A NEUTRAL DENSITY VARIABLE FOR THE WORLDS OCEANS

The use of density surfaces in the analysis of oceanographic data and in models of the ocean circulation is widespread. The present best met hod of fitting these isopycnal surfaces to hydrographic data is based on a linked sequence of potential density surfaces referred to a discr ete set of reference pressures. This method is both rime consuming and cumbersome in its implementation. In this paper the authors introduce a new density variable, neutral density gamma(n), which is a continuo us analog of these discretely referenced potential density surfaces. T he level surfaces of gamma(n) form neutral surfaces, which are the mos t appropriate surfaces within which an ocean model's calculations shou ld be performed or analyzed. The authors have developed a computationa l algorithm for evaluating gamma(n) from a given hydrographic observat ion so that the formation of neutral density surfaces requires a simpl e call to a computational function. Neutral density is of necessity no t only a function of the three state variables: salinity, temperature, and pressure, but also of longitude and latitude. The spatial depende nce of gamma(n) is achieved by accurately labeling a, global hydrograp hic dataset with neutral density. Arbitrary hydrographic data can then be labeled with reference to this global gamma(n) field. The global d ataset is derived from the Levitus climatology of the world's oceans, with minor modifications made to ensure static stability and an adequa te representation of the densest seawater. An initial field of gamma(n ) is obtained by solving, using a combination of numerical techniques, a system of differential equations that describe the fundamental neut ral surface property. This global field of gamma(n) values is further iterated in the characteristic coordinate system of the neutral surfac es to reduce any errors incurred during this solution procedure and to distribute the inherent path-dependent error associated with the defi nition of neutral surfaces over the entire globe. Comparisons are made between neutral surfaces calculated from gamma(n) and the present bes t isopycnal surfaces along independent sections of hydrographic data. The development of this neutral density variable increases the accurac y of the best-practice isopycnal surfaces currently in use but, more i mportantly, provides oceanographers with a much easier method of fitti ng such surfaces to hydrographic data.