Zhang Xiaoming

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  • Thesis
    A model of the equatorial deep jets and the role of the horizontal Coriolis parameter
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1991-01) Zhang, Xiaoming
    This thesis reviews observational and theoretical work on the equatorial deep jets and work related to the study of the role of the horizontal Coriolis parameter. Most existing analytical models interpret the equatorial deep jets as either low frequency, long Rossby waves or stationary,long Kelvin waves generated at or near the ocean surface. These models are unable to answer the question of how wind generated energy propagates down through the equatorial undercurrent and thermocline into the deep ocean. Existing numerical models do not display deep jet features due mainly to their in low vertical resolution and the high eddy viscosity associated with these models. These numerical models also suggest that very tittle energy is able to get into the deep ocean. A natural question is raised: can the equatorial deep jets possibly be interpreted as free, steady inertial motion below the thermocline? We develop a simple model for the deep jets as a free, stationary inertial motion. After scaling the fluid dynamical equations in the appropriate regime, it is found that neither the advective nonlinearity nor the horizontal Coriolis parameter can be neglected. An important conservation equation, the so called potential zonal vorticity conservation equation which governs the equatorial steady and zonal independent equatorial flow is derived. From this conservation principle, an inertial equatorial deep jets model is developed which captures some important features of the deep jets. The horizontal Coriolis parameter is important in this inertial model. The role of the horizontal Coriolis parameter has long been controversial in the literature. We discuss this role for several equatorial flow systems. It is found that the horizontal Coriolis parameter is not significant for inviscid linear equatorial waves due to the presence of stratification in the real ocean. However, when the ratio of momentum eddy viscosity to the density dissipation coefficient becomes small enough, the effect of the horizontal Coriolis parameter becomes more important in a simple viscous model. Some general aspects of this parameter have also been discussed in terms of angular momentum conservation and energy conservation principles. It is suggested that for the ocean circulation of large vertical excursion of the fluid particle, the horizontal Coriolis parameter effect may not be small and should be included in future numerical models.