Spiciness theory revisited, with new views on neutral density, orthogonality, and passiveness

[摘要] This paper clarifies the theoretical basis for constructing spiciness variables optimal for characterising ocean water masses. Three essential ingredients are identified: (1) a material density variable γ that is as neutral as feasible, (2) a material state function ξ independent of γ but otherwise arbitrary, and (3) an empirically determined reference function ξ r ( γ ) of γ representing the imagined behaviour of ξ in a notional spiceless ocean. Ingredient (1) is required because contrary to what is often assumed, it is not the properties imposed on ξ (such as orthogonality) that determine its dynamical inertness but the degree of neutrality of γ . The first key result is that it is the anomaly ξ ′ = ξ - ξ r ( γ ) , rather than ξ , that is the variable most suited for characterising ocean water masses, as originally proposed by McDougall and Giles ( 1987 ) . The second key result is that oceanic sections of normalised ξ ′ appear to be relatively insensitive to the choice of ξ , as first suggested by Jackett and McDougall ( 1985 ) , based on the comparison of very different choices of ξ . It is also argued that the orthogonality of ∇ ξ ′ to ∇ γ in physical space is more germane to spiciness theory than orthogonality in thermohaline space, although how to use it to constrain the choices of ξ and ξ r ( γ ) remains to be fully elucidated. The results are important for they unify the various ways in which spiciness has been defined and used in the literature. They also provide a rigorous theoretical basis justifying the pursuit of a globally defined material density variable maximising neutrality. To illustrate the latter point, this paper proposes a new implementation of the author's recently developed thermodynamic neutral density and explains how to adapt existing definitions of spiciness and spicity to work with it.

[学科分类] 海洋学与技术