Extra long period(20-40 day) oscillations in the mesospheric and lower thermospheric winds: observations in Canada, Europe and Japan, and considerations of possible solar influences

An extra long period (20-40 day) oscillation has been identified in the mes ospheric and lower thermospheric (60-100 km) winds observed simultaneously by radars (MF, LF) at four sites from 70 degreesN to 30 degreesN in the nor thern hemisphere during the winter of 1995/1996. A long-term (1980-1999) in vestigation of this oscillation at Saskatoon and Collm is also carried out to obtain climatological and statistical characteristics. Spectral analysis has shown that this oscillation is a common feature of the winter (Novembe r-March) atmosphere, having strong amplitudes throughout the mesosphere (si milar to 10 m/s) and lower thermosphere (similar to5 m/s), and being much s tronger at mid-low latitudes. Although the oscillation has a climatology si milar to the long period normal mode planetary waves (10-16 day), the phase s at the various sites are very similar, and not consistent with a freely p ropagating wave. Comparisons with geomagnetic/solar wind parameters and sol ar W radiation suggest that the oscillation could be related to the short-t erm solar rotation period (ca. 27 days) in some way. However the range of o bserved wind periods is very broad and this raises questions about this int erpretation. Nevertheless the inter-annual variations of this 20-40 day osc illation indicate a weak Ii-year solar cycle correlation in the mesosphere (positive) and the lower thermosphere (negative). Also, the cross-correlati on between the winds and solar radiation shows significant quasi 27-day cor relation and the wind lags behind the solar radiation a few days in the mes osphere. In general it is implied that the atmosphere could react to the so lar activity in an indirect way due to certain dynamical mechanisms. (C) 20 01 Elsevier Science Ltd. All rights reserved.