To understand and predict changes in the atmosphere and ocean we need need detailed observations. These should cover the whole ocean at sufficient spatial resolution and be frequent enough to show seasonal variations.
Argo floats measure temperature (T) and salinity (S) every 10 days, most to a depth of 2000m. The data tell us about the surface ocean and are used in marine research and, for example, to improve seasonal weather forecasts.
Time series of temperature (left) and salinity (right) from a float in the North Atlantic. For more about how to interpret these plots see
Understanding Argo Data.
Why measure temperature and salinity?
Together temperature and salinity determine the density of seawater. When salinity increases, so does its density; when the water cools it becomes more dense. If surface water cools sufficiently, it may be come dense enough to sink. Similarly, when seawater freezes, the ice leaves behind a salty brine so dense that it sinks right to the deepest parts of the ocean.
When surface water has a similar density to deep water, it is easy for the two to mix. Similarly, when surface water becomes denser than the water below, it sinks. Together differences in temperature and salinity drive the vertical flow of ocean water known as the thermohaline circulation (THC).
The global system of surface and deep ocean currents transports heat from the tropics towards high latitudes and plays an important role of the ocean in the Earth's climate system.
Other Argo measurements
Some Argo floats also measure oxygen, nutrients and other parameters related to the biology and chemistry of the ocean. The development and testing of Argo floats that can do this is carried out in a programme known as Bio-Argo or Biogeochemical Argo.
The ocean's role in our climate is not limited to transporting heat around the world. Microscopic plants known as phytoplankton also play a role by taking up carbon dioxide during photosynthesis. When the plankton die and sink into the deep ocean, their carbon may remain stored there for hundreds of years. This is known as the biological carbon pump. Without it atmospheric carbon dioxide would be about 200 ppm higher than it is, and Earth would be quite a bit warmer.
Bio-Argo measurements allow us to learn more about marine ecology and the ocean's uptake and storage of carbon dioxide.
Argo data from the deep ocean
Most Argo floats go no deeper than 2000m. This is enough to measure variations in the surface ocean and the thermocline, but does not tell us what we need to know about the deep ocean. To fully understand ocean heat transport and the biological carbon pump, we need data right down to 4000-5000m depth.
Most of the information we have about the deep ocean comes from measurements made by research ships. Research cruises are expensive and take a long time, even for a few measurements, so data from the deep ocean are sparse, both in space and time.
As a result we still lack detailed knowledge of how temperature, salinity, oxygen and nutrients vary with time and space in the deep ocean. A programme known as Deep Argo aims to remedy this.
Deep Argo floats must be able to work even under extreme pressure - some 4-500 times higher than atmospheric pressure at the sea surface. The technological challenge involved is considerable, but it is now possible for floats to make continuous measurements all the way from the sea surface to well below 4000m.
The Deep Argo floats are still being tested. Once a larger number can measure below 2000m, they will give us much needed new information about the deep ocean.