What Is Sea Surface Salinity? — How Salty the Ocean Is, River Plumes, and the Global Water Cycle

It is a measure of how much dissolved salt is in the very top layer of the ocean — chiefly sodium and chloride, plus magnesium, sulphate and others. It's usually given in practical salinity units (PSU), which are very close to grams of salt per kilogram of seawater. The global open-ocean average is about 35 PSU, meaning roughly 35 grams of salt in every kilogram of water. Salinity matters because, together with temperature, it sets how dense seawater is — and density differences are one of the forces that drive the slow, deep circulation of the whole ocean (NASA / NOAA salinity science, 2024–2026).

Because salt stays behind when freshwater comes and goes. Where rain falls heavily or great rivers empty into the sea, freshwater dilutes the surface and salinity drops. Where the air is hot and dry and evaporation runs ahead of rainfall, water leaves as vapour but the salt is left behind, so salinity climbs. That's why the rainy equator and the river mouths are fresh, while the subtropical zones around 20–30° north and south — under the dry descending air of the great atmospheric circulation — are the saltiest patches of open ocean (NASA / NOAA, 2024–2026).

The saltiest open ocean sits in the subtropical gyres, around 20–30° north and south, where evaporation dominates — the North Atlantic subtropics are especially salty. Saltier still are the nearly-landlocked Mediterranean and the Red Sea, where intense evaporation and little river input push salinity well above the ocean average. The freshest surface water is in the rainy tropics and near the great river outflows: the Amazon plume spreading across the tropical Atlantic, the Congo, and the Ganges–Brahmaputra freshening the Bay of Bengal. The Arctic surface is also relatively fresh, fed by rivers and melting ice (NASA / NOAA, 2024–2026).

It uses a sequential scale from fresh to salty. Cool purples and blues mark fresher water — below the ocean average — concentrated near river plumes and along the rainy equator. Greens sit near the open-ocean average of about 35 PSU. Yellows mark the saltiest water, in the evaporating subtropical gyres and the Mediterranean and Red Sea. Land and missing data are transparent, so the dark continents show through. The colours come baked into the satellite tiles, so what you see is the published salinity field, not a value we've recoloured.

Salt changes how the sea surface emits faint natural microwave radiation, and dedicated instruments can detect that tiny signal. NASA's SMAP mission — built mainly to measure soil moisture on land — also reads sea-surface salinity over the ocean. The signal is small and noisy, so the data are smoothed into an 8-day running mean rather than a single instant, which is what this layer shows: the latest published 8-day average, updated daily. It's a broad-scale picture of where the ocean is fresher and saltier, not a precise reading at one buoy (NASA SMAP / JPL, 2024–2026).

Salinity is effectively a rain-gauge for the entire planet, painted onto the sea. Because the sea remembers where freshwater entered (rivers, rain, melting ice) and where it left (evaporation), the salinity map traces out Earth's great evaporation and rainfall belts directly. And as the climate warms, that pattern is intensifying — the already-fresh regions are getting fresher and the salty regions saltier, a fingerprint of a speeding-up water cycle that satellites can now watch from space (NASA / NOAA, 2024–2026).

Seawater's density depends on both temperature and salinity, and dense water sinks. In the far North Atlantic, surface water that is both cold and salty becomes dense enough to sink to the deep ocean, helping power the global overturning circulation — the slow 'conveyor belt' that moves heat between the tropics and the poles over centuries. Salinity is one half of what makes that engine run, which is why a freshening of the North Atlantic from melting ice is something climate scientists watch closely. The salinity layer pairs naturally with the Sea Surface Temperature and the Great Ocean Conveyor layers for that reason.

The Amazon discharges an enormous volume of freshwater — by far the largest river outflow on Earth — and that freshwater is lighter than seawater, so it spreads out as a thin lens across the surface of the tropical Atlantic rather than mixing straight down. From orbit that lens reads as a large, fresh (purple/blue) plume reaching hundreds of kilometres offshore and drifting with the currents, sometimes curling far up toward the Caribbean. The Congo and the Ganges–Brahmaputra produce smaller versions of the same signature. They're a vivid reminder that the ocean's surface chemistry is shaped by the land (NASA / NOAA, 2024–2026).

It's live: the tiles come straight from NASA's SMAP sea-surface-salinity product via NASA's imagery service, showing the latest published 8-day running mean, served keyless and free. Two honesty notes. First, satellite salinity is a smoothed, broad-scale measurement — an 8-day average at coarse resolution — so it shows the large patterns (river plumes, salty subtropics, fresh equator) rather than fine detail or a precise value at any one point, and accuracy is lower in very cold polar water and right at the coast. Second, the salinity number you're seeing is NASA's, unmodified; we only place its tiles on the map. Read it next to the Sea Surface Temperature and Great Ocean Conveyor layers for the full density story.