Yao Fu is seeking a motivated graduate student to join his research group at the College of Marine Science, University of South Florida. This position offers an exciting opportunity to study large-scale ocean circulation in the Atlantic Ocean such as the Atlantic Meridional Overturning Circulation (AMOC) and subpolar/subtropical gyre circulation, using observational data and numerical models, with a focus on understanding its variability and role in the climate system.
Hello there! My name is Charlotte and I’m part of the science team onboard the RRS Discovery, on expedition DY181 to the Ellet Array. In this blog I tell you who I am, what I’m doing here, and some anecdotes of my experiences so far… I hope you enjoy reading!
Who Are You/What Do You Do on Land?
I am a postgraduate student on the NERC DTP at the University of Oxford, based in the Department of Earth Sciences. My research involves using model simulations to examine the mechanisms driving ocean circulation in the Arctic, and investigating how these processes will vary with predicted warming and freshening at the sea surface. This is important because water masses modified in the Arctic are ultimately exported into the North Atlantic, and play a significant role in the large-scale ocean circulation, which is fundamental in setting the global distribution of heat and rainfall.
What is your role onboard?
I am part of the physics watch, and a major aspect of my role is taking salinity samples from the CTD. A CTD is a scientific instrument used to study how physical and biogeochemical properties change throughout the water column. It consists of a steel frame (a rosette) fitted with a plethora of sensors to measure salinity (conductivity), temperature, and pressure (depth). Additional sensors are often also included that measure parameters including oxygen, PAR, and transmissivity, and an ADCP (Acoustic Doppler Current Profiler) to measure current speeds using sound waves… more on this later! The rosette can also be fitted with up to 24 x 10L Niskin bottles that are programmed to close at different depths to collect water samples.
When the CTD comes back on deck, the fun begins! First, the chemistry team collect water samples from each of the Niskin bottles, to later analyse in the lab for dissolved oxygen, carbon, and nutrients. It’s important to know the in-situ temperature of the water at the time these samples are collected, and so I follow behind the chemist on watch (usually Zarina) with an empty flask and a thermometer to take a reading. This also helps with cross-calibrating the temperature sensors.
Once the chemistry team is done, I take salinity samples with the remaining water in each of the Niskin bottles. With an empty flask in hand, I first rinse it out three times with water from the corresponding Niskin to prevent any contamination of the sample. I then fill the flask up to just below the neck of the bottle, and use a clean paper towel to thoroughly dry around the opening and inside of the neck, to prevent the formation of salt crystals. With the stopper firmly in and the lid screwed on, I return the sample to it’s crate and repeat again for the next Niskin!
Me taking a salinity sample from one of the Niskin bottles on the CTD
NMF technicians recovering the CTD to deck, ready for sampling!
Another large part of my role onboard has been processing some of the LADCP data in MATLAB. Tangent: I love MATLAB. I don’t know if my fellow scientists on board thought I was being sarcastic when I said this, but I was being genuine… I even have a pair of MATLAB socks! Anyway …
An LADCP (Lowered Acoustic Doppler Current Profiler) is an instrument attached to the CTD rosette that measures full-depth profiles of ocean velocity. In fact, our CTD has two LADCP’s, an “up-looker” and a “down-looker”, which look up and down the water column, respectively. During processing, data from the two ADCPs is combined, meaning we can get a full picture of current velocities throughout each CTD cast.
One result I found particularly interesting occurred during our “Yo-Yo” CTD cast, in which the CTD was lowered to a depth of 400m and brought up to near-surface repeatedly 24 times. This was carried out on the Scottish Hebrides continental shelf in order to study the dynamics of the European Slope Current, which carries warm and salty water northward through the Rockall Trough, influencing the climate across the British Isles and Western Europe. By stacking together velocity-depth profiles for each CTD cast over the duration of the Yo-Yo, we were able to build up a picture of how the current ebbs and flows over a period of a few hours… if you’re as fascinated by this as I am, go read the cruise report when it’s published!
What do you do when not on watch?
There’s plenty to do on board! There’s a bar with lots of games (although somehow, we always end up playing Bananagrams), a DVD room, and even a gym (I don’t go to the gym). Zarina and I have been attempting a 1000-piece jigsaw puzzle that is proving to be quite challenging… stay tuned to hear whether we finish it before we reach Iceland!
