Dianeutral Motion, Water Mass Conversion, and Nonlinear Effects on the Density Ratio in the Pacific Thermocline

Abstract

The averaged hydrography of Levitus has been used to form maps of various quantities that contribute to dianeutral advecion in the Pacific thermocline. On much of the ?26.50? neutral surface cabbeling and thermobaricity contribute between ?0.2 ? 10?7 m s?1 and ?2 ? 10?7 m s?1 to the total dianeutral velocity (assuming a lateral diffusivity of 103 m2 s?1). The dianeutral advection caused by vertical turbulent mixing is also mapped on several surfaces in the Pacific Ocean. Assuming a vertical diffusivity of 10?5 m2 s?1, the typical dianeutral velocity caused by vertical mixing is 10?7 m s?1. This upwelling across the ?26.20? neutral surface, at an average depth of about 200 m, is positive and quite strong throughout the whole equatorial region. There is a striking pattern of strong dianeutral upwelling in the central Pacific on the ?26.50? neutral surface, while in the eastern Pacific on the same surface there is a large region of downwelling. Subsurface water mass conversion is defined as the rate at which fluid properties change on a neutral surface, and the contribution of vertical turbulent mixing to water mass conversion is also evaluated and mapped in the Pacific thermocline. In addition, the source term in the R?, equation caused by the action of vertical shear on the epineutral gradient of salinity or potential temperature is examined. Using the thermal wind relation, this term is shown to be similar to the term responsible for the path-dependence of neutral surfaces. This term is evaluated on several surfaces in the Pacific and found to be small on basin scales, consistent with the relatively high density ratios found in the Pacific. The authors also derive several terms in the R?, equation that arise because of the nonlinear equation of state; these can be as large as the other terms in the equation.