Most of the extra energy in the climate system ends up in the ocean rather than the air. Measuring it requires instruments that sample well below the surface, because the surface alone is a poor guide.
Why the surface reading is not enough
Sea surface temperature describes a thin skin of water that responds quickly to sunshine, wind and passing storms.
That skin can be warm while the water beneath is cold, or cool while a deep reservoir of heat sits below it. Wind mixing rearranges the two within days.
Heat storage is a question about the whole column, so an instrument that only touches the top of it cannot answer the question being asked.
How profiling floats work
The core of the modern network is a fleet of autonomous floats that drift with the currents at depth rather than staying in one place.
On a repeating cycle each float changes its buoyancy, sinks to a set depth, then rises slowly while recording temperature and salinity all the way up.
At the surface it transmits the profile by satellite, then sinks again. No ship is required, which is why coverage extends into oceans that vessels rarely cross.
Why salinity is recorded alongside temperature
Seawater density depends on both temperature and salt content, and density governs whether a layer sinks, rises or resists mixing.
Without salinity, a temperature profile cannot be converted into a statement about how stable the column is or how readily heat will move downward.
The pairing also tracks the freshening of polar water, which alters circulation patterns that redistribute heat over long distances.
How the profiles become a single number
Individual profiles are irregular in space and time, so they are gridded into a common framework before anything is totalled.
Heat content is then computed as the energy stored in a defined depth range across that grid, which is why published figures always name the layer they cover.
Comparisons that ignore the depth range are meaningless, since a shallow layer and a deep one behave differently from year to year.
Why the ocean signal is steadier than the air
Water stores far more heat per unit of volume than air, so its temperature changes slowly and its record carries much less year-to-year noise.
That makes ocean heat content one of the clearest indicators available, because a single unusual season barely disturbs it.
The tradeoff is lag. The ocean responds slowly in both directions, so heat already stored continues to influence the atmosphere for a long time.