Weddell Sea & Filchner Depression
10 December 2012 - 28 January 2013
The main field work of the WEDDELL project was conducted during the cruise The cruise ES060 onboard the Royal Research Ship (RRS) Ernest Shackleton. The cruise was led by Elin Darelius.
The sampling was made using a SBE911plus conductivity-temperature-depth (CTD) profiler equipped with a dual lowered acoustic Doppler Current Profiler (LADCP); ocean microstructure measurements were made using a vertical microstructure profiler (VMP2000). A total of five moorings were deployed which will be recovered in January 2014.
The study site is the Filchner Depression where a large coastal polynya allowed sampling in the vicinity of the Filchner front and on the eastern side of the Depression. The observational program carried out allows for estimates of the warm inflow and an increased understanding of the circulation within the Filchner Depression. In total 115 CTD/LADCP-stations were taken in six sections and at 4 time series (12-14 h long) stations. 35 deep microstructure profiles were made.
Station map here... The map above gives an overview of the occupied stations. .....
Cruise to the Faroe Bank Channel
The cruise HM 2012610 onboard the Research Vessel Håkon Mosby was the main field work of the RCN-funded project OVERFLOW. The physical oceanography field work was carried out through a combination of conventional conductivity-temperature-depth (CTD) measurements, densely instrumented moorings, current profile measurements using lowered acoustic Doppler Current Profiler (LADCP), ocean microstructure measurements and two autonomous gliders. One of the gliders (Gnå- picture below) and one of the moorings (MATS, picture below) were equipped with a suite of turbulence sensors to sample the turbulence levels in the FBC overflow.
The image on the left shows the VMP during deployment. The observational programme was designed to measure turbulence and mixing in the overflow plume which, in addition to the shear-induced mixing at the plume-ambient interface, is hypothesized to be influenced by several processes including mesoscale eddies, secondary circulation and internal waves. The image below shows the Moored Autonomous Turbulence System (MATS) during deployment. It is used to record time series of turbulence at the turbulent plume interface.
Previous measurements of temperature and velocity recorded by moored instruments at the site were dominated by 3-5 day period oscillations. One sampling approach during the cruise was to repeat sections or stations to sufficiently resolve this dominant mode of variability. The hypothesis that this mesoscale variability (eddies) can enhance the mixing rates and the rate of descent of the dense plume will be tested. A secondary, transverse circulation composed of an Ekman transport in the bottom boundary layer and a return flow at the dense layer interface has been previously reported.
During the cruise, attempts were made to resolve the secondary circulation with an aim to quantify its influence on mixing. Finally, data were collected to test the hypothesis that internal-wave turbulence transition is important in the interfacial layer between the dense plume and the ambient. The study site is shown on the left with a subset of stations showing the VMP deployments (red) and Gnå surfacings (blue).
The vertical profiles from the VMP are compared to the microstructure measurements from the glider. The details on the methods and results can be found in Fer et al. (2013). The panel below shows survey mean profiles, with respect to height above mottom, of the dissipation rate of TKE inferred from all VMP casts and the glider (dives, climbs, and their average in black).
Cruise to the Faroe Bank Channel
See a taste of pictures by Kjersti L. Daae here.
In order to study the mixing of the Faroe Bank Channel overflow plume, a physical oceanography cruise was undertaken from 29 May 2008 to 08 June 2008 with main tasks to conduct CTD/LADCP and microstructure measurements from the sill at the exit of the channel to about 150 km downstream. In total, 11 moorings were deployed two weeks prior to the cruise and recovered in mid-July, giving 2 months of time series.
The map above gives an overview of the occupied stations. The triangles mark the location of the moorings. The farthest downstream line (close to CTD section F) of moorings is deployed in collaboration with Detlef Quadfasel (Univ. of Hamburg). In total, we occupied 63 stations with CTD/LADCP casts. CTD system was SBE911+ and we used two 300 kHz RDI Sentinels used in master/slave down/up-looking configuration. 92 microstructure profiles were collected using the microstructure profiler VMP2000 equipped with shear probes and fast response temperature and conductivity sensors. The squares mark the CTD/LADCP/VMP stations. The two circles mark the position of the time series stations (TS1 and TS2) where casts were made approximately every 40 minutes.
The CTD/LADCP data can be downloaded from the Data tab. You can download a pdf of all CTD/LADCP profiles here [1053 KB]. You will need this log file to link the CTD file names and station names with respect to the sections.
The figure below, from Fer et al. (2010), shows contours of a) downchannel velocity, b) stratification (color), and potential density anomaly at 0.1 kg m-3 intervals (gray), and c) dissipation rate of TKE for Sections A to C. Dashed curves delineate the plume interface.
