Seasonal Variations of the MLT tides in 120°E Meridian

Abstract

AbstractThe authors reported in this paper the seasonal variations in major atmospheric tides in the mesosphere and lower thermosphere (MLT) region in the meridian at 120°E. The SABER/TIMED temperature measurements covering November 2004 to January 2006 were used to delineate tidal harmonics through applying the Fourier‐least squares fit and the FFT analysis. Tidal harmonics composing the final apparent tides were examined separately. Investigation results showed that the migrating diurnal harmonic increases with altitude and attain maxima at 97 km height, then decreases rapidly with altitude. It was also found that the other harmonics increase with altitude in the MLT region and attain significant amplitude at 97 km height. Using these harmonics, the diurnal, semidiurnal and terdiurnal tides in the meridian were reconstructed. Particular focus was placed on investigating the contribution of nonmigrating tides for each kind tides of different frequency. The primary results showed that in the meridian, migrating tides play dominant role in characterizing the general features both in space and time for diurnal and semidiurnal tides. Regarding the diurnal tides, contribution of migrating component is most predominant during spring equinox, which is characterized by the amplitude maxima at the equator and that at the tropics for both hemispheres. Moreover, consistency was seen in the comparison among the temporal variation in the reconstructed diurnal tides and the previous result obtained by using meteor radar wind measurements taken at Wuhan (30°N, 114°E). Regarding the contribution of diurnal nonmigrating harmonics, it was found that in addition to the previously observed prominent (1, 0), (1, 2) and (1, –3) mode, (1, –2) mode is also prominent showing amplitude comparable to that of the (1, 0) mode. These four modes contribute together during summer solstice to form an area covering 10°N to 30°S with maximal amplitude 20K appearing at the Equator. Due to the domination of migrating semidiurnal tides, prominent semidiurnal tides occur at the tropical latitudes in both hemispheres. In the northern hemisphere, the semidiurnal tides appear intense amplitudes during autumn equinox with the maximum of 13K. In the southern hemisphere, they are intense in between spring and summer equinox. The influences of the nonmigrating semidiurnal tides appear to be clear in other seasons, a number of amplitude maxima are seen. As confined in 40°S~40°N, the terdiurnal tides exhibit much weak amplitudes. Moreover, the nonmigrating terdiurnal tides are stronger during many times than the migrating tides in amplitude, although the later exhibit regular occurrence at the Equator. Nevertheless, the current results suggested that the nonmigratingt ides are predominant during most times in 2005.