Excitation of the 10-Day and 16-Day Waves

Abstract

The composite analysis of the 10-day and 16-day waves, the observational counterparts of the second antisymmetric and symmetric rotational Hough mode with the zonal wavenumber 1, respectively, were performed as an extension of Cheong and Kimura?s study in order to investigate the global structure and the excitation mechanism. At the 700-hPa level, the composited waves showed a northwest?southeast phase tilt and northward propagation of the wave energy in the Northern Hemisphere (NH) summer, as was the case for the 5-day wave in Cheong and Kimura, while these features are less conspicuous in the NH winter. At the 10-hPa level, no systematic phase variation with latitude was shown due to high level of noise. Consistent to the previous studies, however, it is found that the amplitude in the winter hemisphere is much larger than that in the summer hemisphere. Responses of shallow water model on the sphere (Laplace?s tidal equations) with the zonal-mean flow to the topographic and/or thermal forcing were examined to explain the observed features of composite analysis. Comparison of the forced waves in the model with the composited waves indicates that in the NH summer both the 10-day and 16-day waves are excited mainly by the topographic forcing over Antarctica. In the NH winter, however, the topographic forcing located in the high latitudes of both hemispheres and equatorial thermal forcing are of equal importance for these two modes. When both the topographic and thermal forcing are coexistent simultaneously, the resultant response of the model is much weaker than it is expected to be, due to the cancellation by the destructive interference between them. The extent to which the interference affects the response depends strongly on the frequency and meridional location of the forcing.