A study of abyssal circulation based on the expansion in terms of vertical modes incorporating density diffusion

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The purpose of this project is to clarify the basic dynamics of abyssal circulation by expanding the field in terms of vertical modes and investigating the behavior of each mode.Incorporating both vertical and horizontal diffusion of density makes the obtained model comparable directly with ordinary three-dimensional numerical experiments.In our model,deep western boundary layers can be either in viscous or in diffusive regimes, depending on the stratification and latitudes.Higher modes,for which horizontal diffusion works strongly,tend to have broad diffusive western boundary layers.In the poleward flowing(equator-ward) western boundary layer,deep water upwells(downwells),while the opposite sign of vertical flow is found in the narrow layer adjacent to the western coast.By virtue of density diffusion,deep water can downwell near boundaries and even in interior regions.The structural change of boundary layers and associated appearance of downwelling were explained well in terms of a new concept of diffusion of(the thickness term of)potential vorticity, which applies as well to surface layers of stratified oceans.In an extended model,effects of isolated bottom topography on abyssal circulation were investigated. In particular,by including dissipative terms,we determined the field inside the geostophic island,which had been beyond the scope the characteristic method.A direct method was developed to solve higher modes efficiently.It was used to investigate the vertical structure of ocean circulation.