The source of tropospheric tides

With the method of Hough mode decomposition (HMD), the tidal sources of the three main tidal components, namely, the migrating components DW1 (diurnal westward propagating wavenumber 1) and SW2 (semidiurnal westward propagating wavenumber 2) and the non-migrating component DE3 (diurnal eastward propagating wavenumber 3), at the tropospheric altitudes (1–12 km) and in the latitude range of ±60°, were obtained from National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR) data during the interval from 1988 to 2011. We analyzed these sources in detail at 6 km and obtained the main properties of their yearly variations. The DW1 source was found to present a weak seasonal variation in the lower latitudes (about ±10°–15°). That is, the amplitudes of the DW1 sources were larger in the summer months than in the winter months, and DW1 presented semi-annual variation near the equator (±10°) such that the DW1 source was larger at the equinoxes than at the solstices. In addition, the SW2 source was symmetric and was stronger in the southern hemisphere than in the northern hemisphere. The SW2 source presented remarkable annual and semi-annual variation such that the amplitudes were largest during the March equinox months and larger during the June solstice months. In contrast, DE3 appeared mainly around the equatorial latitudes within about ±30°. The DE3 source presented remarkable semi-annual variation that was larger around the solstices than the equinoxes in the southern hemisphere, and it was opposite in the northern hemisphere. By HMD, we found that the tropospheric tides were primarily dominated by some leading propagating Hough modes, specifically, the (1, 1), (2, 3), and (3, 3) modes; the influences of the other Hough modes were negligible. The consequences of an El Niño–Southern Oscillation modulation of tidal amplitudes for the energy and momentum budgets of the troposphere may now be expected to attract attention. In summary, the above yearly variations of the main tidal sources and the Hough coefficients demonstrate that an HMD analysis can be used to investigate the tropospheric tides.