MF radar observations of seasonal variability of semidiurnal motions in the mesosphere at high northern and southern latitudes

The semidiurnal tide (SDT) is investigated through comparative analysis of horizontal winds measured at Poker Flat (65degreesN, 147degreesW), Andenes (69degreesN, 16degreesE), Davis (69degreesS, 78degreesE), and Rothera (68degreesS, 69degreesW). At the northern hemisphere sites the SDT maximizes around the autumn equinox. Poker Flat and Andenes results from 1999-2001 are used to demonstrate that there is a clear repeatable enhancement in SDT amplitudes around the autumn equinox, and that the maximum is localized in height around 86 km. In the southern hemisphere seasonal dependence of the SDT during 1997-1998 is more complicated, and the autumn enhancement is less pronounced. Many competing mechanisms might contribute to the observed seasonal dependence of the SDT, but this study focuses on the refractive effects of shears in the mean zonal wind and gradients in temperature. The main evidence for a refractive influence is that the seasonal enhancement in the SDT amplitude is accompanied by a dramatic shortening in the wave's vertical scale. This shortening of the vertical scale is consistent with refraction of the SDT energy into the horizontal wind component. Simplified linear tidal theory equations are used to estimate the expected magnitude of the refractive effects using wind and temperature fields observed over Andenes, Norway. The predicted refractive effects are shown to be potentially significant and qualitatively consistent with the observations. In addition to a seasonal dependence, the SDT amplitudes obtained at all the radar sites exhibit a deep amplitude modulation on a time scale characteristic of planetary waves. This sort of modulation has most often been attributed to nonlinear interactions between the tides and planetary waves. We suggest that refraction might instead produce, or at least contribute to, the observed modulation. Although the planetary waves are of weak (< 5 m s(-1)) amplitude, the SDT (particularly the gravest S(2,2) mode) is only marginally propagating at high latitudes. Thus, small perturbations to the background are enough to periodically inhibit propagation of the SDT to higher levels. (C) 2003 Elsevier Science Ltd. All rights reserved.