Internal Salt Content: A Useful Framework for Understanding the Oceanic Branch of the Water Cycle

Significance Statement Recent efforts to measure changing rainfall patterns have focused on sea surface salinity. This presents a number of challenges because salinity is determined by surface freshwater fluxes as well as circulation and mixing within the ocean, which depend on salinity gradients. We introduce the concepts of internal salt and internal fresh water, which measure the salt and freshwater content associated with variations in salinity within water masses in the ocean. We present precise budgets of internal salt and freshwater that we use to identify the oceanic pathways through which precipitation added in the subpolar and tropical regions is redistributed to balance evaporation in the subtropics. Future studies will investigate the response of circulation and mixing to long-term water cycle change. The global water cycle is dominated by an atmospheric branch that transfers freshwater away from subtropical regions and an oceanic branch that returns that freshwater from subpolar and tropical regions. Salt content is commonly used to understand the oceanic branch because surface freshwater fluxes leave an imprint on ocean salinity. However, freshwater fluxes do not actually change the amount of salt in the ocean and-in the mean-no salt is transported meridionally by ocean circulation. To study the processes that determine ocean salinity, we introduce a new variable internal salt along with its counterpart internal fresh water. Precise budgets for internal salt in salinity coordinates relate meridional and diahaline transport to surface freshwater forcing, ocean circulation, and mixing and reveal the pathway of freshwater in the ocean. We apply this framework to a 1 degrees global ocean model. We find that for freshwater to be exported from the ocean's tropical and subpolar regions to the subtropics, salt must be mixed across the salinity surfaces that bound those regions. In the tropics, this mixing is achieved by parameterized vertical mixing, along-isopycnal mixing, and numerical mixing associated with truncation errors in the model's advection scheme, whereas along-isopycnal mixing dominates at high latitudes. We analyze the internal freshwater budgets of the Indo-Pacific and Atlantic Ocean basins and identify the transport pathways between them that redistribute freshwater added through precipitation, balancing asymmetries in freshwater forcing between the basins.

Automated summary

This study introduces the concepts of internal salt and internal freshwater to measure the salt and freshwater content associated with variations in salinity within water masses in the ocean. Precise budgets for internal salt in salinity coordinates are used to reveal the pathway of freshwater in the ocean and identify different mechanisms of freshwater transport in different regions.