Sea surface temperature

The sea surface temperature (often abbreviated SST). is, in the definition of the German Weather Service, the water temperature one meter below the immediate sea surface. More generally, the term is used for temperatures at depths of one micrometer to a few meters.

The sea surface temperature is an important meteorological and climatological parameter, as it determines both the thermohaline circulation of the ocean and its heat exchange with the Earth's atmosphere. The complete heat content of the oceans is determined, among other things, with submersible probes and abbreviated as OHC (ocean heat content). The heat content of the oceans has gained importance in climatology in recent years.

Important parameters that are directly or indirectly based on sea surface temperature and its spatial differences are the Southern Oscillation Index and the North Atlantic Oscillation. The SST therefore plays a major role for El Niño and La Niña as well as for monsoon phenomena, especially the Indian monsoon.

There are several ways to measure SST, and there can be sometimes considerable differences in measurement accuracy between these methods, as they are subject to errors of different magnitudes. Initially, direct measurement with thermometers was used, either manually by measuring on a water sample (bucket) or automatically with ships. However, the data have considerable inaccuracies because, for example, measurements are not always taken at the same depth due to a different draught or inconsistent position of the measuring device. Better and much more reliable data is therefore obtained from fixed buoys. Their measurement data are usually transmitted via satellites and automatically evaluated. The advantage is not only that the depth of the measurement is always the same, but also that the position is fixed. This provides a continuous series of measurements for defined points and avoids distortion of the data set due to spatial and temporal differences in the data situation, depending on whether or not a ship with measuring equipment is present at a particular location. However, another problem is that the buoys are only suitable for high seas to a very limited extent and therefore often only cover the coastal picture.

Since the 1980s, therefore, satellite measurements have been increasingly used, which have the advantage of covering the entire area in almost the same time, in contrast to the above point measurements. Here, the ocean is scanned with electromagnetic radiation in the infrared wavelength range (see image description of the figure above right). The importance of satellite measurements can be seen in direct comparison with their alternatives. The satellites provide a high-resolution overall view in a comparatively very short period of time. For example, it takes a ship travelling at ten knots about ten years to cover the same section as a satellite does within just two minutes. However, measuring SST by satellite also has disadvantages. It only records the uppermost millimetre of the ocean and therefore does not represent the real SST, due to the depth-dependent warming effect of the sun, cooling at night and surface evaporation. The comparability of direct temperature measurements by buoys and ships with the satellite data is therefore very limited, which leads to considerable evaluation problems in the case of relevant temperature differences of often one tenth of a degree. In addition, satellite measurements are disturbed by cloud cover and may therefore themselves show inconsistencies if these disturbances are not compensated for. However, these problems are minor compared to the advantages of satellite-based measurement. A uniform SST data set is regularly obtained from satellite and in-situ measurements.