Besides that, I enjoy spending time up on the bridge or out on deck with a pair of binoculars looking for marine mammals! So far, we’ve encountered dolphins, pilot whales, and even some sperm whales!
I also love watching the waves rising, swirling, and crashing against my cabin porthole! The colours of the sea and sky are spectacularly beautiful, and I feel incredibly inspired to make some artwork when I return home!
Some images I took of the waves crashing against my cabin porthole!
What are your favourite moments so far?
As we were leaving Aberdeen, we were accompanied by a pod of dolphins playing off the bow of the ship! I’ve never seen real-life dolphins in the wild before, so watching them leap and play was particularly special!
Transiting past the Scottish Islands was truly magical. I spent a lot of time in Scotland trudging through bogs and clambering over boulders whilst on fieldwork during my undergrad, so seeing the islands from afar has given me a fresh perspective and deepened my appreciation of this unique geological landscape!
The Isle of Mull, viewed from my cabin porthole!
A view of the Scottish Isles we encountered whilst traversing through the Firth of Lorn
Ultimately though, my favourite thing has been getting involved in the science! Knowing that I’ve made even just a small contribution towards advancing our understanding of the ocean in the Subpolar North Atlantic has been exceptionally rewarding, and I am so grateful to have had this phenomenal opportunity! Thank you to everyone involved who helped make it happen!
Follow me on Twitter/X: @char_marris and Instagram: @charlottegoes2sea
The RRS Discovery on a rare sunny day in the Irminger Sea, taken by the U. Miami drone.
Femke den Ouden – Student Utrecht University
When I got an email from my Marine Sciences Master (Utrecht University) coordinator that you could sign up to join the DY182 cruise on the RRS Discovery, I didn’t hesitate. My main scientific interest is physical oceanography and the cryosphere and I just finished a course in which we elaborately discussed the AMOC and the OSNAP project. How amazing is it to actually experience a project and region like this, instead of only seeing it on a lecture screen?! When I heard that I would also be able to write my master thesis with NIOZ on the OSNAP data, I was even more excited to be able to experience the data collection hands on. I bought all the clothing needed for the expected cold and wind in the North Atlantic, did my safety training and medical exam and flew, together with my fellow students, to Reykjavik to start this adventure!
Group photo!
The DY182 cruise is shared between the University of Miami (moorings in the Icelandic basin, east of the Mid-Atlantic Ridge) and NIOZ (moorings in the Irminger sea, west of the Mid-Atlantic ridge). The University of Miami team is full of experienced seafarers: Tiago, chief scientist for the first time(lobbyist for a Brazilian barbeque on deck); Bill, the most experienced oceanographer and one of the founders of OSNAP (and the best cribbage teacher); and oceanographers and technicians Eduardo (who looks like a superhero in his safety harness), Cedric (likes to drink an IPA each night), Joe (No need for a spare anchor this time) and Houraa (showing exponential growth in her darts skills). From the Netherlands (NIOZ and Utrecht University) we came with a relatively inexperienced team. However, I feel like we bring a lot of enthusiasm and energy to the ship. Sasha, PhD student and the only one with real seagoing experience (can finally use his bike helmet); Emma, working on her virtual ship classroom (voluntarily wakes up at 4 in the morning); Claudia, having a do-over for the 2022 cruise (has a hate relationship with Microcats and a love relationship with Iceland); Laura, journalist of NRC writing an article on the AMOC (whose favourite moment of the day is sitting at the back deck with a book and cup of tea); Dave, NIOZ technician who impressed the whole ship with his efficient recoveries and deployments (can be found in the gym each day); and then lastly the students Aart (programming specialist and foosball star player) Ann (LOVES cleaning buoys with slimy marine life), Josefien (always does a happy dance when walking along the amazing food buffet), and me, Femke (the sports instructor of our team). And of course, the amazing technicians Ellis, Dean, Dave, John and Tom, and the rest of the crew!
