This thesis investigates the circulation and mixing of estuaries, particularly estuaries that are driven by surface buoyancy fluxes. A numerical model is used to perform a hind-cast simulation of the exchange flow of the Persian Gulf. The exchange flow of the Persian Gulf with the Indian Ocean is investigated and quantified by using a thermohaline framework. Furthermore, the salt mixing is analyzed which can be diagnosed from the properties of the exchange flow and the driving surface fluxes.