Climate research in the tropical Atlantic is trying to find answers to several pressing questions of great social, economic and environmental importance: What are the dominant climate-related fluctuations in the tropics, what mechanisms drive them and what processes are responsible for such changes? Is prediction possible at all, and if so, what processes must it be correctly reproduced or parameterized in the forecast models? What impact do climate fluctuations have on biogeochemistry and marine life? While these questions are also of great interest in purely natural climate variations, they are especially relevant in the tropics regarding the local impact of anthropogenic climate change: What are the local effects of global warming and ocean acidification on climate variability, biogeochemistry and natural resources in the ocean? Which interactions between climate change and biogeochemistry must be included in the analysis of future climate scenarios? The physics of the ocean play a central role in all of these questions, including processes on all spatial and temporal scales, from the ocean-wide wind and density driven currents, planetary waves, to the small-scale mixing by internal waves and turbulence. They also include the exchange of heat, fresh water and momentum between ocean and atmosphere. The research conducted by the Physical Oceanography Department at GEOMAR | Helmholtz Centre for Ocean Research in Kiel in the tropical Atlantic is concerned specifically with tropical circulation fluctuations, causes of variations in sea surface temperature, and the supply of oxygen to the oxygen-deprived areas.
The dominant climate phenomenon in the tropical Atlantic is a north/south movement of the tropical rain belt, which on average extends from equatorial South America across the Atlantic to the southern part of West Africa. This rain band is caused by the convergence of the northeast and southeast trades and is commonly referred to as the Inter Tropical Convergence Zone. It moves with the sun to the north in early boreal summer and reaches its northernmost position in August. Then it moves back south as the season progresses. The associated rain areas are displaced not only over the ocean but also over continental Africa where they are referred to as the African monsoon. It is characterized by a two-time reversal of winds during the course of a year, leading to the formation of rainy and dry seasons. Despite the dominant annual cycle in the Atlantic, sub-Saharan Africa experiences strong multi-year to multi-decadal climate variability with great impact on water supply, agriculture, tropical diseases, and infrastructure of the respective countries.
While long-term, decadal and multi-decadal fluctuations may be associated with the general temperature gradient between the North and South Atlantic, for example due to changes in the thermohaline circulation, year-to-year fluctuations may depend more on the surface temperature of the adjacent seas. With the appropriate prediction of sea surface temperatures, it would thus be possible, in principle, to forecast the rainfall, its starting date and strength, not only for the next rainy season but for the next few years as well – a tremendous benefit for the people living in these regions. The atmospheric and oceanic processes responsible for the change in sea surface temperature, however, are still not understood well enough to be implemented correctly in models to allow an accurate prediction of the African monsoon.
Furthermore, there are always discoveries to be made of entirely new processes and their impact on climate, for example the deep equatorial jets. Our moored data have demonstrated that these deep jets propagate to the surface and cause a 4.5-year cycle of sea surface temperatures (Brandt et al. 2011). Their inclusion in climate models may facilitate more reliable predictions in the future.
In addition to its role in climate, the ocean is also an important source of food for the population in coastal states. The upwelling areas off West Africa are among the most productive and richest fishing areas in the world ocean. The interaction of wind and Earth's rotation causes the trade winds to blow parallel to the coast of West Africa, forcing the surface waters to the west, away from the coast. This water is resupplied along the shelf edge from below, feeding the plankton bloom with nutrients and allowing the establishment of an entire food chain, from phytoplankton to the great predators of the tropical oceans, such as tuna and swordfish. The high production in the upwelling areas, combined with the generally weak circulation in the eastern tropical oceans, has a serious impact on the oxygen content in the sea. The increased oxygen utilization caused by the decay of biological material, combined with a reduced supply of oxygen by mixing and currents, creates extended oxygen minimum zones beneath the surface layer in the eastern tropical oceans. Since a large part of the oxygen supply is provided by easterly currents from the western boundary, the Pacific represents an extreme example of extensive oxygen-free zones. The oxygen supply paths in the Atlantic Ocean, on the other hand, are much shorter and therefore more effective, causing the oxygen saturation to barely drop below 10 percent. Therefore, no oxygen-free regions exist in the Atlantic. In recent years, however, an expansion of these oxygen minimum zones in the tropical oceans was found on a global scale (Stramma et al. 2008). The absolute oxygen concentrations are declining, and the boundaries of oxygen minimum zones are expanding, thereby increasing the volume of oxygen-depleted water in the ocean. For any near-surface marine life which depends on oxygen, this means a reduction of habitat, and in some shelf areas it may lead to dead zones near the seabed. The reasons for this change are largely unknown: global warming and its impact on stratification and oxygen solubility in the ocean, ocean acidification by increased CO2 in the ocean, or circulation fluctuations could play a pivotal role.
L. Stramma (2008): Expanding oxygen-minimum zones in the tropical oceans. Science, 320, 655–658.
P. Brandt et al. (2011): Interannual atmospheric variability forced by the deep equatorial Atlantic Ocean. Nature, 473, 497-500.
Prof. Dr. Peter Brandt
Dr. Lothar Stramma
Dr. Marcus Dengler