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

Yandong Lang1, Geoff Stanley2, Trevor J McDougall2 and Paul M Barker3, (1)University of New South Wales, School of mathematics and statistics, Sydney, NSW, Australia, (2)University of New South Wales, Sydney, NSW, Australia, (3)University of New South Wales, School of Mathematics and Statistics, Sydney, NSW, Australia
We present a new method to label the neutral density of an arbitrary water parcel that depends only on its salinity, potential temperature, latitude, and longitude. In particular, its neutral density is independent of the pressure or depth of the parcel, and is therefore independent of heave in observations or high-resolution models. To do so, we move the parcel, adiabatically and isentropically like a Submesoscale Coherent Vortex (SCV), to its level of neutral buoyancy in a nearby water column of a climatological atlas. The parcel's neutral density γSCV is interpolated from pre-labelled neutral density values surrounding this resting location in the climatological atlas. This method is similar to γn of Jackett and McDougall (1997): their discretization of the neutral relationship equated two parcel's potential density referenced to their average pressure, whereas our discretization equates two parcel's potential density referenced to the pressure of the climatological parcel. We compare the material derivative of γSCV with that of γn, finding the contributions from irreversible mixing are similar, that lateral advection materially changes γSCV by roughly twice the amount it changes γn, and that (by construction) vertical advection has no material effect on γSCV. We find similar variations of γn and γSCV on the ω-surfaces of Klocker et al. (2009). Finally, we find that isosurfaces of γn and γSCV deviate from the neutral tangent plane to a similar degree.