Diagnosing the relation between ocean circulation, mixing and water-mass transformation from an ocean hydrography and air-sea fluxes

摘要

The aim of this thesis is to understand the relationship between surface freshwater andudheat fluxes, (interior) ocean mixing and the resulting changes in the ocean circulationudand distribution of water-masses.udThe ocean circulation is analysed in Absolute Salinity (SA) and Conservative Temperatureud(SA) coordinates. It is separated into 1) an advective component related to geographicaluddisplacements in the direction normal to SA and ⊖ iso-surfaces, and quantifiedudby the advective thermohaline streamfunction Ψadv/SA⊖, and 2) into a local component, relatedudto local changes in SA and ⊖ values, without a geographical displacement, andudquantified by the local (temporal) thermohaline streamfunction Ψloc/SA⊖. In this decomposition,udthe sum of the advective and local components of the circulation is given by theuddiathermohaline streamfunction Ψdia/SA⊖ and is directly related to the salt and heat udfluxes of the surface forcing and ocean mixing. Interpretations of the streamfunctions is givenudand it is argued that the diathermohaline streamfunction provides a powerful tool for theudanalysis of and comparison amongst numerical ocean models and observational-basedudgridded climatologies.udThe relation between Ψdia/SA⊖ and fluxes of salt and heat is expressed as the ThermohalineudInverse Method (THIM). The THIM uses conservation statements for volume, salt andudheat in (SA, ⊖) coordinates to express the unknown Ψdia/SA⊖ as surface freshwater and heat fluxes, and mixing parameterised by a down-gradient epineutral diffusion coefficient, and an isotropic down-gradient turbulent diffusion coeffcient of small scale mixing processes.udThe resulting system of equations that is solved in the THIM is tested against a numericaludmodel and shown to provide accurate estimates of the unknowns (Ψdia/SA⊖, and theudepineutral and small-scale diffusion coefficients).udThe THIM has been applied to observations to obtain constrained estimates of theudepineutral and small-scale diffusion coefficients and Ψdia/SA⊖. New insights in Ψdia/SA⊖ areudrevealed and the estimate of small-scale diffusion coefficient compares well with previousudestimates. The estimates of the epineutral diffusion coefficient is about 50 times smallerudthan those typically used in coarse resolution climate models, suggesting that either theudsurfaces fluxes that are used under-estimate the production of epineutral anomalies ofudSA and ⊖) or the epineutral diffusion coefficients commonly used in climate models areudtoo large.udThe geometry of interior ocean mixing is analysed and it is found that under the smallslopeudapproximation there is a small gradient of tracer in a direction in which there isudno actual epineutral gradient of tracer. The difference between the correct epineutral tracer gradient and the small-slope approximation to it, is quantified and it is shown that it points in the direction of the thermal wind. The fraction of the epineutral flux in this direction is very small and is negligible for all foreseeable applications. Smallscaleudmixing processes act to diffuse tracers isotropically (i.e. directionally uniformly inudspace), hence it is a misnomer to call this process 'dianeutral diffusion'. Both realisationsudaffect the diffusion tensor, and a more concise diffusion tensor is derived for use in oceanudmodels.udThe techniques, diagnostics and insights presented in this thesis lead to increased understandingudof the relationship between ocean circulation and water-mass transformation due to ocean mixing and surface uduxes, and result in enhanced ability to model the ocean and its role in the climate system.