The Lagrangian-Mean Motions Forced by Steady, Dissipating Equatorial Waves: Part I

[摘要] The Eulerian- and the Lagrangian-mean flows induced by steady, dissipating equatorial waves are discussed. The waves are assumed to be excited by the bottom corrugation and to be dissipated by Newtonian cooling and Rayleigh friction with an equal relaxation time. Four wave modes considered are Kelvin waves, mixed Rossby-gravity waves, Rossby waves with n=1 (n is the meridional mode number, see Matsuno (1966)) and westward propagating inertio-gravity waves with n=1.For all equatorial normal modes, both of the Eulerian- and the Lagrangian-mean meridional circulations do not get across the equator. This result follows from the symmetry properties of equatorial waves, and can hold even if the feed-back effects of the wave-induced mean zonal flow are taken into account. In other words, in order for air parcels to go across the equator, an asymmetric configuration of the mean zonal flow is necessary.In case of Kelvin waves, it is shown that the Lagrangian-mean meridional circulations are identical with the Eulerian-mean counterparts, and have downward flows above the equator. It is also shown that, in the Boussinesq limit, both the Eulerian- and the Lagrangian-mean meridional circulations cannot be caused by Kelvin waves.The present results show that throughout all the cases of modes, the Lagrangian-mean meridional circulations have simpler structures than the Eulerian-mean counterparts. For example, in case of mixed Rossby-gravity wave, the Eulerian-mean meridional circulation has two cells opposite to each other in one hemisphere, while the Lagrangian-mean one has only one cell in the same hemisphere.In addition, it is emphasized that, in the case of mixed Rossby-gravity waves, the deviation from the geostrop.hic balance between the Eulerian-mean zonal flow and pressure gradient is remarkably large as in the case of westward propagating inertio-gravity wave with n=1.

[学科分类] 大气科学