Ultraviolet light from the sun has a critical influence on life in, and the chemistry of, our oceans. Consequently, these effects have to be reflected as accurately as possible in climate models, to deliver accurate forecasts. Experts from the Alfred Wegener Institute (AWI) and the Institute of Environmental Physics (IUP) at the University of Bremen have now succeeded in using readings taken by the TROPOMI sensor on board the satellite Sentinel-5P to calculate the diffusion of UV light in the oceans. The study, which was just released in the journal Frontiers of Marine Science, shows that the new method can yield global, seamless and precise data on UV light in the oceans.
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Sunlight plays a central role in the oceans of the world. It warms the upper layers of the water and consequently has a substantial influence on the global climate. Moreover, countless single-celled algae use the light for photosynthesis to produce tremendous amounts of biomass. As such, solar energy forms the basis of life for nearly all marine organisms – from tiny amphipods to massive whales.
![[Translate to English:] Triaxus](/fileadmin/_processed_/8/2/csm_20180508_Triaxus_JessicaVial_8bbd06bbcb.jpg "Triaxus")
“At all levels of marine and climate research, highly precise data on sunlight is indispensable,” says Astrid Bracher from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI). “Climate models are only reliable when we have comprehensive information on the intensity, absorption and penetration depth of sunlight in the oceans, allowing us to make correct statements on the oceans’ important services – like primary production on the part of phytoplankton. In the visible spectrum of light, the sensors now used on numerous satellites offer a good basis of data. In the shortwave ultraviolet range of the spectrum, until recently there was no such basis. As a result, information on the UV light in the oceans had to be indirectly derived from the satellite data on the visible light, which involved empirical methods and often produced imprecise results.”
In the oceans, too, UV light can harm the cells of microalgae and microorganisms. In addition, it can change the chemistry of the oceans: it affects the bioavailability of important trace metals like iron and copper, as well as the exchange of trace gases between the water and atmosphere. “If our goal is that all these effects are precisely reflected, for instance in climate models, we have to do away with the uncertainties in the available data,” says Bracher. “In our study, we show a new method that can achieve exactly that.”
For the study, which was financed by the European Space Agency (ESA), the team of researchers led by Bracher including the study’s first author, Julia Oelker, used data from the TROPOMI spectrometer on board the Sentinel-5 Precursor satellite. Sentinel-5P is an Earth observation satellite launched by the ESA in 2017 as part of the European Union’s Copernicus Programme. Its TROPOMI (TROPOspheric MOnitoring Instrument) sensor is one of the very first satellite sensors to deliver with high spatial resolution data on the ultraviolet spectrum and can detect the UV light reflected back from the oceans with a high spectral resolution of 0.5 nanometres.
![[Translate to English:] Einsetzen Radiometer Ramses (Trios)](/fileadmin/_processed_/f/5/csm_20180517_RamsesEinsetzen_JessicaVial_791cb747d9.jpg "Launch Radiometer Ramses (Trios)")
On the basis of this data, the study team calculated the “Diffuse Attenuation Coefficient” (Kd) for UV light, which represents the extent to which light is scattered with increasing water depth. “The calculated values for the Kd match very well with the data gathered directly in the Atlantic during a Polarstern expedition in 2018,” the AWI expert explains. “The majority of this high-resolution spectral data, from UV light to infrared, was continually measured with a radiometer mounted on a platform that could be raised or lowered to water depths of up to 200 metres – called Triaxus. In the course of several transects along the ship’s route from South America to the English Channel, the platform was towed behind the ship. The good matching shows that our method could potentially yield reliable data on the UV light in the oceans for roughly 95 percent of the world’s waters. Especially in terms of climate modelling, this could help to substantially improve projections.” The ESA plans to launch additional Sentinel satellites with UV-light spectrometers from 2023. In this way, over the next few years the logistical basis for applying the new method globally and consistently will be established.
Original Publication:
Oelker J., Losa S. N., Richter A., Bracher A. (2022) TROPOMI-retrieved underwater light attenuation in three spectral regions in the ultraviolet to blue. Frontiers in Marine Science 9. doi: 10.3389/fmars.2022.787992