How waves and mixing determine coastal upwelling systems

They are among the most productive and biodiverse areas of the ocean: the coastal upwelling regions located along the eastern borders of the Atlantic and Pacific Oceans. There, equatorial winds blow near-surface waters away from the coast. This brings cold, nutrient-rich water from the depths to the surface, inducing the growth of phytoplankton and providing the basis of a rich marine ecosystem in these regions.

However, in some tropical regions, productivity is high even when favorable upwelling winds are weak. An international team of researchers has studied the physical mechanisms behind upwelling off the Angolan coast. They found that the combination of trapped coastal waves and increased mixing on the shelf controls productivity in this system. Their conclusions, published in the journal Scientists progresscould help predict the strength of seasonal productivity spikes.

“Productivity in the upwelling region off Angola shows strong seasonal fluctuations,” explains corresponding author Mareike Körner, Ph.D. student at the Physical Oceanography Research Unit of the GEOMAR Helmholtz Center for ocean research in Kiel. “The main upwelling season occurs during the austral winter, from July to September. During this period, primary productivity is very high in the waters off the Angolan coast and, therefore, fishing is important.”

Waves within the ocean play a crucial role for productivity, causing the up and down movement of cold, nutrient-rich water on seasonal timescales. These waves are not generated locally off the coast of Angola but come from the equator. There, seasonal wind fluctuations create waves that move eastward along the equator.

Once they reach the eastern edge of the equatorial Atlantic, they excite trapped coastal waves, which propagate poleward along the African coast. On their way, these trapped coastal waves carry nutrient-rich waters to the Angolan plateau. Strong tidal mixing on the shelf brings nutrients to the surface, where a bloom of phytoplankton is induced. These plankton blooms can vary from year to year, depending on the intensity and arrival time of trapped coastal waves.

For their study, the researchers combined hydrographic, oxygen, nitrate and satellite data, as well as a regional ocean model.

Körner says: “Upwelling off Angola is caused by waves that are excited at the equator and then propagate along the African coast. This offers potential for predicting the strength and timing of peak biological productivity off Angola on seasonal timescales. »

A better understanding of the driving mechanisms of this coastal upwelling system in southwest Africa is also crucial for assessing possible future changes, such as the effects of climate change or other human impacts, in this important ecosystem. marine.

At GEOMAR “Upwelling in the Atlantic Ocean” is a research focus within the GEOMAR 2030 strategy. Since 2013, GEOMAR has been carrying out research in this area and has established in-depth cooperation with Angolan colleagues.

Seven research campaigns carried out by the Physical Oceanography Research Unit have provided extensive data on the mixing and distribution of nutrients on the shelf. In addition, an underground anchorage has been collecting data on various parameters such as current speed, temperature, salinity and oxygen since 2013.