Historical data in various domains within the Gulf of California are used to describe the seasonal balance of the average salinity, ⟨S⟩. The difference of evaporation minus precipitation, E - P, is positive along the course of a year. This produces a positive salinity anomaly, S', which is exported to the Pacific Ocean through the mouth of the gulf. Even though E - P has strong annual and semiannual cycles, it is not enough to explain the seasonal variation of the transport of S'. A linear one-dimensional nondiffusive inhomogeneous two-layer model suggests that the seasonal balance of ⟨S⟩ is largely controlled by the Pacific Ocean, which excites a baroclinic wave at the mouth of the gulf. Advection due to this wave is the main carrier of S' within the gulf, and the associated water rearrangement produces a considerable change in ⟨S⟩. The Pacific Ocean has been previously shown to maintain the seasonal heat balance through a similar mechanism. This paper thus adds importance to the Pacific Ocean influence on the determination of the gulf's seasonal dynamics and thermodynamics.