What Is an Argo Float — and What Does “Measured, Not Modelled” Mean?

Almost everything you can put on an ocean map is, in some sense, a guess. Sea-surface temperature comes from satellites reading the skin of the water. Currents and the temperature of the deep are model outputs — physics-based estimates, because no satellite can see a kilometre down. They are good guesses, carefully made. But they are guesses.

Argo is the exception. An Argo float is a real instrument, physically in the water, recording real numbers at real depths. That is the whole point of this layer: it is the ocean measured, not modelled — and it is the ground truth that the models are checked against.

What a float actually is

An Argo float is an autonomous robot about the size of an adult person. It has no propeller and cannot steer. What it can do is change its own buoyancy — pumping oil between an internal reservoir and an external bladder to make itself a little heavier or lighter than the water around it. That single trick is enough to make it rise and sink on command.

Left to drift, a float spends most of its life "parked" around 1,000 metres, carried wherever the deep current takes it. Then, on a cycle of about ten days, it runs its routine.

How one float reads the deep

1. It dives. From its parking depth the float sinks to about 2,000 metres — into the cold, dark, high-pressure deep.
2. It rises and measures. Slowly ascending, it samples temperature and salinity against pressure (which stands in for depth) at hundreds of levels. That stack of readings is a profile — the vertical column you see when you tap a float on the map.
3. It surfaces and sends. At the top it fixes its position by satellite, transmits the profile, and then sinks back down to start again.

Each float repeats this for years until its battery finally runs out. The result is a steady drip of vertical slices through the ocean, from all over the planet, arriving continuously.

Core floats and BGC floats

Two kinds drift out there, and the map colours them differently.

Core floats (shown in blue) measure the physics: temperature and salinity. These two numbers, with depth, describe how heavy the water is, how it layers, and how it moves — the engine of ocean circulation and a huge part of how the planet stores heat.

BGC floats (biogeochemical, shown in green) carry extra sensors for dissolved oxygen, nitrate, pH and chlorophyll. They read not just the physics but the chemistry — how much the ocean is breathing, where life is feeding, and how fast seawater is acidifying as it absorbs our carbon dioxide.

Why "measured, not modelled" matters

Below the surface, the ocean is mostly invisible to us. Satellites stop at the skin. So the deep currents, the heat hidden hundreds of metres down, the slow changes in saltiness — all of it has to be inferred unless something is actually down there taking the reading. Argo is that something.

Because the floats measure directly, their profiles are the reference standard. Climate and ocean models are tuned to match Argo; estimates of how much heat the ocean has absorbed lean heavily on it; weather and seasonal forecasts that depend on ocean state use it. When this map shows you a float's temperature curve bending through the thermocline, that bend is not a simulation — it is the shape of the real water that float swam through, on the date stamped beside it.

What it can't (yet) do

Argo is not the whole ocean. The standard array profiles to about 2,000 m, which is deep but not the bottom; a newer Deep Argo effort is pushing specialised floats to 4,000–6,000 m, and the hadal trenches remain beyond the routine network. Coastal seas, under-ice regions and the very deepest water are still thin in coverage. So a float's profile is an honest, complete record of its column to 2,000 m — and silent about everything below that.

That honesty is the point. This layer doesn't invent a number where there isn't one. It shows you exactly what the instrument saw, dated to the day it saw it — and where the measurement stops, so does the line.

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Float positions and profiles on LiveEarthViewer come from Argovis (University of Colorado Boulder), distributing data from the international Argo program — a free public good, used with attribution. Figures reflect the array as of June 2026.