Glider Mission in the Baltic Sea
In an interview oceanographer Johannes Karstensen from the "GEOMAR Helmholtz-Zentrum für Ozeanforschung" talks about the first Glider Mission in the Baltic Sea.
Mr. Karstensen, you participated in the first glider mission in the Baltic Sea. What was it about?
The mission had two goals: on the one hand we wanted to better understand which processes, especially wind situations, control the upwelling near the coast at the time series station "Boknis Eck". On the other hand we wanted to show that autonomous operating Gliders can be deployed in the Baltic Sea. In the past, attempts had failed to do so.
What were the goals of your campaign in the Baltic Sea?
The impact of costal upwelling is can be felt by anyone who want to go swimming in summer. When jumping into the water during offshore wind situations one finds out that it is horribly cold. The reason is usually in the wind - which pushes the warm water away from the coast towards the open sea and in response, colder water from greater depth rises to the surface.
As this process takes place on the rotating earth, it is usually most efficient if the wind blows parallel to the coast, namely to the north in the northern hemisphere with the coast to the west. With such northward winds the "Coriolis force" causes the water to be pushed away towards the east –theoretically.
Being close to the coast, having very shallow water and a snapped off coastline as it is the case with "Boknis Eck", local effects play a role and the theory cannot be transferred 1:1.
By using the measurements carried out by the glider and wind data from a nearby meteorological station (Kiel Leuchtturm), we could investigate the connection between wind direction and wind strength and the sea surface temperature.
Which positive and negative effects does the "upwelling" you depicted have?
What is a certain "handicap" swimmers is in fact a blessing for the ecosystem – especially in summer, plants in the light and warm surface water run out of nutrients and die. However, the upwelling carries new nutrients from the deeper layers into the light-flooded upper water layers – plant growth is supported and provides the basis for the existence of further links of the food web – up to the human. For sure that is the reason that marine scientists worked on the phenomenon 100 years ago already.
The most widespread coastal upwellings are found at the tropical and subtropical east coasts, where trade winds generate upwelling conditions continuously – therefore Mauretania, Peru, Chile and Venezuela are regions extremely rich in fish.
Why exactly did you choose the area of "Boknis Eck" for your survey?
"Boknis Eck" is the ideal "test area" for us – we know the region very well, because the GEOMAR (editor’s note/Helmholtz-Zentrum für Ozeanforschung Kiel) operates a "time series station" since the mid 50s there.
This "station" is not by any means a "building" but rather an area marked by four yellow buoys as a "prohibited zone". About once a month the station is visited by a ship from the GEOMAR and physical, biological and chemical probes are taken from the water column. It is inaccessible for shipping and therefore the glider could be operated safely.
Because of our research we could precisely determine critical wind direction and wind strength driving upwelling at Boknis Eck. Luckily wind data is available since the early 1980s so we could reconstruct the upwelling activity for the last 30 years. The results allowed us to classify the monthly Boknis Eck Station cruises into "normal", meaning without upwelling, and potentially "abnormal", meaning with upwelling. This information is of use when further interpreting the cruise data.
What is a glider, how does it operate and what does it measure?
A glider is an autonomous and mobile measuring platform. It has no propeller, but rather moves through specific "density" modifications. With the aid of a pump, it changes its volume (gets bigger or smaller), while keeping its weight. This is similar to the situation when exhaling strongly in a swimming pool and starting to descend. The glider pumps oil out of a rubber bladder that is located in the water into its pressure housing – then it descends. When it pumps the oil out of the pressure housing into the rubber bladder it ascends.
During the up-and-down movement small wings at the shaft cause the movement to go forward as well. For one meter up (or down) it moves about two to three meters forward. The direction is controlled by the glider with a small rudder at the stern. Every time it reaches the surface, it sticks out an antenna to determine its position via GPS and to establish contact with the control center at GEOMAR via Iridium-satellite telephone. In this way, data is exchanged and new course data is assigned to the glider. At "Boknis Eck" we made the glider navigate back and forth between two points close together – as if we did the sampling at quasi one point.
Which problems did you struggle with?
The glider drifted into a very shallow part of the prohibited zone a few times probably due to temporary increasing currents. This led to the glider “landing” on the ground and not appearing for a few hours. The first time we were pretty concerned and thought something might have happened and it was a big relief when receiving messages again. Apart from that everything went surprisingly well and we could show that glider operations in the Baltic are possible.
Are there other test series planned in the Baltic Sea or other oceans in the future?
The Baltic Sea is not our main research area at GEOMAR – that is the Atlantic where we have carried many glider missions already. At the moment an experiment is running which is about coastal upwelling as well – certainly on a larger scale, off the coast of Western Africa. But we also did glider missions in the South Pacific in the context of the large scale upwelling off Peru.
The GEOMAR belongs to the Helmholtz-Community. To what extend is a data exchange or collaboration in general taking place with other centers?
The glider data is sent to us during the mission already and processed immediately in a way that they can be viewed online. Furthermore most data is send automatically to a European data center that subsequently provide access to the data for further analysis, especially ocean forecasting, for everyone interested.
The Glider-user-community in Germany has a group, the "German Glider Group (GGG)" that meets once a year to discuss experiences in dealing with gliders on a national basis – one topic appearing on the agenda regularly is for example the permission of glider deployments in the North and Baltic Sea. On an international basis, in Europe as well as worldwide, we belong to the "Everyone Gliding Observatories" EGO-group, where relevant topics are discussed in detail. In both groups our Helmholtz-colleagues of the Helmholtz-Center Geesthacht and the Alfred-Wegner-Institute are represented, who use this technology very effectively as well.
Scientists from the University in Kiel and the GEOMAR Helmholtz-Zentrum für Ozeanforschung Kiel taking samples at the station "Boknis Eck" (www.bokniseck.de Ostsee) in June 2014 (Film: GEOMAR).
Questions were ask by Karl Dzuba for the Earth System Knowledge Platform.