The mooring data revealed oscillations in temperature and velocity at 3-5 day time scales. The results are described in detail in Darelius et al. (2011). The figure below from that paper shows an example from mooring C2 (near mid section E). The temperature scale (color) varies from -1 (blue) to 8oC (red). The low-passed velocity anomalies are superimposed (black sticks) together with the time-averaged currents at those levels (white arrows).
Field Work from Barneo Ice Camp
As a part of the field work programme of my NORKLIMA young investigator project “Ocean mixing in the Arctic”, together with Anders Sirevaag, we conducted turbulence and current measurements from drifting ice camp Barneo (or Borneo), about 110 km from the North Pole. See also the 2007 field work description below. Ice Camp Barneo was established on about 2m thick first year ice around 89N, but by the time of our arrival it had drifted southwest to about 88.5N, 7E. We occupied the ice camp from 17 to 27 April. The drifts of 2007 and 2008 are shown below together with the three stations occupied across the Lomonosov Ridge.
We got all our equipment set up in a separate heated tent. Our sampling included a microstructure profiler measuring oceanic turbulence as well as precision temperature and salinity, and a 200-m deep mooring suspended through the ice, equipped with temperature recorders every 10 m, supplemented by several conductivity (salinity) recorders and acoustic current profiler sampling between 80-160 m. We also launched XCPs (eXpendable Current Meters) that profile relative horizontal currents down to 2000 m depth.
During our stay we performed continuous turbulence profiling down to 500 m which resulted in an hourly time series as the ice drifted southwest and we deployed XCPs every 8 hours. In addition to our programme, we also collaborated with scientists from the University of Washington and assisted them with their Twin Otter based CTD/XCP survey in the Arctic and with the deployment of a cluster of instruments that measured turbulence in the under-ice boundary layer close to our measurement location.
In the profiles, a persistent, but laterally incoherent thermohaline staircase structure is identified in the 200–260 m depth range. The figure below shows an example of the staircase in an arbitrary subset of the profiles. A detailed analysis on the role of double-diffusive mixing is presented in Sirevaag and Fer (2012).
On 25 April we took off with the Mi-8 helicopter to the Lomonosov Ridge in order to find out if this large topographic feature which separates the Arctic basins introduces mixing in the water column. Unfortunately 3 hours flight time was insufficient for a well-sampled section, since the base drifted very far south, however, we occupied three stations across the ridge, conducting three turbulence profiler and five XCP deployments from refrozen leads with thin ice.
All in all, our stay at Barneo was successful with high data recovery rate. We got 6 days of continuous turbulence profiling (about a 100 km long stretch of data!), 8 days of mooring data and deployed 20 XCPs.
The slides below give a taste of the preliminary data.
Field Work from Barneo Ice Camp
As a part of the field work programme of my NORKLIMA young investigator project “Ocean mixing in the Arctic”, I conducted turbulence and current measurements from drifting ice camp Barneo (or Borneo), about 110 km from the North Pole. The camp drifted from approximately 89degN at the Greenwich meridian towards southwest (Amundsen Basin) at a mean speed of 12 cm/s. Borneo is a tourist facility operated by the Russian company Polus. Scientists from all over the world make use of the costly opportunity and logistics provided by Polus, particularly during IPY. Borneo is accessible from Longyearbyen by the Russian jet Antenov in 3.5 hours. Flights are regular in ideal conditions. Due to a storm early in April the runway on the ice was damaged and flights were postponed. My arrival to Borneo was delayed by 6 days until a new runway was constructed. This was a big loss on my planned 12 days field work and I negotiated to extend my stay until they tear down the camp giving me in total 10 days. The sampling I planned included time series of turbulence profiles and background ocean current in the upper 500 m at the ice camp interrupted by a 5 station section across the Lomonosov Ridge to be accessed by helicopter (estimated 2 days work). I postponed the helicopter section to year 2008 and concentrated on the time series measurements.
Ice was about 2 m thick. After long hours of labor, my lab - a Sami style tent - was up with a clear hole for the MSS microstructure profiler. On my third day, I was set and started collecting turbulence profiles.
Image on the right shows the upper end of the MSS profiler (MSS90L S/N 033, see the manufacturer's web site) with orange buoyancy elements and green “hair”. This hydrohole is the gateway to 4500 m deep Arctic Ocean. The buoyancy of the profiler is such that it falls at a rate of about 0.6 m/s. The orange tether is for data transfer and recovery. The winch has about 700 m cable spooled. It is a quiet winch and requires little power (0.4 kW). The winch is controlled by a practical hand-held control (image on the right). I feed cable sufficiently fast to keep the instrument nearly free-falling, and record data only during the down cast. In total, I collected 103 microstructure profiles down to 500 m depth, approximately every hour for five sets of about 15 -20 hour duration. A single profile to 500 m depth takes about 30 min.
