Topographic effects on the solar semidiurnal surface tide simulated in a very fine resolution general circulation model

Abstract

We present an examination of the solar tidal variation of surface pressure in integrations conducted with the Atmospheric GCM for the Earth Simulator (AFES) global general circulation model run at very fine resolution (roughly 10 km horizontal grid spacing and 96 numerical levels from the ground up to 0.1 hPa pressure). The basic features of the observed diurnal and semidiurnal surface pressure oscillations are reasonably well simulated by the model, although simulated amplitudes of the semidiurnal oscillation have an overall enhancement of about 25% over those observed, a deficiency which is reasonably attributed to the effects of the upper boundary condition in the model. The focus of our analysis is the local‐/regional‐scale modulation of the semidiurnal tidal oscillation in the tropics and subtropics associated with high and steep topography. The results show that the first‐order effect of high topography is a reduction in the semidiurnal pressure amplitude with surface elevation, a feature that is consistent with our understanding of the semidiurnal tide as a vertically propagating inertia‐gravity wave primarily excited in the ozone layer. We also find evidence in the model for systematically weak semidiurnal pressure amplitudes to the west of very high and steep topography, which is reasonably attributed to a shadowing effect of topography on the global‐scale westward propagating tide. Support for these effects is presented in our analysis of previously published station observations of the semidiurnal pressure oscillation. In addition, we present new determinations of the semidiurnal pressure oscillation based on barometric data from a special array of nine sensors established on the island of Hawaii, representing an unprecedented sampling of surface tides over a large range of elevations in a relatively small geographical region. The results of the AFES simulations agree quite well with these new detailed observations in Hawaii.