The last November 22th the Copernicus Sentinel-3A satellite captured a long stream of discolored water over the entire Adriatic Sea coast of Italy. This singular sight from the outer space is fascinating and the wide impacted area is perfectly exploited by the natural-color RGB image showed below. What’s that about and how it came into being? At a first glance it seems to be related to a massive bloom of phytoplankton or suspended matters over the sea’s surface. Indeed all those episodes are connected to an already known phenomenon: the upwelling, a periodic and current event that occurs over the italian adriatic coasts and other parts around the World.
Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler and nutrient-rich water towards the sea surface, replacing the warmer and drained surface water. The nutrient-rich upwelled water stimulates the growth and reproduction of primary producers such as phytoplankton: this explains the massive bloom showed above. Due to the biomass of phytoplankton and presence of cool water in these regions, upwelling zones can be identified by cool sea surface temperatures and high concentrations of chlorophylla.
OLCI instrument, thanks to its large variety of processed data, allows us to study and better understand the nature of this biological phenomenon. Indeed the instrument carried by Sentinel-3A can detect the clorofilla, in order to evaluate eventual phytoplankton blooms. The image below compares the natural-colour image with the distribution of algal pigment concentration, processed with OLCI data. The concentration is expressed in mg/m^3 and is greater in the waters near the coastlines, i.e. the water masses appearing milky and discolored in the natural colors image. This is the first evidence of the upwelling, namely the presence of phytoplankton and nutrient-rich water.
Left: Natural Colors Image – Right: Algal Pigment Concentration
The second evidence of the upwelling can be reached in the ascent of deep and cold water, caused by the wind. In the case of the Adriatic Sea the wind blows in parallel way with respect to the coastline and involves the movement of big masses of water, masses ascending from the seabed to replace the warmer waters on the sea surface. The image below has been captured by the SLSTR instrument, detecting the sea surface temperature and so showing that the water masses near the coast, namely the ones involved in the upwelling, are colder than the open sea.
Upwelling near the coast is also associated with elevated amounts of suspensates, which are not only planktonic: upwelled surface waters are indeed enriched in organic or inorganic suspended matter also due to river flowing into the sea or to winds from the hinterland carrying many sediments. OLCI instruments can detect the total suspended matters, a water quality parameter used for example to assess the quality of wastewater or water in general. The suspended matters is espressed in g/m^3 and is showed in the left part of the image below: the upwelled waters are rich in sediments and this is the third evidence of this phenomenon.
Left: Total Suspended Matter- Right: Algal Pigment Concentration
At the end the animated gif below shows the variation of the upwelling effect over the Adriatic waters near the italian coastlines thanks to several consecutive images acquired in the last months, then highlighting the periodicity of this event.
At the end here below an animated gif showing the 3 different effects of the upwelling compared to the natural colors images
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 Sarhan T, Lafuente JG, Vargas M, Vargas JM, Plaza F. (1999). Upwelling mechanisms in the northwestern Alboran Sea. Journal of Marine Systems, 23: 317-331.
 The influence of upwelling on suspended matter and shelf sediments off southeastern Brazil. C. P. Summerhayes U. de Melo H. T. Barretto
 Distribution of suspended matter in a coastal upwelling area. Satellite data and in situ measurements. J-M.Froidefond P.Castaing J-M.Jouanneau