Relationship between Vertical, Diapycnal, and Isopycnal Velocity and Mixing in the Ocean General Circulation

摘要

On ocean general circulation scales, vertical velocity 'w' is extremely small, (0.00001) smaller than the horizontal velocity in scale. Nevertheless, it is of great importance to advective-diffusive and vorticity balances, To better understand the relationship between the cross-isopycnal(diapycnal) velocity, the mixing and w, in a nonideal ocean in which some mixing and density sources are present, the velocity vector is decomposed into isooycnal and diapycnal components. Applying this decomposition to the exact continuity (mass conservation) equation, it is shown that while the diapycnal divergence is in principle the first correction to the isopycnal divergence for general circulation scales, the observational uncertainties in isopycnal velocity are large enough that the diapycnal divergence cannot in practice be determined. Using these results, the horizontally averaged near-vertical (diapycnal) velocity computed by Wunsch et al. in a model of the South Pacific (28-43 deg S) is reconsidered. Calculation of diapycnal velocity from isopycnal mass convergence is not consistent with uncertainties in isopycnal velocity and that the implied mixing cannot be accounted for with simple diffusion models and salt-fingering alone.