Besides the fact that the whales have been exceptionally absent this cruise, it has been amazing so far! We have had some very sunny days during which we could chill at the front or back deck, some great sunsets and sunrises, and not many fogs for which the Irminger sea is famous. I am very aware that I got very spoiled to have the Discovery as the ship for my first cruise. The ship is super stable so it took me only 2 days to grow my sea legs. Furthermore, we have the extreme luxury of CTD operators from the ship, which means that they do most of the CTD work. This gives us more time to learn about the moorings and their data. The food here is amazing, Peter, Neil, Tina and Tony make sure that all our hearts and stomachs desires are met. Luckily there is a relatively large sports gym, otherwise I would gain a kilo per week due to their amazing cooking and hospitality. The whole crew is amazing, they have made me feel welcome since day 1 and the Discovery has really started to feel like home in these past weeks.
Early morning deployment of a NIOZ mooring.
Femke and Ann programming and reading out instruments of the moorings.
Josefien’s typical day at the RRS Discovery
7:15 Waking up My alarm goes off. The science cabins for the students are located on the Main Deck, which is on the same level as the sea, so I wake up with the sounds of waves splashing against my two small round windows.
7:30 Breakfast The galley (the ship’s kitchen) is two floors up. Here, you find your typical English breakfast made by Peter and Neil. At 8:00 the first monitor check needs to be done, which only takes a couple minutes and needs to be done every two hours.
8:30 Science meeting After breakfast we go one floor down to head towards the Main Lab. We meet up with the scientists, students, technicians and captain to discuss today’s research plan based on the weather forecast.
9:00 Mooring recovery After spotting the big yellow buoys in the sea, the mooring is attached to the ship and cautiously lifted out of the water. Dave, our technician, carefully detaches each instrument; this takes a pretty long time since the mooring can be up to 2500 m. We help with cleaning the instruments (after two years in the ocean the devices are covered in algae and some slimy worms) and bring them inside to stop the logging and start reading out the data. In the rush of the mooring recovery, I forgot the monitor checks at 10:00, oops…
11:30 Lunch. A nice and warm lunch is prepared for everybody. Way better than the slices of bread I would eat in the Netherlands! With so many options, your plate could look completely different from your neighbour’s.
13:30 CTD CTD time! The CTD Rosette collects water samples at different depths and measures the salinity, temperature and pressure on its way. We log the timing and coordinate position. Meanwhile, we begin programming the Microcats – devices that measure salinity, temperature, and conductivity – for a ‘Caldip.’ This calibration process compares the CTD measurements with those from the Microcats to determine their placement on the mooring line for deployment. Once the CTD is back on deck, we collect water samples and the technicians analyse them in the Salinity Lab.
15:30 Plankton We deploy a plankton net to a depth of 50 meters. After the cruise, DNA analysis will be conducted on the collected plankton. To prevent contamination with human DNA, we handle the plankton net only with plastic gloves. In addition to lot of plankton, the catch also included fish larvae and miniature sea snails.
16:00 Sports Together with Femke, Emma, Laura and technician Ellis, I am doing an intense workout in The Hold below the deck. Exercising is a lot harder on a moving ship, but way more fun!
17:30 Dinner and spare time With the whole crew we have a nice dinner, especially the desserts are amazing. After dinner, we hang out in the bar, have a beer (max two per day is allowed), and play some games. Especially foosball and darts are popular. Unfortunately, Aart is on the winning hand with foosball… After watching an episode of Game of Thrones with some people, I go to my cabin to sleep.
Daily plankton sampling.
Laura Bergshoef – Journalist NRC
A day on the ship is never boring. Around us it might seem as an endless emptiness: just ocean, clouds and I only saw one other ship during the cruise. But inside the ship it’s never silent. Constantly you will hear ventilators, men with helmets talking though radio’s, rolling bottles and stories about pirates. I have been wandering around the ship a lot during day and night (and even saw the northern lights). Before coming on board I expected that life on sea will be lonely as we are so remotely. But the opposite is true. On the RRS Discovery – crossing the North Atlantic – we all eat, sport, watch Olympics, play games together as one big family on a Christmas holiday.
Surprisingly for me was that at night, the ship is controlled in complete darkness. Looking outside from the bridge, it felt like driving a car at night without any light.
As a journalist I always try to get as close to my topic as possible. When Femke de Jong and I arranged a time and date for our newspaper interview and she joked that if I wanted to come as close to her work as possible I should join this cruise on the North Atlantic, I thought ‘why not’. As a climate scientist/oceanographer by training and author joining a cruise like this for a story is a dream come true. I think it’s one of the most special stories I will ever write.
