Global Structures and Multi-Temporal Variabilities of MLT Migrating Diurnal Tide

Migrating diurnal tide in the MLT region is examined by the application of Hough mode decomposition with the tide delineated from the SABER/TIMED temperatures over 2002--2006. The decomposition results show that in the height range 60--100km, the (1, 1) mode is the most predominant among eight leading Hough modes including four propagating and four trapped modes. It exhibits a sustained maximum at 97km and significant semi-annual oscillation. Additionally, a novel feature of inter-annual variation with period of about two years is clearly seen in the (1, 1) mode, e.g., repeated maxima are seen at the March equinox of 2002, 2004 and 2006, respectively. This feature is further manifested by the tidal amplitudes in the height range 70--100km in the height-time cross-section at the equator. It is likely of the QBO as the height range just coincides to where the zonal mean zonal winds derived by using the UARS data exhibiting the QBO. The other results show that the (1, 2) mode is important at <80km exhibiting comparable amplitude to that of the (1, 1) mode, and in particular the nearly anti-correlation with the (1, 1) mode. The tide at about 85km is suggested of rather complex as the four trapped modes exhibit maximum at these heights, which indicates the presence of local excitations or sources at below.