Investigation of deep-water mixing in the Baltic Sea with the help of a tracer release experiment

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The purposeful release of a long-lived tracer is an elegant and accurate method to estimate integrated parameters of mixing and dispersal in the ocean. In this project, a tracer release experiment is proposed in order to investigate deep-water mixing in the central Baltic Sea. In collaboration with partners from the US, Sweden, and Poland, a long-lived tracer will be injected in August 2007 in the deep water of the Eastern Gotland Basin, and its spreading will be observed during a series of subsequent cruises in 2007 and 2008. The experiment will be accompanied by moored instrument arrays, large-scale hydrographic surveys, and detailed turbulence measurements aimed at an improved understanding of a number of long-standing research questions: What is the effective vertical transport of dissolved substances between the usually deoxygenated deep layers and the upper layers of the central Baltic during stagnation periods? Which mixing mechanisms (internal-wave mixing, boundary mixing, gravity current entrainment, etc.) can be identified in order to explain the effective mixing of the tracer, and what is their relative importance? In a separate proposal, submitted to the DFG in August 2006, a scientific consortium of three German institutes has suggested the acquisition of an Ocean Tracer Injection System (OTIS), which is the key component of any tracer release experiment. In the OTIS-proposal, a series of upcoming tracer release experiments has been sketched, the first of which is described in detail in the following.