First Long-Term Use of Argo Floats in the Baltic Sea (2013)

T. Purokoski (1), E. Aro (2), A. Nummelin (1,3), P. Roiha and S. Siiriä (1)

(1) Finnish Meteorological Institute, Finland ; (2) Aalto University, Finland ;
(3) University of Bergen, Norway

Full paper: Purokoski T., Aro E., Nummelin A., Siiriä S. 2013, Pushing the Boundaries: The First Time Use of an ARGO-Float in the Baltic Sea, Conference: The 4th Euro-Argo Science Meeting and Workshop on the Arctic and sub-Polar North Atlantic, At Southampton, UK, doi: 10.13140/2.1.3431.8080.
Context

In 2010, Finnish Meteorological Institute (FMI) decided to test the suitability of Argo floats for the shallow and low-salinity 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 (Fig. 1). 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. In wintertime, the northern parts of the Baltic Sea freeze, and during the coldest winters the entire Baltic Sea can be ice-covered.

The first long-term deployment in 2012 took place in the Gulf of Bothnia.

Figure 1. Map of the Baltic Sea. The first long-term deployment in 2012 took place in the Gulf of Bothnia.
(Credit:  Norman Einstein)

Data & Method

In January 2011, FMI received two APEX floats specifically balanced for use in sea areas around Finland. Both of the floats were equipped with basic Sea-Bird Electronics Inc. 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.

In general, in the shallow Baltic Sea the operating environment is completely different compared to deep oceans. 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.  Bottom contact is to be avoided because if the bottom is muddy the float might get stuck or be damaged by the impact.

Results

Initial testing was carried out in 2011. In May 2012, after a final, short 24-hour test from R/V Aranda, one of the floats was deployed for a longer mission. The specific sea area chosen for the mission was the Bothnian Sea, which is the southern part of the Gulf of Bothnia. The optimistic goal was to keep it in water until September, but there was a concern that it would drift to shore much earlier. However, it turned out that the float exceeded all the expectations, operating flawlessly until December, when it was recovered by a Finnish Coast Guard search and rescue helicopter.

During its six and half month mission, the float transmitted more than 200 daily vertical CTD profiles to FMI (Figs. 2 - 3). The average parking and profile depth varied between 50 -80 m. The longest daily distance the float drifted was more than 20 kilometers. The drift trajectory followed quite closely the isobaths in the slope area of the deep basin. Thus, there seemed to be little water exchange between the open sea and the coastal region, at least in the deeper layers. In the middle of the basin, the drift path was more variable. During the summertime, the currents were weak, but in the fall, with stronger winds and mixed water mass, the currents increased considerably.

Figures 2-3. Time series of temperature and salinity. Summertime warming of the surface water and
water mass mixing in the fall can clearly be seen.

Modification of the Diving Algorithm

Some float firmware development has also taken place during the project. Since the diving control algorithm of the Argo float was developed for deep oceans, it is generally too slow for the shallow Baltic Sea. Therefore, a joint development project with Aalto University was carried out with the aim of modifying the APEX float firmware and thus enabling it to settle to the target dive depth quicker. The algorithm for controlling the dive has been made faster by a factor of four. Furthermore, the accuracy at which the set dive depth is maintained has been doubled. These modifications were tested in summer 2012 and verified to work as planned.

In summer 2013, the modified float was deployed to the Bothnian Sea in the beginning of June (Fig. 4).  The average parking and profile depth was about 90 m. During its four month mission it transmitted over 100 daily profiles to shore. It was recovered in October by R/V Aranda. The faster diving algorithm worked very well, and the float was able to reach its target dive depth much quicker than the standard float.

Routes of the first three Argo floats deployed in the Baltic Sea during 2012-2013

Figure 4. Routes of the first three Argo floats deployed in the Baltic Sea during 2012-2013.  Red line: deployment in 2012. Red dot indicates the recovery point. Black line: modified float in 2013. Black dot indicates the recovery point. Green line: bio-optical float still measuring at the time of writing. Green dot is the latest position.
(Credit:  Simo-Matti Siiriä)

Current activities

In 2013 our Baltic Argo fleet was expanded by two bio-optical APEX floats. These floats have an Aanderaa oxygen optode and Wetlabs FLBB-AP2 for measuring chlorophyll and turbidity. The other float was deployed in August 2013 into the Eastern Gotland Basin.  This is a large central basin in the Baltic Sea with anoxic (possibly hydrogen-sulfide) deep water. This float is still active after almost 80 profiles. The average parking and profile depth is about 125 m.

The other bio-optical float was deployed this May to the Bothnian Sea. At the same time also a standard float was deployed in the same area. This standard float is the same one which was used for half a year already in 2012. It had gone through service and sensor calibration and was thus ready for redeployment.

Besides environmental monitoring and model development, the floats will be used for specific measurement campaigns. For example, when a storm approaches the floats can be commanded to perform multiple profiles per day. This will enable researchers to gain valuable insight into the speed at which the storm mixes the water mass and the maximum depth of the mixing.  In this kind of situation, the float is the instrument of choice since, for example, research vessels will not be able to lower instruments during storms.

Conclusions

The experience gained so far indicates that Argo floats are useful and valuable tools for monitoring the Baltic Sea. The floats can be used for environmental monitoring, model verification, data assimilation and specific measurement campaigns. They are reliable and can also be used during storms. However, their operation in shallow and well-mixed water mass requires almost daily operator involvement and active control of the floats.

References
  • A. Westerlund, P. Rocha : Comparing FMI Baltic Sea NEMO results to profiles from Argo floats, 2014 NEMO Users Meeting, Grenoble, 7 July 2014.  Poster presentation.
  • P. Roiha, S. Siiriä, A. Nummelin, E. Aro, T. Purokoski, A. Westerlund : Baltic Sea ARGO experiments: developments and new directions , BOOS-HIROMB seminar, Riga, 8 May 2014. Oral presentation
  • Petra Roiha, Simo-Matti Siiriä, Aleksi Nummelin, Eemeli Aro and Tero Purokoski: What can Argos tell us on the Processes in Baltic Sea?, Geophysical Research Abstracts, Vol. 16, EGU2014-10917-1, 2014, EGU General Assembly 2014
  • Purokoski, T., E. Aro, and A. Nummelin, 2013: First Long-Term Deployment of Argo Float in Baltic Sea, Sea Technol., 54(10), 41-44
  • Petra Roiha, Simo Siiriä, Aleksi Nummelin, Eemeli Aro and Tero Purokoski : Argo Experiments and Developments in Baltic Sea, Abstract in The Book of Abstracts of The Future of Operational Oceanography 2013 - Hamburg, Germany, 8-10 October 2013
  • Tero M.T Purokoski, Eemeli Aro, Aleksi Nummelin, Petra E Roiha and Simo-Matti Siiriä : Pushing the boundaries: the first time use of an ARGO-float in the Baltic Sea, in Proc. 4th Euro-Argo Science Meeting and Workshop on the Arctic and sub-Polar North Atlantic, Southampton, UK, 18 - 20 June 2013, 2013, 6 p.
  • Petra Roiha, A. Nummelin, S. Siiriä and T. Purokoski : First long-term ARGO float experiment in the Baltic Sea , in Proc. 7th Study Conference on BALTEX, Borgholm, Sweden, 10-14 June 2013         
  • Petra Roiha, Tero Purokoski, Simo Siiriä : Finnish Meteorological Institute’s Argo Float Experiments in the Baltic Sea, News from BOOS, vol. 3, Spring 2013, 5-6