The R/V Armstrong along the southeast Greenland coast. Drone photo by Croy Carlin.
By Aurora Roth
On September 1, almost halfway through the cruise, we passed through Ikerasassuaq (“big sound”, alternately known as Prince Christian Sound). It’s a long narrow channel that demarcates the north end of Uummannarsuaq (“large heart-shaped place” or Cape Farewell), at the southern tip of Kalaallit Nunnat (Greenland). Travelling through this fjord marked our transition from work on the southeast coast and the Cape Farewell mooring array to work on the southwest coast and the Labrador Sea mooring array. Until this, we had been moving up and down the southeast coast (see map for cruise track), completing hydrographic surveys, taking water samples, and replacing long-term instruments that will live in the depths of the ocean taking measurements for the next two years. The coasts, seas, glaciers, histories, and settlements of the southeast and southwest regions are wildly different, and going through Ikerasassuaq felt like going through a portal to a different world.
Map of South Greenland showing some Greenlandic place names discussed in this post. Colors indicate sea floor bathymetry (from BedMachinev4 (Morlighem et al., 2017)) and show troughs extending from fjords on the continental shelf. Blue stars indicate towns. White is the Greenland Ice Sheet extent and grey is ice-free land above sea level. Red dots show Cape Farewell Mooring Array (on the east) and Labrador Sea Mooring Array (on the west). Map created with the help of Jamie Holte.
We followed a cargo ship into Ikerasassuaq, bound for Qaqortoq (“white”), the largest town in the south of Greenland. We saw the lights of Aappilattoq (“the reddish place”), the first town we’ve seen as we stopped in the sound for the night. In contrast, the southeast coast has no towns, though there are archeological sites of older settlements. The southeast coast, with its steep mountains and glaciers plunging into the ocean, isn’t the best place for permanent towns, but it used to be travelled often by people from further north around Ammassalik (“the place with capelin”). From Ammassalik, people would travel south and around the bottom of Greenland to the west coast to trade and meet up with West Greenlanders. As the east coast was colonized, a Danish trading post and mission were set up in Ammassalik. Travelling along this part of the southeast coast stopped as trading with Denmark replaced travelling to trade with West Greenland. As a consequence, East Greenlanders became even more isolated from the populous western side (Gulløv 1995). Now, the few people that follow this coast and navigate around the icebergs here are from adventure sailing and climbing expeditions and the occasional research vessel, like us. For almost two weeks we didn’t see any other vessels. There is also a lasting separation between East Greenland and West Greenland, and East Greenlanders are often marginalized, with less access to resources, fewer education opportunities, and less power in the national government.
The R/V Armstrong making our way through narrow Ikerasassuaq – moving from the southeast coast to the southwest coast.
I’ve been wondering what knowledge, place names, and connections have been lost here along this stretch of coast as a result of colonization. Documented Greenlandic place names are sparse here, but what I could find (mostly from a report by the Danish Geodata Agency, reviewed by Oqaasileriffik, the Greenlandic Language Secretariat) made this coast feel more animated. The place names show how Greenlanders gave their attention to and were in a relationship with this stretch of coastline – Timmiarmiut fjord and glacier means “habitation of the birds”. Sikuijivitteq means “where it is never free of ice”. Puisortoq glacier means “how often something comes to the surface” but also “puisi” is a word for seal and perhaps this is related? Anoritooq glacier and fjord, meaning “where it is very windy”.
Our furthest north point was Timmiarmiut fjord. We surveyed the underwater trough that extends beyond the fjord onto the continental shelf. This trough, and others like it, were made over 10,000 years ago when the sea level was lower and glaciers of the ice sheet extended further. After sending our instruments down into the water above the trough, we watched squiggly lines form on the screens in the lab. These data tell us that cold water with a temperature below 0°C was following the trough out into the ocean but near the surface, and warmer water (3-4°C) from the North Atlantic and Gulf Stream was flowing into the bottom of the trough. We call the cold water “Polar Water” and it’s a mix of glacial ice melt, sea ice melt, and water from the Arctic Ocean. Its freezing point is below 0°C because it’s salty ocean water despite being less salty than other ocean waters. The Polar Water and warmer Atlantic water come together and intermingle at different depths, trying to figure out where to go as they meet each other and mix. We collected data at other fjord outlets and troughs as we came back south along the coast. Each time we saw this layering of colder, fresher water, sitting above warmer, saltier water and saw thin layers of each water type trying to weave together. The outcome of this meeting and weaving is what controls how much warm water can come into fjords and contact glaciers causing further melt, and how much glacial meltwater can then come out of fjords and mix into the North Atlantic Ocean, affecting large-scale ocean currents and climate.
