Diapyknische Vermischungsprozesse in den Auftriebsgebieten des tropischen Atlantiks
Final Report Abstract
In the tropical regions of our planet, climate is very sensitive to changes in the oceans’ surface temperature. In the eastern equatorial Atlantic and in the oceanic region off Northwest Africa, sea surface temperature changes drastically on seasonal and year-to-year time scales, thereby affecting the distribution of precipitation over Africa and northeastern South America. Additionally, elevated primary production occurs in these two oceanic upwelling regions and they are known to be a source of greenhouse gases for the atmosphere. Although it was assumed that oceanic processes play an important role for the variability of sea surface temperature in those regions, the processes controlling sea surface temperature were not fully understood. In this project, we investigated the contribution of mixing by mechanical turbulence in the deeper ocean to the heat loss of the sea surface and to the flux of nutrients and greenhouse gases from the deep ocean to the sea surface. To obtain accurate datasets of turbulence from the deeper ocean, existing instrumentation was improved and a measurement campaign consisting of 20 research cruises was conducted. Our measurements showed that both upwelling regions are mixing hot spots and turbulence below the sea surface is much larger than in other regions of the ocean. The processes sustaining turbulence in the two regions, however, differ. While tidal currents interacting with the continental slope are the major energy source for turbulence in the upwelling region off Northwest Africa, zonal currents in the equatorial regions and large-scale oceanic equatorial waves feed energy to turbulence in the equatorial upwelling region. For the equatorial upwelling region, our data analysis showed that mixing below the sea surface is the dominant mechanism for cooling the sea surface in boreal summer, when sea surface temperature drops by about 6°C within two months. Similarly, mixing is also the controlling mechanism for transporting nutrients and greenhouse gases to the sea surface. Present climate models as well as general ocean circulation models are not capable of resolving mixing processes in the ocean. Thus, these processes need to be parameterized using other modeled variables. We tested several parameterizations used in today’s climate and ocean models against the data collected in the upwelling regions. The results showed that only few of the parameterizations have skill in accurately describing mixing in the ocean. Apart from advancing understanding of the physical processes occurring in the upwelling regions of the tropical Atlantic, our results will help improving climate and general ocean circulation models to a more realistic representation of sea surface temperature in the tropics, particularly in the upwelling regions. A somewhat unexpected development during the course of the project was the acceptance of our measurement techniques and observational strategies by the scientific community. The measurement program of this project was originally planned to include turbulence observations during eight field campaigns to the equatorial and the Mauritanian upwelling region. By the end of the project, the Emmy Noether junior research group had completed 20 field campaigns – more than twice as many as planned. On 26th October 2007, a press releases was issued by IFM-GEOMAR that was printed in several local newspapers in Germany. The title of the contribution in German language was “Kaltes Wasser am Äquator beeinflusst Monsun über Westafrika”. A second press release was issued on 16th November 2009 by GEOMAR. It was titled “Kleine Ursache – große Wirkung”. It was also adopted by several regional and nationwide newspapers in Germany. The text can be found online at http://www.geomar.de/news/article/kleine-ursache-grosse-wirkung-1/.
Publications
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(2008), Oxygen tongues and zonal currents in the equatorial Atlantic, J. Geophys. Res.
Brandt, P., V. Hormann, B. Bourles, J. Fischer, F. Schott, L. Stramma, and M. Dengler
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(2008), Stacked jets in the deep equatorial Atlantic Ocean, J. Geophys. Res., 113, C04003
Eden, C., and M. Dengler
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(2008), The oxygen minimum zone in the North Atlantic south and east of the Cape Verde Islands, J. Geophys. Res.
Stramma, L., P. Brandt, J. Schafstall, F. Schott, J. Fischer, and A. Körtzinger
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(2010), Tidal induced mixing and diapycnal nutrient fluxes in the Mauritanian upwelling region, J. Geophys. Res., 115, C10014
Schafstall, J., M. Dengler, P. Brandt, and H. Bange
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(2010), Turbulente Vermischungsprozesse und Zirkulation im Auftriebsgebiet vor Nordwestafrika, Ph.D. Thesis, Mathematisch-Naturwissenschaftlichen Fakultät der Christian-Albrechts-Universität zu Kiel, 219 pp.
Schafstall, J.
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(2010), Upwelling and associated heat flux in the equatorial Atlantic inferred from helium isotope disequilibrium, J. Geophys. Res., 115, C08021
Rhein, M., M. Dengler, J. Sültenfuß, R. Hummels, S. Hüttl-Kabus and B. Bourles
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(2011), A one dimensional modeling study of the diurnal cycle in the tropical Atlantic at the PIRATA buoys during the EGEE3 campaign, Ocean Dyn., 61, 1–20
Wade, M., G. Caniaux, Y. DuPenhoat, M. Dengler, H. Giordani and R. Hummels
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(2011), Equatorial upper-ocean dynamics and their interaction with the West African monsoon, Atmospheric Science Letters, 12, 24-30
Brandt, P., G. Caniaux, B. Bourles, A. Lazar, M. Dengler, A. Funk, V. Hormann, H. Giordani and F. Marin
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(2011), Interannual atmospheric variability forced by the deep equatorial Atlantic Ocean, Nature, 473, 497-500
Brandt, P., A. Funk, V. Hormann, M. Dengler, R. Greatbatch and J. Toole
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(2012), Diapycnal diffusivity at the upper boundary of the tropical North Atlantic oxygen minimum zone, J. Geophys. Res., 117, C09016
Banyte, D., T. Tanhua, M. Visbeck, D. W. R. Wallace, J. Karstensen, G. Krahmann, A. Schneider, L. Stramma and M. Dengler
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(2012), On the variability of turbulent mixing within the upper layers of the Atlantic Cold Tongue region, Ph.D. Thesis, Mathematisch-Naturwissenschaftlichen Fakultät der Christian-Albrechts-Universität zu Kiel; 153 pp.
Hummels, R.
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(2012), Sea-to-air and diapycnal nitrous oxide fluxes in the eastern tropical North Atlantic Ocean, Biogeosciences, 9, 957-964
Kock, A., J. Schafstall, M. Dengler, P. Brandt, and H. W. Bange
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(2013) Diapycnal oxygen supply to the tropical North Atlantic oxygen minimum zone Biogeosciences (BG), 10 . pp. 5079-5093
Fischer, T., Banyte, D., Brandt, P., Dengler, M., Krahmann, G., Tanhua, T. und Visbeck, M.
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(2013), Seasonal and regional variability of upper ocean diapycnal heat flux in the Atlantic Cold Tongue, Prog. Oceanogr., 111, 52-74
Hummels, R., M. Dengler, and B. Bourlés
