Student Cruise In Faroese Waters – Leg I April 21, 2006Posted by Putri in Berlayar.
Observations form the basis of all natural sciences and are the ultimate truth against which all theories have to be tested. This is in particular so in geophysical systems such as the ocean and the atmosphere that are highly non-linear and where wellfounded theories have been developed only during the past decades. An education in observational techniques and in data analysis is thus a fundamental part of the curriculum of students of natural sciences.
Since the 1990s the Institut für Meereskunde, Hamburg, and the Geofysisk Institut, Bergen train their physical oceanography students in marine fieldwork during cruises with their research vessels. Recently the Niels Bohr Institutet for Astronomi, Fysik og Geofysik, København and the Institut für Umweltphysik, Bremen joined these training programmes, which are now being carried out at a rate of two per year. In February 2001 a cruise with RV HÅKON MOSBY went to several west Norwegian fjords and for July 2001 a cruise with SV KOMMANDOR JACK went to the region around the Faroe Islands. SV KOMMANDOR JACK, the former RV VALDIVIA of the University of Hamburg, was used for the student cruise Faroese waters in July 2001. There were two legs of one week each, allowing 16 students to participate.
The Cruise started in Torshavn on the Faroe Islands July 12, 2001 (leg 1) and ended in Edinburgh, Scotland, on July 27, 2001 (leg2). The in-between cruise stop for the change of the studet crews in Torshavn on July 20, 2001. See Site Map in figure 3 for all section during the cruise.
July 11, 2001 : go to Kopenhagen
July 12, 2001 : go from Kopenhagen to Torshavn on the Faroe Islands.
July 13, 2001 : start the cruise at 24 o.clock.
July 14, 2001 : start with CTD in station 101 at 6 am until station 115 at 12 pm
July 15, 2001 : use CTD for station 116 until station 129 and Mooring in station 126
July 16, 2001 : CTD for station 130 until 141, and Mooring in station 136 and 137
July 17, 2001 : CTD for station 142 until 154, and Mooring in station 148 and 149
July 18, 2001 : CTD for station 155 until 161
July 19, 2001 : the end of leg I at 6 am o.clock in Torshavn.
The cruise continue on leg II.
Every afternoon the participant discuss about the cruise, data, or the
result. The last day we wrote all of the results on leg I.
Participant in Leg 1
Prof. Detlef Quadfasel, Gudrid Eiriksdottir Johansen, Mai-Britt Kronborg, Mutiara Putri, Karin Magretha Husgard Larsen, Silke Hinze, Dagmar Hainbucher, Maike Breiholz, Voltaire Velazco, Abdul Mumem Al Raei, Christian Mertens.
The following instruments were used onboard SV KOMMANDOR JACK.
The conductivity, temperature and depth sonde measures these parameters in the water column. The CTD used, a SeaBird SBE9/11 model, also measures the oxygen content in the water. At a station, that is a measurement location, the CTD is lowered on a wire down into the water, to just above the bottom, and up again. The data are transmitted on board the ship via the conducting cable and stored in a computer. From pressure, temperature, and conductivity data the salinity and the density of the water can be calculated.
Rosette Water sampler: The CTD is mounted on a stainless steel frame that also houses Niskin type water samplers. These are initially open at both ends, but can be closed at selected depths to preserve a water sample and bring it up on deck. From these water samples salinities and oxygen content is determined in the laboratory and used to calibrate the CTD sensors.
An RD Instruments Acoustic Doppler Current Profiler is attached to the hull of the ship and continuously measures the velocity at different depths while the ship is moving. It uses Doppler effect for determining the water currents. The ADCP transmits acoustic pulses along four beams arranged quadratically in a plane, facing 30° from this plane. The sound pulse is not reflected by the water itself, but by particles moving with the current. Due to the movement of the reflecting particles, the returning sound wave has a different frequency than the transmitted one; this is the Doppler shift. By using this Doppler effect, the horizontal and vertical currents at selected depth bins can be estimated.
This is a small CTD without a pressure sensor that measures temperature and salinity of the near surface water while the vessel is moving.
With Guildline salinometer the water samples are analysed for their salinity. The instrument compares the conductivity of the sample with that of Standard Sea Water which salinity has been determined very accurately in the laboratory.
The Global Positioning Navigation System is used to determine the ships location at a rate of one Hz. It is a satellite-based system that uses a method similar to cross-bearing of up to 24 satellites to determine the position.
Results and Analysis
1. Water masses and large scale circulation
"A little drop of water a little grin of sand, make the mighty Ocean and the pleasant Land". With a hard work and different strong weather around the clock, we had finally the following results :
The North Atlantic Water at the surface is characterised by temperatures higher than 7. C and a salinity higher than 35.15. Especially in the eastern part of the section the interface between these two water masses is sloping down towards the east, which due to geostrophy indicates a northward transport of the North Atlantic Water in the upper layer and a southward transport of the overflow water. Along the ridge we found five eddies showing as peaks of large thicknesses of overflow water.