From velocity measurements taken with Acoustic Doppler Current Profilers (ADCPs), we started to map out how these troughs make the East Greenland Coastal Current wiggle and turn, further complicating this meeting of different waters. Even when hundreds of meters deep, the ocean can “feel” the bottom topography and responds to it by changing course. Sometimes in the data, we could see eddies generated from the troughs that were swirling in circles, causing water to mix. To make things even more complicated, we experienced two days of strong winds while collecting data. The winds pushed the surface of the ocean around causing waves and more mixing of water. When the winds blow from north to south, this sets up the physics of the ocean to push the warmer water closer to the coast. When I step back and take this all in, there is so much movement and action at every scale. Everything affects everything else. It’s sort of amazing that we can make any sense of the data we collect at all, that we can figure out stories of what’s happening here and why it matters.
Many of the glaciers along the southeast coast have transitioned in the last two decades from ending in the ocean to now ending on land, retreating up into valleys, deflating from their moraines, and separating into different branches. When a glacier loses its direct connection with the ocean, it changes how the glacier meltwater mixes nutrients into the ocean and it changes the marine ecosystem. One of the things we’re doing on this cruise is collecting water samples to analyze for nutrients (like nitrate, phosphate, silicate, etc) to understand this story better. The Greenlandic place names speak to these ecosystems supported by glaciers, of the birds and seals (even the winds which are impacted by glaciers). I wonder how long the place names that remain will continue to describe these places, or even if they still do.
We are here now in Kalaallit Nunaat, as western-trained scientists trying to understand certain stories of this place. Greenlandic people and culture have stories of this place too that are equally important. Sometimes glimpses of those stories are found in the Greenlandic place names. The small act of putting in effort and time to figure out place names, and use them in conversation, outreach, or academic publications, is a small act of acknowledging Greenlandic presence and relationship here. It’s also an act that acknowledges what was violently lost due to colonization and what stories and names have persisted despite it.
Learn More
Greenland Pilot – Explanations of the Place Names from the Danish Geodata Agency
Hans Christian Gulløv. Olden times in Southeast Greenland: New archaeological investigations and the oral tradition. Études Inuit Studies Vol. 19, No. 1, Archéologie, Art, Ethnicité (1995), pp. 3-36.
Morlighem M. et al., (2017), BedMachine v3: Complete bed topography and ocean bathymetry mapping of Greenland from multi-beam echo sounding combined with mass conservation, Geophys. Res. Lett., 44, doi:10.1002/2017GL074954, http://onlinelibrary.wiley.com/doi/10.1002/2017GL074954/full
Our cruise is now about halfway through, and spirits are high as we steadily make progress on accomplishing our cruise objectives. Yesterday the final University of Miami mooring was deployed in the Iceland Basin, completing the turn-around operations on all of the seven moorings deployed along the eastern flank of the Reykjanes Ridge. We have now turned the mooring operations over to the NIOZ team, who at this moment are deploying their westernmost Irminger Sea mooring (see photo), and if all goes well will follow that with the deployment of their second (of five) moorings this afternoon.
It is a rather nice morning in the Irminger Sea, with relatively calm conditions and the sun even peeking through the clouds from time to time. Apart from a following swell that is occasionally putting some water on the aft deck, it is a fine day for mooring work, seriously challenging our usual mantra that “it is nicer in the Iceland Basin”. Our mooring operations there were mostly in overcast and often cold, drizzly conditions and so the weather today in the Irminger Sea is a pleasant surprise (however, not to be discussed further..). The only major glitch we have encountered so far is that one of the U. Miami moorings unexpectedly returned to the surface after its deployment, while communicating with the acoustic mooring releases to put them into “sleep mode” for the next two years until we come back again, and that mooring had to be recovered and redeployed again, basically costing us a day of ship time. This is one of the reasons, besides unpredictable weather, that we build a little extra time into our cruises for contingencies.
