Seasonal and Interannual Variations of Global Tides in the Mesosphere and Lower Thermosphere Neutral Winds: I. Diurnal Tides.

In the mesosphere and lower thermosphere, diurnal tides are responsible for the dynamics and structures at low latitudes since they have largest amplitudes there. Based on the 20‐year (2002−2021) observations from Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics Doppler interferometer (TIDI), we investigate the seasonal variations of three diurnal tidal components (DW1, DE3, DW2), and their responses to stratospheric quasi‐biennial oscillation (SQBO) and solar cycle globally. The results show that: (a) DW1, DE3 and DW2 show prominent semiannual (SAO) and annual oscillations (AO) in their peak regions where the ratio of their annual mean amplitude to their maximum annual mean amplitude is larger than 0.8. DW1 also exhibit strong terannual oscillations (TAO) especially in meridional wind. (b) The responses of the amplitudes of seasonal variations of DW1, DE3 and DW2 to SQBO and solar cycle are comparable in magnitude to those of their annual mean amplitudes (for response to solar cycle, even stronger in some cases), and thus cannot be neglected. (c) In their respective peak regions, the responses of annual mean amplitudes of these diurnal tidal components to SQBO are uniformly positive except DW2, and their responses to solar cycle are uniformly negative. For these diurnal tides, the amplitudes of the dominant seasonal variations exhibit consistent response patterns with those of annual means to the SQBO/solar cycle. (d) Empirical formulas are given, which well describe the seasonal and interannual variations of dominant diurnal tidal components in their peak regions. Plain Language Summary: Diurnal tides, which are variations of atmospheric properties with periods of 24 hr, are among the most important waves in the mesosphere and lower thermosphere (MLT). Diurnal tide is normally represented as the sum of various components that have different structures in the longitude × time dimension. Using the 20‐year observations (2002−2021) of global neutral horizontal winds in the MLT by TIMED Doppler interferometer (TIDI), we investigate the global structure and variability of three of these diurnal tidal components (DW1, DE3, DW2) in neutral winds in the MLT in this work. We found strong seasonal variations at semiannual (SAO), annual (AO), and terannual (TAO) periods. We then focus on the responses of the annual means and seasonal variations to two long‐term external forcings known to affect tides: the stratospheric quasi‐biennial oscillation (SQBO) and the solar cycle. The seasonal variations show responses to the SQBO and solar cycle that are comparable in magnitude to those of the annual means. For these diurnal tides, the response patterns of the dominant seasonal variations to the SQBO/solar cycle are similar to those of the annual means. Finally, we provide empirical formulas, which describe well the seasonal and interannual variations of dominant tidal components. Key Points: Responses of seasonal variations of global tidal winds to the stratospheric quasi‐biennial oscillation (SQBO) and solar cycle are revealedFor each diurnal tide, the modulation of the dominant seasonal variations by the SQBO/solar cycle is similar to that of the annual meansEmpirical formulas are given, which well describe the seasonal and interannual variations of dominant diurnal tides in the peak regions [ABSTRACT FROM AUTHOR]