger/weaker during solar maximum. In the following this connection is investigated in some more detail.
In Figure 1 the time series at 95 km altitude are shown. It can be seen that, whatever months of the spring/summer season are chosen, there is a positive trend during the period under consideration. This trend is supposed to belong to a shrinking of the mesosphere due to long-term cooling, which leads to a decent of the lower thermosphere westerly jet and, at a given height, to an increase of the westerly wind.
It can also be see from Figure 1, that a possible solar cycle dependence is preferrably found in spring (lowermost panel). This can also be seen from the spectra shown in Figure 2. This Figure on the left-hand side shows the detrended time series, and the corresponding spectra on the right-hand side. For the spring months, a clear 11-year peak is visible, which is weaker, and accompanied by a 5-6 year oscillation in summer.
Conclusions
Model calculations have shown that a possible response of the middle atmosphere on varying solar activity consist of an increase of the mesospheric jets, with the effect decreasing with height. This is exactly what is seen in the Collm winds. During spring, generally easterly winds are measured even at mesopause region heights, while in summer the lower thermospheric westerlies reach down below 95 km. Therefore the mesospheric 11-year cycle is still seen in spring, but not any more in summer. However, this conclusion bases upon data from roughly only 2 solar cycles. More measurements are necessary to confirm these.
References
Jacobi, Ch., 1998: On
the solar cycle dependence of winds and planetary waves as seen from midlatitude
D1 LF mesopause region wind measurements. Ann.
Geophysicae 16, 1534 - 1543.
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Last modification: 27.7.2001