The U.S. and Dutch teams continue to work very well together, and this is particularly evident on deck during the mooring operations. During the first half of the cruise, when the U. Miami mooring work was taking place, the two lead technicians from NIOZ (Leon Wuis and Toon Koopman) ran the mooring winch for the U. Miami team, who were busy loading instruments and floats onto the mooring line as it was paid out. Now the lead U. Miami technicians (Eduardo Jardim and Cedric Guigand) are doing the same for the Dutch team as they recover and deploy their moorings. It is always interesting to see how different research groups approach their mooring operations, with somewhat different mooring designs and different methods of deployment. For example, the NIOZ team uses a “mooring table” (pictured in the photos) that has a built-in stopper system for changing mooring wires and attaching instruments, whereas for the U. Miami moorings this is all done at deck level using a traditional double line stopper system.
The mooring winch system we are using is a specially designed system called a LEBUS (aka “double barrel”) winch system (see photo) that is somewhat complicated but highly efficient for performing rapid mooring turnaround operations. It takes two people to run it with a third assisting, and with at four people on the aft deck making mooring connections, and several others bringing instruments in and out from the lab, it requires a large team and a carefully choreographed operation.
With the change-over in mooring operations, the night-time CTD watches have also changed at mid-cruise to free up personnel from the Dutch team for their mooring work. During the first half of the cruise, these watches were mostly manned by NIOZ scientists but now they are being mostly manned by the U. Miami team. Everyone is adapting to these changes well and pitching in wherever needed. It has been a great pleasure sailing with this entire team thus far, and – as we often say – “so far so good”, with our fingers crossed.
A simple Python code that computes the meridional overturning circulation (MOC) from a model dataset interpolated onto the OSNAP section. More information can be found on the o-snap website under the tabs Outreach/For Modelers and the code can be downloaded here.
The deadline to submit your abstract is *3 August (23:59 EDT/03:59 GMT)*, instructions are found here.
We hope to meet you virtually or in person in December. Please don’t hesitate to forward this call to other colleagues who may be interested.
Raquel, Elizabeth, Yao and Dariia
*OS026 – Variability and controls of ocean climate revealed by long-term multidisciplinary eulerian observatories*
The global ocean absorbs, stores, and redistributes vast amounts of heat and carbon and is therefore the main driver of climate regulation. It means that human-induced forcing is superimposed on ocean natural variability, and that this variable forcing involves complex interactions and feedbacks of physical, chemical and biological processes from the air-sea interface to the sea-floor. To understand these complex relationships between ocean processes, their forcings, and effects, long term time-series of Essential Ocean Variables (EOV) from fixed point (Eulerian) multidisciplinary observatories, monitoring processes that take place over much shorter periods of time, are critical.
In this session, we aim to share most recent research based on long term OceanSITES data sets or on long term time-series from eulerian multidisciplinary observatories not included in the network addressing air/sea exchange processes such as heat and freshwater fluxes, and ocean carbon and oxygen update; ocean transport, but also other biogeochemical, biological, and deep ocean processes. We welcome contributions which describe ocean variability at different time-scales from large-scale climatic fluctuations to hurricanes, and examine their governing mechanisms and environmental implications. Submissions focused on processes-oriented studies integrating different observing platforms (e.g. satellite, Argo floats, gliders, …) with long-term eulerian observation (moorings, ship-based time-series) are also encouraged as well as studies combining observations and modelling.
*Conveners:*Raquel Somavilla (Spanish Institute of Oceanography), Elizabeth Shadwick (CSIRO Marine and Atmospheric Research), Yao Fu (Georgia Institute of Technology), and Dariia Atamanchuk (Dalhousie University)
*Index Terms: *4215 Climate and interannual variability, 4805 Biogeochemical cycles, processes, and modeling, 4504 Air/sea interactions, 4532 General circulation
As revealed by the first 21-month record (2014-2016) from the OSNAP array [Lozier et al. 2019], the subpolar overturning occurs primarily in the eastern subpolar region of the North Atlantic Ocean. This finding contrasts the view of Labrador Sea variability dominating the overturning circulation variability across a range of latitudes via the southward propagation of density anomalies formed by convection in the basin interior. The small overturning in the Labrador Sea was surprising since there was strong convection in the Labrador Sea during the 2014-2016 winters. However, given the scale of the expected delay in the overturning response, the 2014-2016 time series may have been too short to pick up this response. In short, the signals in the Labrador Sea may take a few years to imprint on the OSNAP array.
