Internal salt content: a useful framework for understanding the oceanic branch of the water cycle

Abstract The global water cycle is dominated by an atmospheric branch which transfers fresh water away from subtropical regions and an oceanic branch which returns that fresh water 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 which determine ocean salinity we introduce a new variable: “internal salt” and 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 fresh water in the ocean. We apply this framework to a 1° global ocean model. We find that in order for fresh water 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, while 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 which redistribute fresh water added through precipitation, balancing asymmetries in freshwater forcing between the basins.