Investigation of deep-water mixing in the Baltic Sea with the help of a tracer release experiment
Zusammenfassung der Projektergebnisse
In this joint research initiative between two Leibniz-Institutes (IOW and IFM-GEOMAR), we have studied deep-water mixing processes in the central Baltic Sea, using a combined approach including a tracer release experiment, ship-based hydrographic and turbulence measurements, and long-term moored instrumentation. More specifically, our goals have been: (i) to quantify global vertical mixing in the deep Eastern Gotland basin by means of a tracer release experiment; (ii) to identify the physical processes responsible for the observed basin-scale mixing rates, in particular to distinguish between the contributions of local vertical mixing and boundary mixing processes; and (iii) to determine the particle affinity of the tracers used in our experiment. Investigation of the spreading of a small amount of the inert tracer gas CF3SF5, injected in September 2007 in the deep-water of the Gotland Basin, during 5 subsequent ship surveys until February 2009 allowed us to compute horizontal and vertical mixing coefficients, and to draw conclusions about the relative importance of boundary and interior mixing processes. Before BaTRE, the compound CF3SF5 had only been used once in a small-scale pilot study, and never in anoxic/sulfidic waters, such that our experiment could also provide some relevant information about the long-term stability of this new tracer (the standard for future tracer experiments in the ocean) under extreme conditions. Lateral tracer homogenization within the basin was found to occur on a time scale of several months, corresponding to a lateral diffusivity of the order of 10 m2 s-1 for scales of the order of 10 km. Vertical exchange coefficients were of the order of 10-5 m2 s-1 on the average with a pronounced seasonal variability. As one of the main results of this project, the tracer experiment has revealed that mixing processes near the lateral slopes of the basin provide the by far largest contribution to overall vertical deep-water mixing. This new picture of boundary-driven deep-water mixing in the central Baltic Sea has some important implications for the interpretation of mixing transport pathways of ecologically relevant substances, especially for those with a source at the sediment (phosphate, H2S, etc.). The analysis of the physical processes responsible for the observed mixing yielded interesting and somewhat unexpected results as well. Contrary to the situation in the open ocean, and different from previous speculations for the Baltic Sea, internal waves were found to provide only a negligible contribution to the overall mixing. Topographic waves and a deep rim current in the basin were identified as the major energy sources for deep-water mixing. The indirect (but strong) evidence for boundary mixing as the dominant mixing process already inferred from the tracer experiment was further corroborated by direct turbulence microstructure measurements, revealing highly turbulent bottom boundary layers with thicknesses reaching several tens of meters. Since CF3SF5 is a relatively new oceanic tracer, and only little is known about its behavior in waters with high particle load, laboratory experiments were carried out to assess the CF3SF5 affinity to particles. Particles typically found in the Baltic Sea (quartz, calcium carbonate, clay minerals) and particles of organic origins representing typical plankton groups (diatoms, dinoflagelates and blue algae) were exposed to the tracer over different time periods at different temperatures. No affinity of the tracer to quartz was seen, whereas for the other particles a weak affinity was documented. Despite their tremendous importance for the functioning of the ecosystem, deep-water mixing processes in the Baltic Sea were previously not well explored, and known as a notoriously weak point in regional numerical circulations models. The results from this project therefore provide an important step towards a better understanding of deep-water mixing processes, their overall impact on the ecosystem, and their representation in numerical models.
Projektbezogene Publikationen (Auswahl)
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BaTRE: a Trace Release Experiment in the deep Gotland Basin. Swedish Society for Marine Sciences, Marine Science Conference 2009, 18-19 November 2009, Lulea, Sweden
Tanhua,T. G. Rehder, P. Holterman, J. J. Waniek, L. Umlauf, O. Schmale, V. Mohrholz
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Hunting a new oceanic tracer, Eos, 89(43), 419-420, 2008
Umlauf, L., T. Tanhua, J. J. Waniek, O. Schmale, P. Holtermann, and G. Rehder
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Energetics of mixing in a stratified basin without tides. EGU General Assembly, May 3-7 2010, Vienna, Austria
Holtermann, P., L. Umlauf, and T. Tanhua
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Mixing rates in the deep Baltic Sea revealed by a tracer release experiment. Ocean Science Meeting, February 22-26, 2010, Portland, OR, USA
Tanhua,T. G. Rehder, P. Holterman, J. J. Waniek, L. Umlauf, O. Schmale, V. Mohrholz
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The Baltic Sea Tracer Release Experiment: Mixing rates and mixing processes. Baltic Sea Science Congress (BSSC) 2011, 22-26 August 2011, St. Petersburg, Russia
Holtermann, P., L. Umlauf