A TS-diagram based on all the observations shows the distribution of the main water masses and how they mix. In this area three main water masses are found: The southward flowing Norwegian Sea Arctic Intermediate Water (NSAIW), the Modified North Atlantic Water (MNAW) which flows northward, and the Modified East Icelandic Water (MEIW) in the western part of the section near Iceland. Mixing of the above mentioned water masses is seen to take place. The MEIW originates from the Arctic Ocean and is transported southward via the East Greenland and East Icelandic currents. In the TS diagram the temperature is seen to be higher than the 1-3.C mentioned in the definitions, this is probably due to summer heating.
The cold overflow water is characterised with temperatures below 2.C and salinities below 35.0 . This water is most often seen on the ridge in the deeper parts, but in one case it is also seen to be hugged onto the western side of the deepest trench. In this trench the thickness of the overflow water is approximately 150 m.
In the north on the north-south section the MNAW is seen as a relative thin layer compared to further south, where it has a thickness of approximately 600 m. North of the ridge overflow water is seen to pile up. As seen in the along ridge section overflow water crosses the ridge mainly through the deeper trenches. In the southern part of the north-south section two branches of overflow water are seen, one part between station 44 and 46 with a thickness of 10-20 m and another branch from station 41 and southward. We believe that these two branches stem from the two eastern most trenches in the ridge (centered around stations 5 and 10).
Station Map. The red point represent the stations, where the CTD used just one time, the green point where the CTD used two times, and the star represent the mooring stations. The observation had been measured between 13 – 18 July, 2001.
TS-Diagram including all of the observations. Red represent characteristics from the along ridge section (station 1-35, except 25,26,27). Pink represents the three western most stations (station 25-27), while blue and green represents the rest of the stations north and south of the ridge. The water masses in this area are MNAW (Temperature : 7.0 – 8.5°C and Salinity : 35.10 – 35.30), MEIW(Temperature : 1.0 – 3°C and Salinity : 34.70 – 34.90), and NSAIW (Temperature : -0.5 – +0.5°C and Salinity : 34.87 – 34.90), following the definition of Hansen and Østerhus (2000).
Potential Temperature (a) and Salinity (b) along the Faroe Ridge Section (Station 1 . 35) in 14 . 15 July, 2001.
Potential Temperature (a) and Salinity (b) across the Faroe Ridge Section (Station 54-50 in north, Station 15-19 on the ridge, and station 47-38 in south) in 16 .17 July, 2001.
2. Time and mesoscale variations
At the Iceland-Faroe Front we can expect to observe meanders and eddies. Eddies arise from unstable meanders and are typically on the order of 15 to 70 km wide and can have effects down to 400 m. South of the front cyclonic eddies with cold cores and upwelling in the centre may form, while north of the front anticyclonic warm core eddies with downwelling may form.
On the figure showing the along ridge section the domes of cold water can be indications of cyclonic eddies south of the front. The peak centred at station 23 reaches to the surface and will show up on a satellite picture. The other domes may be intermediate eddies. On the north-south section figure a warm depression is centred at station 58. This could be an anticyclonic eddy.
The stations plotted in green on the station map have been run twice with 3 days interval to observe time variations. The temperature distribution has changed a lot indicating variability within a few days in this region – see figure of repeated sections.
The goal of making the north-south section from station 50 to 54 was to investigate the structure of the big cold dome centred at station 16 on the along ridge section. Unfortunately we cannot really observe this on the section, both because the section is to short, but also because of time variations.
Temperature distribu-tions from the green section on the section plot. Upper axis shows station numbers. The upper figure shows the first along ridge section and we observe between station 12 and 17 a big blob of cold overflow water (< 2° C), which is about 150 meters thick. The lower figure shows the same section taken 3 days later. Now the overflow is spread out over the passage, and has a height of about 50 meters. The cold dome observed at station 16 on the upper figure can no longer been seen on the lower figure, but instead we observe 2 smaller domes. This indicates that these eddies have moved. One could speculate that these eddies are a mechanism for carrying the overflow water across the ridge – the cyclonic eddies upwell the cold water above sill level, and when these eddies move across the ridge, the overflow water is heavier than the ambient water and sinks.
– Compared to previous studies our observations show very large layer thickness of overflow water at the ridge. Likewise the layer of the downslope branches of overflow water is also thicker.
– The observed slope of the interface between the upper layer and the overflow layer in both sections indicates that the surface current crosses the ridge with an angle towards north, and not as an anticyclonic current following the topography. This can also be seen in the current patterns derived from satellite-tracked drifters (see the regional conditions on this homepage .
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