Continuous observations from the OSNAP array are now available for a 4-year period. The 4-year time series (2014-2018) of the estimated overturning across the section is shown in the figure below. Here the strength of the overturning circulation is defined as the maximum of the streamfunction in density coordinates, and is estimated across the full array, OSNAP West (Labrador Sea) and East (eastern subpolar gyre between Greenland and Scotland), separately. As shown in Lozier et al. [2019], this decomposition allows us to make relative comparisons of the overturning contributions within the subpolar subbasins. Clearly, there is a consistent pattern throughout the records: the overturning circulation takes place primarily across the OSNAP East section in the eastern subpolar region. The extended time series proves the validity of the first 21-month record and supports a diminish contribution from the Labrador Sea, over the observational time scales.
30-day time series of the strength of the meridional overturning circulation (MOC) across the full OSNAP array, and the OSNAP East and West sections, separately. Shading indicates the uncertainty in each 30-day estimate, obtained from Monte Carlo simulations. The 4-year mean MOC is 16.6 Sv# (uncertainty of 0.7 Sv; standard deviation of 3.5 Sv) across the full array, 16.8 Sv (uncertainty of 0.6 Sv; standard deviation of 2.8 Sv) across OSNAP East, and 2.6 Sv (uncertainty of 0.3 Sv; standard deviation of 1.2 Sv) across OSNAP West. # 1 Sv= 106 m3 s-1
Further analysis has been performed to assess the role of Labrador Sea and Irminger Sea convection in creating western boundary density anomalies in the region and the contributions of those boundary anomalies to the subpolar overturning variability [Li et al. 2020]. Results from this analysis shed light on the causes of North Atlantic overturning variability downstream from the subpolar region.
Of course, a question remains as to which subpolar basin(s) holds the key to the overturning variability over interannual and longer time scales. Needless to say, maintaining high-quality trans-basin observations is critical for tackling this question.
Finally, we are pleased to announce that all of the seven 2020 OSNAP cruises have been carried out successfully. Special thanks to the PIs from the observational groups who worked tirelessly in (re)planning the cruises to accommodate the extraordinary challenges caused by the pandemic. All cruises had to reduce their working time at the mooring sites because of the need for more transit time to and from their home port. All observational groups outperformed expectations: everyone was safe and the data return was superb. We also applaud the captain, the crew and everyone on the science teams! With all of these efforts, we will be adding another two years of transport estimates in 2021 without significant delays.
References
Lozier, M.S., F. Li, S. Bacon, F. Bahr, A.S. Bower, S.A. Cunningham, M.F. de Jong, L. de Steur, B. deYoung, J. Fischer, S.F. Gary, B.J.W. Greenan, N.P. Holliday, A. Houk, L. Houpert, M.E. Inall, W.E. Johns, H.L. Johnson, C. Johnson, J. Karstensen, G. Koman, I.A. Le Bras, X. Lin, N. Mackay, D.P. Marshall, H. Mercier, M. Oltmanns, R.S. Pickart, A.L. Ramsey, D. Rayner, F. Straneo, V. Thierry, D.J. Torres, R.G. Williams, C. Wilson, J. Yang, I. Yashayaev, J. Zhao, A Sea Change in Our View of Overturning in the Subpolar North Atlantic, Science, 363(6426), 516-521 (2019).
Li, F., M.S. Lozier, S. Bacon, A. Bower, S.A. Cunningham, M.F. de Jong, B. deYoung, N. Fraser, N. Fried, G. Han, N.P. Holliday, J. Holte, L. Houpert, M.E. Inall, W.E. Johns, S. Jones, C. Johnson, J. Karstensen, I.A. LeBras, P. Lherminier, X. Lin, H. Mercier, M. Oltmanns, A. Pacini, T. Petit, R.S. Pickart, D. Rayner, F. Straneo, V. Thierry, M. Visbeck, I. Yashayaev, C. Zhou, Subpolar North Atlantic Western Boundary Density Anomalies and the Meridional Overturning Circulation, submitted to Nature Communications (2020).
