Abstract River runoff, precipitation and evaporation are drivers of the overturning circulation and hence water mass formation in marginal seas and estuaries. Systems with river runoff and strong evaporation can form a salinity maximum between the freshwater sources and the adjacent ocean, a so‐called salt plug. In this study, we explore the diahaline overturning circulation and the associated diahaline mixing in the Persian Gulf as a representative salt plug estuary. The overturning structure combines features of classical and inverse estuaries: Salt mixing per salinity class is the product of salinity and river runoff at low salinities, and salinity and evaporation at the high salinities of the salt plugs. Overall, riverine freshwater and hypersaline water are transformed by mixing toward the salinity of the outflowing water. For the case of the Persian Gulf, the dynamically important separation of the surface and bottom layers in summer is highlighted by the overturning analysis.

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