The image on the left below shows the leading edge of the profiler with sensors and the probe guard. Two shear-probes and FP07 thermisor are protected by caps. Already at cast number 3, the tip of one shear probe broke and the profiler leaked seawater severely damaging the electronics. Below image on the right shows this broken shear probe and damaged connectors.
To sample the background ocean current, I deployed a 75 kHz Longranger acoustic Doppler current profiler (ADCP). The Longranger required a separate hydrohole of about 1 m cross-section, which I failed to auger. Thanks to the French group who left a nearby large hole where they operated an ROV, I deployed the current profiler for the last 4 days of my stay at Barneo. ADCP sampled in beam coordinates in the upper 400-500 m at 8 m vertical resolution. Longranger is a heavy instrument and I thank Sergey Pisarev for help during deployment and Rick ?? during recovery. We suspended the ADCP from a tripod and lowered the transducers approximately about 1 m below the ice. A differential GPS compass sampled the navigation which I used to convert the data to absolute current. The picture on the left was taken during the recovery.
In addition to current profiles in the upper 400-500 m from the moored ADCP, I dropped 9 expendable current profilers (XCPs) that measures relative current down to 1800-2000 m depth. 2 out of 9 failed. An example XCP ......
The data set will be used to quantify and discuss the internal wave field, mixing and vertical heat flux from turbulence and double-diffusion.
Cruise to West Spitsbergen and Yermak Plateau
See a taste of pictures by Kjersti L. Daae here.
Download the cruise data report (96pp) [pdf – 5.5 MB].
To conduct field work in relation to the project “CuNoS: Current profiling north of Svalbard- An extension of the North Pole Environmental Observatory” we were at sea between 10 July – 4 August 2007, on board Håkon Mosby.
(21 July-4 August) to conduct fine-scale and microstructure survey along the path of West Spitsbergen Current (WSC) with particular attention at the Yermak Plateau. The experiment was designed to collect a data set that would help to better understand the processes involved in mixing of WSC and in the topographic influence of Yermak Plateau, including internal waves.
An ambitious crowd including myself, Ragnheid Skogseth, Florian Geyer, Kjersti L. Daae, Steinar Myking, and Frank Cleveland steamed north to the Yermak Plateau and met the ice edge on 23 July, frustratingly more south than previous week’s ice-charts suggested.
The map above gives an overview of the occupied stations. Sections are labelled A to J with start and end station numbers indicated. Microstructure profiling was made at stations marked by black crosses. Standard CTD profiling was made at stations marked by red circles. Day-long stations 1 to 5 are marked. Short-term mooring was deployed at station 5 (red square). We deployed a heavily equipped mooring to sample for the cruise-duration. We occupied 5 stations of 24 hour duration each, repeating turbulence profiling at 1 hour intervals. We dropped 15 expendable current profilers (XCPs) recording ocean currents down to 1500 m depth. The XCP capability is a result of the CuNoS project. We worked sections across Yermak Plateau and across WSC along 80N, 79N30, 79N and 78N, the latter two extending into Kongsfjorden and Isfjorden. Overall, a high-pressure system sitting somewhere nearby gave us very pleasant and comfortable working conditions. Hard-working cruise participants appreciated a quick stop at Ny-Ålesund, but we failed to have the shop opened, as Mosby did not qualify a cruise ship. In addition to the mooring data, we collected 185 CTD profiles, 222 microstructure profiles, 15 XCP drops, and estimated one week duration of ship-ADCP.
Cruise with the Ice-breaker Coast Guard Svalbard
19 - 31 March 2007. The cruise started in Longyearbyen and ended in Tromsø. A complete report on the cruise can be accessed here, through Lars H. Smedsrud's web page.
We had about 10 days in the sea-ice south-east of Svalbard and occupuied three ice-stations with about 24 h duration.
In addition to the eddy-correlation turbulence measurements conducted by Miles McPhee, microstructure profiles were collected covering the full depth at the three ice stations: Station 1 in Van mijenfjorden fast-ice; Station 2 near Freemansundet fast-ice; and Station 3 from drifting ice east of Freemansundet in western Barents Sea. An MSS90L loosely tethered free-fall profiler was used with a noise level for dissipation of about 3x10^-9 W/kg. Ice deployments were made from a hydrohole in the ice, within 30 m from TIC location and at least 150 m away from the research vessel.
Background currents in the upper 30-60 m (depending on the scatterer strength) were monitored by acoustic Doppler profiler Aanderaa RDCP600 at Stations 1 & 2 and Sontek at Station 3, suspended from ice looking downwards. Additional microstructure profiles were collected from K.V. Svalbard along a section in the Storfjorden polynya and at several stations in the vicinity of the Storfjorden sill.