Seawater is a dilute salt solution. The salt or salinity is reduced or elevated as freshwater is added or removed, respectively, through precipitation, evaporation, and sea-ice melting and freezing, as well as river runoff from land. If the freshwater inventory within the ocean water column remains in quasi-steady state, imbalances of sea-air flux of freshwater at specific sites are compensated with freshwater convergence or divergence by ocean currents and mixing, including eddies and wind-induced Ekman transport. Climate fluctuations alter the hydrological cycle. On land, these modifications are manifested as droughts in one region and floods in another. At sea, they alter the ocean’s freshwater inventory and salinity. Considering that the ocean is about 71% of Earth’s surface and that the hydrological cycle, including its relationship to latent heat transport, is so central to the climate system, the marine component of the hydrological cycle is surprisingly poorly observed. Sea-surface salinity (SSS) serves as a proxy for the marine hydrological cycle.