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First Long-Term Deployment Of Argo Float in Baltic Sea

By Tero Purokoski • Eemeli Aro • Aleksi Nummelin

Map of the Baltic Sea. The long-term deployment took place in the Gulf of Bothnia. (Credit: Norman Einstein)
In order to acquire more oceanic observations from vast, undersampled sea areas, the Argo program was initiated in 1999. Today, there are more than 3,000 free-drifting, profiling Argo floats in the world's oceans. They measure temperature and salinity of the upper ocean. Some floats also have an oxygen sensor attached, and the newest models can be equipped with bio-optical sensors. All the data are transmitted by satellite and made available in a public database. Thus, continuous measurement data become available from areas where sampling otherwise would be sparse.

The Argo float is capable of moving vertically in the ocean by changing its effective density. This is achieved by pumping oil into and out of a flexible bladder. This expansion and deflation of the bladder changes the volume of the float and thus its density, as the mass of the float stays constant.

Usually the floats drift for about 10 days at a depth of 1 to 2 kilometers. After this, they ascend to the surface. While ascending, they collect profiles of temperature and salinity. Once the float is at the surface, the measurement data, as well as the float's GPS position, is transmitted to shore via satellite. Then, the float dives again. Currently, almost all deployed floats are in the deep oceans, with only a few operating in shallow, marginal seas.

The Finnish Meteorological Institute (FMI) began oceanic Argo operations in 2009 by purchasing two floats. To date, FMI has deployed eight floats altogether in the Nordic Seas.

In 2010, FMI decided to test the suitability of Argo floats for the Baltic Sea. This was motivated by the need to get more data in an economical way from sea areas not often visited by a research vessel. Data were needed for environmental monitoring and for model development.

The Baltic Sea is situated in northern Europe and is one of the largest brackish water bodies in the world. Currents are weak and tides are negligible in the Baltic Sea. Its bottom topography can be described as irregular with an average depth of 54 meters. It is connected with the Atlantic through the narrow and shallow Danish straits. There is also a large volume of incoming freshwater through river runoff that creates a low-salinity water layer on top of the saltier bottom water.

The salinity of surface water in the northern Baltic Sea, i.e., in the Gulf of Bothnia, can drop to 5 practical salinity units or even lower. During the fall, the water mass in the Gulf of Bothnia mixes completely down to 70 to 80 meters depth. In wintertime, the northern parts of the Baltic Sea freeze, and during the coldest winters the entire Baltic Sea can be ice-covered.

After contacting the float manufacturers, it became evident that no one else was using the floats in a Baltic Sea-like environment, i.e., in a shallow and low-salinity sea. In general, floats are designed to be used in deep oceans where there is no danger of hitting the bottom or being hit by a ship while at the surface. Normally, there is no need to change the diving parameters during the mission, and thus only one-way satellite transmission from float to shore is sufficient. However, in the Baltic Sea the operating environment is completely different. With the aim of preventing the float from hitting the bottom, a much more active involvement with the float's mission is required. Therefore, to be able to change the diving parameters of the float, two-way satellite communication and short dive cycles are absolute requirements.

Initial Testing
In January 2011, FMI received two floats specifically balanced for use in sea areas around Finland. These APEX (Autonomous Profiling Explorer) floats were manufactured by Teledyne Webb Research Corp. (East Falmouth, Massachusetts). Both of the floats were equipped with basic Sea-Bird Electronics Inc. (Bellevue, Washington) SBE 41CP CTD sensors and two-way Iridium satellite transmitters. Since these were the first floats to be used in the Baltic Sea, there was no prior experience in the diving behavior of the floats in low-salinity Baltic waters. Thus, a cautious path was selected to verify the diving dynamics of the floats.

During the spring of 2011, the satellite communication system between the floats and the FMI-based RUDICS (Router'based Unrestricted Digital Interworking Connectivity Solution) server was built and tested. Once the communication system was operational, one float was deployed for a short mission. The deployment area chosen was relatively deep, with a level bottom. The goal of this test was to establish the amount of overshooting the float does before it settles to its designated dive depth. Since the diving algorithm of the float is designed for the oceans, it is generally too slow for shallow sea areas where the bottom is relatively close to chosen dive depths. To continue this article please click here.

Tero Purokoski works at the Finnish Meteorological Institute in the Marine Technology Services group as a technical expert. He has been involved in numerous oceanographic instrumentation projects and has considerable sea-going experience. He holds a master's degree in aerospace engineering from Aalto University.

Eemeli Aro is a doctoral student at the Finnish Centre of Excellence in Generic Intelligent Machines Research at Aalto University in Helsinki, Finland. His thesis topic is the self-localization and self-coordination of an autonomous group of underwater robotic floats.

Aleksi Nummelin is currently a doctoral student at the University of Bergen, Norway. Previously, he worked at the hydrodynamics group at FMI and participated in the first Argo missions in the Baltic. His research interests include the ocean's role in climate dynamics and the interplay between small- and large-scale dynamics.

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