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Why does heavy precipitation lower the salinity of surface seawater?

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Diagram illustrating Earth's environmental spheres, with labeled sections showing the geosphere (land and crust), biosphere (living things), atmosphere (air), and hydrosphere (water), each extending vertically from the Earth's surface. — hydrosphere Earth's environment includes the atmosphere, the hydrosphere, the lithosphere, and the biosphere.

Heavy precipitation lowers the salinity of surface seawater because it acts as a process of dilution. Salinity, which is the concentration of dissolved salts in water, is determined by the balance of processes that add or remove fresh water at the Earth's surface.12

When heavy precipitation occurs, it adds a significant volume of fresh water to the ocean surface. This influx of fresh water dilutes the existing concentration of salts, thereby depressing the salinity levels in that specific region. Conversely, in areas where evaporation rates are high, fresh water is removed from the surface, which concentrates the remaining salts and leads to elevated salinity. This relationship is clearly observable on a global scale; for example, the intertropical convergence zone, characterized by high levels of rainfall, leaves a distinct imprint on the oceans as a latitudinal band of lower surface salinity.12