Abstract. A lunar tidal model similar to that of Forbes [1982] is used to investigate the effects of changes in background zonal winds on the structure of the lunar tide in the upper atmosphere. It is found that changes in the winds have a greater effect than changes in the background temperature profile with latitude. Observed changes in the structure of the lunar tide from year to year at Saskatoon may be explained by introducing stratospheric warmings into the model. The addition of an associated mesospheric cooling gives an even better fit to the observed phase data. It is also seen that northern hemisphere warmings are predicted to give considerable amplitude changes in southern hemisphere lunar tides. It is expected that much of the difficulty in measuring lunar tides in the upper atmosphere is due to changes in the tidal parameters at these levels brought about by variations in the underlying atmosphere.
[1] The relationship between sudden stratospheric warmings (SSWs) and large-amplitude lunar tides in the equatorial electrojet (EEJ) is studied. Analysis of ground magnetometer data shows that the lunar tide in the EEJ is maximum during the northern winter season except in the Pacific Ocean region. Since SSWs are also a northern winter phenomenon, it is suggested that the relation between the large lunar tide in the EEJ and the SSW may possibly be coincidental. The lunar tide in the geomagnetic variations at Huancayo is anomalously large compared with other EEJ stations. An examination of geomagnetic variations at EEJ stations during SSW events shows that afternoon counter-electrojets are frequently present at new moon and full moon, though the relationship is sometimes broken. The observation of large lunar EEJs when no SSW is present and of various different delay times suggests that other atmospheric processes are likely to be in play.
Days of reversed equatorial electrojet were identified at Trivandrum, India, and, in order to seek a global wind change in the dynamo region responsible for these events, upper atmosphere winds at a height of 99 km at Saskatoon, Canada, were examined. During northern winter, and also in April and September, the reversed electrojets were accompanied by reversals of the eastward wind at Saskatoon. The winter events generally occurred during a stratwarm. No changes in the winds were detected during summer reversed electrojets.
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