This thesis presents oceanic budgets of potential temperature, salinity and buoyancy as well as a novel way of diagnosing water-mass transformation in salinity-temperature space. The buoyancy of seawater is given by a nonlinear function of temperature, salinity and pressure and much of the work in this thesis revolves around how fluxes of heat and salinity influence the buoyancy of seawater through these nonlinearities.

Another large part of the material in this thesis is aimed at quantifying the relative importance of different processes for the vertical transport of heat and salinity in the ocean. Careful analysis of those fluxes in an ocean model reveal the different effects of e.g. advection, diffusion and penetrative shortwave radiation. An interesting finding is that the diffusive fluxes due to isoneutral diffusion (diffusion along density surfaces) and dianeutral diffusion (diffusion across density surfaces) have opposing effects on the oceanic heat and salinity budgets.

The final major topic of this thesis is water-mass transformation. A quantitative framework for the study of water-mass transformation in salinity-temperature space is introduced. A continuity equation is also derived for salinity-temperature space, which can be used to calculate the time rate of change of volume in a small salinity-temperature interval. The water-mass transformation framework is applied in an ocean general circulation model, and it is shown how the volume distribution in salinity-temperature space is affected by the different tracer fluxes in the model. It is also shown how the transformation framework is related to earlier work on thermohaline streamfunctions.