Overview of the Dependence of Atmospheric Tide Amplitudes on the Phases of Natural Tropical Oscillations based on MUAM simulations

The dependence of atmospheric tide amplitudes on the phases of long-period tropical oscillations, specifically the Quasi-Biennial Oscillation (QBO) of zonal wind in the equatorial stratosphere and the El Niño–Southern Oscillation (ENSO), is examined. Numerical simulations of global atmospheric circulation are conducted using the MUAM nonlinear mechanistic atmospheric model under various scenarios incorporating different combinations of QBO/ENSO phases. The structures of migrating diurnal and semidiurnal tides with zonal wave numbers 1 and 2, respectively, as well as non-migrating diurnal and semidiurnal tides with zonal wave numbers 2 and 1, respectively, are calculated. The analysis is focused on the boreal winter season (January - February), the period of peak wave activity for planetary waves (PWs) that are involved in the nonlinear generation of non-migrating tides. The results demonstrate, in particular, that the migrating diurnal tide (DT1) is amplified during the westerly QBO phase (wQBO) and under La Niña conditions. For the semidiurnal migrating tide (SDT2), ENSO effects are found to be more pronounced than those of the QBO. During El Niño, the tide’s amplitude decreases in the equatorial region while increasing to the North and South of it, regardless of the QBO phase. Changes in non-migrating tides differ from those of migrating tides with similar periods, which is attributed to the altered wave activity of the stationary PW with zonal wave number 1 (SPW1). Nonlinear interactions between primary migrating tides and this wave generate non-migrating tides. The effect of strengthening/weakening of non-migrating diurnal tide (DT2) generation for different combinations of QBO/ENSO is demonstrated explicitly by considering the terms responsible for the nonlinear interaction of PW1 and DT1 in the balance equation of perturbed potential enstrophy. The numerical simulations performed under “idealized” conditions, isolating the effects of QBO and ENSO, allowed for the differentiation of the influences of these two oscillations. Such separation is challenging with observational data due to limited time series, which restricts sample size and thereby limits the statistical capacity needed to distinguish between these phenomena having close periods.