A pressure-invariant Neutral Density variable for the World’s Oceans

Abstract

We present a new method to calculate the neutral density of an arbitrary water parcel. Using this method the value of neutral density depends only on the parcel’s salinity, temperature, latitude, and longitude, and is independent of the pressure (or depth) of the parcel, and is therefore independent of heave in observations or high-resolution models. In this method we move the parcel adiabatically and isentropically like a Submesoscale Coherent Vortex (SCV), to its level of neutral buoyancy on four nearby water columns of a climatological atlas. The parcel’s neutral density γSCV is interpolated from pre-labelled neutral density values at these four reference locations in the climatological atlas. This method is similar to the neutral density variable, γn, of Jackett and McDougall: their discretization of the neutral relationship equated the potential density of two parcels referenced to their average pressure, whereas our discretization equates the parcels’ potential density referenced to the pressure of the climatological parcel. We calculate the numerical differences between γSCV and γn and we find similar variations of γn and γSCVon the ω-surfaces of Klocker, McDougall and Jackett. We also find that isosurfaces of γn and γSCV deviate from the neutral tangent plane by similar amounts. We compare the material derivative of γSCV with that of γn, finding their total material derivatives are of a similar magnitude.