Seasonal Variations of the MLT tides in 120°E Meridian

Abstract

The 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.

Global structures of the DE3 tide

“…In the mid-mesosphere, the DE3 tide always exhibits antisymmetry about the equator, the feature has been reported by the previous researches [1][2][3][4][5] . Based on the results of HMD analysis applied with the DE3 tide at 86 km, Forbes and Wu [4] argue that the anti-symmetric feature is due to the influence of the second propagating Hough mode (−3, 4) whose amplitude can be maintained at a sustained level although significantly weaker than that of the (−3, 3) mode.…”

“…The SABER/TIMED samples in the latitude range from 52° in one hemisphere to 83° in the other hemisphere in one yaw period of 60-day length, however, the scheme of binning 60-day measurements can provide sufficient LST sampling at the latitudes of 40° equatorward in the two hemispheres [5] . In the current research, 120-day SABER temperatures were binned to maintain sufficient LST sampling at high latitudes in both hemispheres.…”

“…This is consistent to that reported in previous studies (confer ref. [5] and the references therein). In addition to this, Figure 3 tially enhanced maxima in 2002 and 2004, respectively.…”

“…Although these amplitudes are weaker than the migrating diurnal tide with maximal amplitude of 8 K, maximal influence of the tide may prevail over that of the migrating diurnal as the two tides exhibit different annual variation. In a recent research using one year satellite data, Chen and Lu [5] report that 9 K of the maximal DE3 amplitude is observed at 97 km, which suggests that maximal DE3 activity might be at an even higher altitude. The importance of the influence of the tide is highlighted as it is considered as a robust candidate that can propagate into the upper atmosphere thus being responsible for causing ionospheric disturbances (please refer to Wan et al [6] and the references therein).…”

The Result of Intensive Radiosonde Observation over Wuhan

“…to the global tides, localized excitation sources, such as diurnal activity of cloud convection or heat exchange near the Earth's surface, can also produce diurnal oscillations, which can be treated as nonmigrating tides [8,9] . Statistical analysis of nonmigrating tides was extensively conducted by Haurwitz [10] by using large amount of surface data.…”