Diahaline transport and the global water cycle

Global evaporation and precipitation complete the atmospheric branch of the water cycle, which is predicted to increase under climate change. The water cycle is difficult to observe directly, so recent efforts have focused on its relationship with ocean salinity: the high salinity subtropical ocean experiences net evaporation while the low salinity tropics and poles experience net precipitation. For a quasi-stationary distribution of salinity to be maintained in the ocean, large amounts of salt must be transported away from the evaporation dominated regions of the ocean, crossing surfaces of constant salinity. We present an approach, based on Water Mass Transformation theory, to study the processes which transport salt across isohaline surfaces. Using a salinity coordinate we isolate the contribution of parameterised diabatic processes and the effective mixing from the numerical advection scheme in the ACCESS-OM2 global ocean model. We find a maximum forcing of 8.72×10⁷kg sec^-1 , while the parameterised mixing processes which occur at a maximum of 3.15×10⁷kg sec^-1 vertical mixing and 3.76×10⁷kg sec^-1 along isopycnal mixing accompany 2.44× 10^7×10⁷kg sec^-1 effective mixing from numerical advection. We extend this salinity coordinate analysis to latitude-salinity coordinate and introduce a salt function which quantifies the meridional transport of salt. We demonstrate that meridional salt transport describes the pathways taken by freshwater during the oceanic component of the water cycle. Using this relationship, we describe how these methods can be used to study long-term water cycle change.