Intra-seasonal oscillations observed in the MLT region above UK (52°N, 2°W) and ESRANGE (68°N, 21°E)

Analyses of intra-seasonal oscillations (i.e., oscillations with periods of similar to30-100 days) in the MLT region made by meteor radars at middle (UK, five years of data) and high latitudes (Esrange, three years of data) are presented in this work. There is clear evidence of similar to75-day variability in the mean winds. This intra-seasonal oscillation appears to be a prominent feature of the mean zonal circulation and was observed quite regularly in the time intervals December 1996-September 2001 over UK and October 1999-December 2002 over Esrange. The intra-seasonal oscillations observed over Esrange are coherent in both amplitude and phase over the height range of similar to80-100 km covered by the radar. They are especially strong in the range 90-97 km, weaker near 87-88 km and stronger again near 81 km. At these lower heights the intra-seasonal oscillations are usually of longer period (similar to80 days) than, and are out of phase with, the oscillations at the upper heights. We also note that this oscillation is a common feature in the meridional winds over the UK. It is present, but intermittent, in the meridional winds over Esrange.