Not very hot!
But not very surprising either, when you consider that the average depth of the oceans is 3,682 metres and that only the first 200 metres are bathed in sunlight. In the ocean, the water is mainly heated by the sun’s rays. As the temperature of the water is scientifically linked to the amount of heat contained in a drop, these first 200 metres constitute what is known as the “euphotic” layer, the layer to which the light penetrates and where photosynthesis can develop.
Logically enough, the warmest waters are found on the surface of equatorial regions, where solar heating is at its maximum. On the other hand, the poles are home to the coldest waters, which can sometimes reach negative temperatures in winter. In the western equatorial Pacific, oceanographers refer to a “warm pool” of water, which extends over several kilometres and reaches temperatures close to 30°C.
Oceanographers are continually trying to measure temperature because it is an essential physical property of seawater. In the ocean, salinity and water temperature constitute what are known as ‘tracers’: together they allow the density of seawater to be calculated, the density that constitutes the identity card of each of the masses of water that perpetually stir the oceans.