OSNAP scientist, Femke de Jong, and a team of NIOZ scientists are currently at sea on a research cruise aboard the RV Pelagia. They will be servicing moorings and deploying drifters in the Irminger Sea. Follow their progress on the cruise blog posted here.
While the rest of the OSNAP team has been busy preparing the first continuous time series of the AMOC in the high-latitude North Atlantic from the OSNAP array measurements (link to Science paper), the Bower Lab has been steadily processing data from more than 120 deep-sea floats that were released in the deep currents of the AMOC between 2014 and 2017 as part of “OSNAP Floats”. These so-called RAFOS floats (RAFOS stands for RAnging and Fixing Of Sound) were tracked continuously underwater using an array of 13 moored sound beacons spread throughout the subpolar region. Once a day during each float’s mission, the float “listened” for signals from the sound beacons, and internally recorded the time it heard each one (usually only 2-4 beacons were in range on any given day). After two years of collecting these “times-of-arrival” from the sound beacons, each float dropped a ballast weight, popped to the surface and transmitted all its stored data via satellite. We have been reconstructing each float trajectory by converting the acoustic travel times to distances, and triangulating float position for every day. It is always an exciting moment when the complete trajectory pops onto the computer screen, usually revealing some incredibly interesting pattern of the flow deep in the ocean.
Nearly every diagram of the pathways of the lower limb of the AMOC in the subpolar North Atlantic show continuous lines of flow snaking around the rim of its three sub-basins:the Labrador, Irminger and Iceland Basins (e.g., Fig. 1). This view of the deep circulation has been shaped largely by our observations of the subpolar ocean around its edges, which almost all show a relatively strong deep boundary current transporting modified versions of the dense overflows (Iceland-Scotland Overflow Water and Denmark Strait Overflow Water) that feed the lower limb of the AMOC. When we track the pathways of individual fluid parcels, or large groups of them, we see a somewhat different view of the pathways of the deep AMOC (Fig. 2). This “Lagrangian” view—by which we mean the ocean circulation as observed by following individual fluid parcels, in contrast to the “Eulerian” view, which is constructed from current measurements at fixed locations—highlights the turbulent nature of the currents in some regions of the deep ocean. The float trajectories show a much messier view because deep ocean currents are not always orderly streams of water flowing smoothly along, but rather consist of blobs or patches of water 10-100 km in diameter swirling around like slow-motion hurricanes.
In some regions, the Lagrangian and Eulerian views look similar, at least on the large scale. For example, most (but not all) of the floats released east of Greenland more or less follow the boundary around the southern tip of Greenland. In other regions, however, the boundary-following pipe-like view seems completely inadequate to describe the deep pathways of the AMOC. For example, floats coming through the Charlie-Gibbs Fracture Zone (a deep gap in the mid-Atlantic ridge that separates the Iceland and Irminger Basins) do not, in general, follow the boundary of the Irminger Basin. Instead, the floats spread westward and even southward in a somewhat chaotic fashion. Also, some floats released in the deep boundary current transporting Iceland-Scotland Overflow Water southward in the Iceland Basin never make it to the Irminger Basin through gaps in the ridge, but instead drift southward within the eastern North Atlantic, apparently having escaped from the deep boundary current system altogether.
So why do we care what pathways these deep waters take? For one practical reason—if we want to accurately monitor the strength of the deep AMOC, we better have a good idea where its major branches are so we can place our instruments judiciously. Also, the pathways of these deep water masses have an impact on how they are modified. A branch that carries deep waters along a slow, meandering interior pathway may lead to more stirring and modification of the water properties than would occur along a more direct, boundary-following path. This in turn affects how heat and other properties are re-distributed throughout the ocean.
The Bower Lab is looking forward to the upcoming General Assembly of the European Geosciences Union in Vienna, Austria next week, where we will have the opportunity to showcase the many and varied discoveries emerging from OSNAP Floats.
Fig 1. Recent diagram illustrating the circulation of the deep currents of the subpolar North Atlantic, From Daniault et al. (2016)
Fig. 2: Trajectories of 123 RAFOS floats, tracked at depths from 1800 m to 2800 m between 2014 and 2018. Colors indicate individual float tracks.
Daniault, N. et al., 2016. The northern North Atlantic Ocean mean circulation in the early 21st century. Progress in Oceanography 146, 142-158
