Profiling Argo floats
Argo floats are autonomous measuring instruments with a length of 1.6 to 2.0 m and weight between 20 and 30 kg. Most of these floats are drifting for most of their time at a depth of 1.000 m, the so-called parking depth.
Nearly all floats have a measuring cycle of 10 days which starts with a further descend from their parking depth to 2.000 m. From there they rise slowly to the surface measuring temperature and conductivity of sea water and the pressure of the water column (depth).
Floats are de- or ascending by changing their volume, i.e. their density. Therefore they pump mineral oil from their pressure case into an external synthetic bubble under the float or from the bubble into the pressure case. After surfacing at sea surface the floats transmit their data via satellite to global data centers. Without limitations, all data are available for everybody free of charge.
Generally, Argo floats are deployed by research vessels and start profiling every 10 days until the batteries are exhausted. Since the beginning of Argo, the average life time of a float extended from four to six years. Currently, about 4.000 floats are drifting in the world ocean providing temperature and salinity profiles from the upper 2.000 m of the ocean for marine and climate research. This map illustrates the current status of the Argo programme. The map shows the positions of all floats which transmitted data within the last 30 days, colour coded with respect to the country which provided and deployed the float.
EuroArgo – The European contribution to the international Argo programme
European states have contributed to the international climate monitoring programme in the framework of their own national programmes. In July 2014 the first European Research Infrastructure Consortium (ERIC) for ocean monitoring, the so-called EuroArgo-ERIC, was established by the European Union for a better coordination and support of national activities. The Federal Republic of Germany is a foundation member of the EuroArgo-ERIC and one of currently 12 contracting parties of this consortium. The objective is a long-term and sustainable development of the European contributions to Argo. This is the only way to secure European support of Argo with the aim to supply and deploy a quarter of all floats (250 floats per year) for the Argo programme. The ERIC also allows a coordinated European data processing and quality control.
The EuroArgo-ERIC consortium will also be engaged in the advancement of Argo technology. One important item is the monitoring of ocean at depth of more than 2.000 m. Therefore, current pilot projects are testing floats diving down to 4.000 m and 6.000 m. The EuroArgo-ERIC also coordinates the implementation of floats equipped with sensors for the detection of chemical and biological parameters for the monitoring and assessment of deep ecosystems. A further aim of the technical development is the expansion of float measurements into ice-covered polar regions.
BSH is significantly involved in the management of EuroArgo and the further development of the Argo programme. It coordinates and maintains the German Argo contribution and secures the long-term data acquisition. Focal regions of the about 150 German floats are the North Atlantic, the Arctic Ocean and the Mediterranean. BSH deploys about 50 floats per year. Other German marine institutes (inter alia University of Hamburg, Alfred-Wegener-Institute for Polar- und Marine Research and Helmholtz-Center for Ocean Research Kiel (GEOMAR)) support the program with floats financed by third-party funds.
An overview of the German Argo activities is provided by this brochure.
BSH activities in the framework of Argo
The BSH Argo team consists of four staff members, scientists and technicians (see contacts). BSH supplies around 50 floats per year for array maintenance of the and for current research and observing programmes.
All floats delivered to BSH or directly to research vessels have to undergo a functional checkup by our technical staff. BSH is then disseminating the floats on selected research vessels. Therefore, the proposed cruise tracks of national and international research vessels are checked with regard to their working areas. If the working areas coincide with planned deployment positions, BSH contacts the chief scientists and ask them to deploy the floats during their cruise. The willingness of the chief scientists to deploy the floats is a fundamental precondition for the long-term maintenance of the global array and is gratefully acknowledged.
A further focus of BSH is data processing and data quality control. In cooperation with the Argo data centers incoming data are decoded within 24 hours. On longer time scales the data profiles have to undergo quality checks and the drift tracks must be monitored. If a float which was deployed on the high seas drift into national territorial waters, the respective national authorities have to be informed.
BSH also supports university partners (e.g. by undertaking the data control), is national focal point in case of arising problems and organises regular national user meetings. BSH also represents national Argo interests within European and international committees and is engaged in projects for the technical and scientific advancement of Argo.
The use of Argo data at BSH
One of the legal tasks of BSH is the monitoring and assessment of changes in the marine environment in the North Sea and Baltic. The North Sea is a shallow shelf sea whose physical status, primarily characterised by temperature and salinity, is to a large extent determined by the exchange of water masses with the Atlantic at its open northern boundary. There is also a link to the Atlantic via the English Channel which is much smaller but important for the shallow southern North Sea. The main inflow paths of Atlantic water are the Fair-Isle Channel between the Orkneys and the Shetlands, the East Shetland shelf and the western slope of the deep Norwegian Trench. Over these pathways the Atlantic exerts a significant influence on the oceanographic conditions of the North Sea.
To understand and assess the long-term changes in the North Sea, the changes in the bordering Atlantic must be considered. Here, Argo floats provide year-round comprehensive reliable temperature, salinity and density data as well as information about the large-scale circulation in the parking depth of the floats (generally 1.000 m). Changes in the Atlantic and the North Sea with time scales up to several 10 years are caused by climate change as well as by natural variability partly forced by atmosphere.
One example of the usage of Argo data at BSH is shown by the results of a study of the source water masses of the North Sea inflow from the Atlantic via the northern boundary and the English Channel. Two areas have been defined for this study: one area at the northern boundary of the North Sea and a second one at the outer entrance of the Channel (black polygons in the figures). For the period from 2.000 to 2015 all floats which entered the polygons have been registered and the deployment positions of these floats have been identified in order to determine their source regions (yellow circles in the figures). The analysis reveals that the floats/water masses came from the subpolar gyre as well as from sub-tropical regions.
Current deployment plans for BSH floats
There are two criteria for planning the float deployment: On one hand the maintenance of the global array must be secured, i.e., there must be at least one profile per month in every 3°×3° area. On the other hand floats have to be deployed where they are required for current research and observing programmes. BSH focuses in the framework of EuroArgo on the Southern Atlantic and the High Latitudes as well as the filling of current gaps in the North Atlantic.
The figure shows the probable position of BSH floats to be deployed by research vessels during the next months. Detailed information is available within this table.
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