Physical Processes Controlling the Tide in the Tropical Lower Atmosphere Investigated Using a Comprehensive Numerical Model

Abstract

Abstract The lower-atmospheric circulation in the tropics is strongly influenced by large-scale daily variations referred to as atmospheric solar tides. Most earlier studies have used simplified linear theory to explain daily variations in the tropics. The present study employs a comprehensive limited-area atmospheric model and revisits some longstanding issues related to atmospheric tidal dynamics. The tides in the tropical lower atmosphere are realistically simulated in the control experiment with a near-global (75°S–75°N) version of the model. Sensitivity experiments with different aspects of the solar heating suppressed showed that the semidiurnal (S2) tide near the surface can be attributed roughly equally to stratospheric and tropospheric direct solar heating and that the diurnal (S1) tide is excited almost entirely by tropospheric direct solar heating as well as solar heating of Earth’s surface. Linear theory with forcing only by direct radiative heating predicts the phase of the S2 barometric oscillation should be ~0910 LT versus the ~0945 LT phase seen in low-latitude observations. The roles of (i) convective and latent heating and (ii) mechanical dissipation, in determining the S2 phase, are assessed in the model. It is found that the former effect delays the phase by ~25 min and the latter by ~5 min; these two effects together explain the observed phase. When the model is run in limited-area domains comparable to those used in typical regional climate studies the S2, but not S1, tide is found to be significantly weaker than observed, even using atmospheric reanalysis data to drive the lateral boundaries.