mon océan & moi

What temperature is and why is it important to measure it ?

Intuitively, the temperature is directly linked to the feeling of warm and cold. Scientifically spoken, its definition is much more complex. The unit used is the Celsius degree (°C), but it is also possible to find the Fahrenheit degree (°F) or the Kelvin (K).

Temperature measures help to identify waters, understand some currents and explain the development of some plankton species. At the global scale, ocean temperature plays an important role on climate.

How to measure it ?

In the ocean, the temperature is measured by sensors called “thermistors”, and they are generally coupled with others sensors that measure salinity and pressure. We find them on most instruments used for oceanographic observations: gliders, profiling floats, buoys etc.

Since 1980s, and thanks to the satellites, it’s also possible to measure indirectly the sea surface temperature (SST). This kind of data is very useful for studying oceanic circulation and currents. 

How is distributed in the ocean ?

• Vertical profiles

Normally, in the oceans, the temperature decrease from the surface to the sea bed. This decrease is not constant and depends on the depth. It is possible to noticing a layer where the temperature drops very quickly: the thermocline.

Illustrations bellow shown vertical profiles of temperature, characteristic of an area characterized by a strong seasonality (for example Mediterranean sea).  Focusing on surface layer, we can see low temperature in winter (Fig 1B), relatively constant over 50 meters. In fact, in winter, when there is wind and cool temperature, the water at the surface becomes denser. These create vertical water thinking inducing mixing and homogenization of parameters such as temperature.  The area where parameters are homogenate is called mixing layer.
Conversely, in summer, sea surface temperature increase, and can reach more than 20°C (Fig 1A). In this condition mixing is not observed. This is the stratified period.

In both season, the thermocline is located about a hundred meters, and separate warm surface water and cold deep water. Note that the deep water temperature  is more or less constant.

• Satellite images of SST
This picture (élément graphique ci-dessous) come from the American satellite MODIS, which measure SST every day.  Maximum temperatures are observed along the Ecuador and the lowest values are close to the poles.  In this example, temperatures vary from -2°C to +35°C, in May 2001.