Abstract
Abstract In the Antarctic upper mesosphere and lower thermosphere around 90 km, meteor radar observations at the South Pole have detected a significant semidiurnal wind component in summer which is found to be non-migrating with zonal wavenumber s = 1. It has been surmised that this component might possibly be excited through the non-linear interaction of the migrating semidiurnal tide with stationary planetary waves with zonal wavenumber s = 1 prevailing at stratospheric heights. The Kyushu University GCM has been successful in elucidating very unambiguously this conjecture. In the present paper, linearized steady tidal modeling is carried out in this connection to reproduce, by a fairly simplified but explicit model, trans-equatorial propagation of non-migrating semidiurnal tide forced in the opposite winter hemisphere and to compare latitudinal structures of migrating and non-linearly excited intermittent tides in view of polar latitudes where non-migrating tide tends to dominate over migrating tide. It is also shown that the re-analysis meteorological data for almost 10 years clearly supports the well-known N-S asymmetry in stationary planetary wave activity in winter polar stratospheric regions, possibly due to the difference in surface topography between two hemispheres. We suggest that there might be significant asymmetry in the summertime enhancement of semidiurnal non-migrating tide between both polar regions. This phenomenon may be in a context similar with the N-S asymmetry of gravity wave activity in the polar regions, possibly giving rise to N-S disparity of cold summertime mesopause temperature. Clarification awaits intensive bi-polar studies by coordinated radar and optical observations which are running both in the Arctic and Antarctic regions.
Highlights
Semidiurnal 12-h oscillations are still a dominant atmospheric wave in the high-latitude mesosphere and lower thermosphere in that non-migrating or zonally symmetric (s = 0) tides have non-zero wind value near the poles
Discussions on the expected asymmetry in semidiurnal non-migrating tide at both polar regions are given in the last section
Asymmetry in the zonal wavenumber s = 1 semidiurnal tide in summer between northern and southern hemispheres, if it exists, very unambiguously supports a suggested scenario by Forbes et al (1995) and Yamashita et al (2002) that the zonal wavenumber s = 1 semidiurnal tide prevalent more in the Antarctic than in the Arctic summer at meteor heights is due to the interaction of the migrating semidiurnal tide and stationary planetary waves of wavenumber s = 1 which prevails in winter northern hemisphere
Summary
Semidiurnal 12-h oscillations are still a dominant atmospheric wave in the high-latitude mesosphere and lower thermosphere in that non-migrating (zonal wavenumber s = 1) or zonally symmetric (s = 0) tides have non-zero wind value near the poles. Yamashita et al (2002) performed a comprehensive Kyushu University-GCM calculation and conÀrmed that the non-linear interaction with stationary planetary waves is really taking place, contributing to generation of the s = 1 semidiurnal component They calculated EP Áuxes and, together with the phase structure of this component, suggested that this is excited in the winter hemisphere and propagates across the equator up to mesosphere and to lower thermosphere heights in the summer hemisphere, giving rise to summertime enhancement at polar MLT regions. Discussions on the expected asymmetry in semidiurnal non-migrating tide at both polar regions are given in the last section
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