OSNAP/NIOZ Cruise Blog

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: https://www.nioz.nl/en/blog/niozatsea-i-osnap-2020

OSNAP at Ocean Sciences Meeting 2020

If you’re at Ocean Sciences this week, and curious to learn more about ongoing OSNAP research, there are numerous opportunities. Follow the link below to find a list of oral and poster presentations featuring OSNAP related topics:

OSNAP at Upcoming Meetings

Postdoctoral position – IFREMER

Applications for an 18-month postdoctoral position are invited from candidates with a recent PhD in physical oceanography or similar. The successful candidate will be based in IFREMER, Brest (France), and will join the“Ocean and Climate” team of the Laboratory of Physical and Spatial Oceanography (LOPS).

Project summaryThe sampling of the deep ocean (>2000 m) is primarily relying on intermittent shipborne surveys along sparse trans-basin hydrographic sections that cannot capture the full spectrum of oceanic variability. To overcome this issue and reduce the associated uncertainties on heat and freshwater ocean budgets, IFREMER and LOPS contribute to an international effort for the vertical extension of the Argo network towards the abyss – the Deep Argo network – by maintaining a regional pilot array in the subpolar North Atlantic. This region is key in the climate system since it is here that surface climate signals can enter the deep ocean interior, through the formation and vertical sinking of the North Atlantic Deep Water (NADW).

As part of the European project H2020 Euro-Argo RISE, the proposed work will combine newly available Deep-Argo profiles (Figure) and other observational datasets to obtain a better quantification and understanding of NADW intra-seasonal to decadal variability in the Irminger Sea. In parallel, the quality of Deep Argo data will be evaluated through an inter comparison of three different deep sensors mounted on a single three-headed float (Figure). On top of informing on the variability of the abyssal North Atlantic and its causes, these activities will provide a significant valorization of the Deep Argo dataset as well as crucial recommendation to the international community on technological choices for the global extension of the network.

Contract duration

18 months (open until filled)


  • PhD in physical oceanography or related discipline (meteorology, applied mathematics)
  • Experience in analysis of observed data sets and/or model outputs
  • Good programming skills (Matlab, Python, or equivalent).
  • Skills in writing peer-reviewed scientific papers (English)

Interested candidates should send the following documents to damien.desbruyeres@ifremer.fr andvirginie.thierry@ifremer.fr: Curriculum Vitae (including publication record), a statement of previous research and motivations for the present project, and contacts of at least two referees). Candidates must be within 6 years after PhD defense and must not have been employed as a postdoctoral fellow by Ifremer before.

Starting date before 31st December 2019


A closed heat budget for the mid-latitude North Atlantic?!

by Nick Foukal

Now that the OSNAP and RAPID arrays are running concurrently, an obvious question arises: can we close a heat budget for the mid-latitude North Atlantic? A heat budget is a very simple concept – let us pretend that the ocean between RAPID and OSNAP is a box with an ocean flux coming into the southern boundary (at RAPID), another going out of the northern boundary (at OSNAP), and surface fluxes exiting through the top (Fig. 1). The sum of the oceanic fluxes and surface fluxes should equal the change in the temperature of the box, meaning the heat budget is “closed” (see equation below). This is a useful exercise because determining whether the ocean temperature variability is caused by ocean dynamics or surface fluxes gives us a better idea of how the system will evolve in the future. Though this task of closing heat budgets may seem incredibly simple to the lay audience, it has never been done before for a region as large and important as the mid-latitude North Atlantic.

Ocean temperature variability = surface heat flux + ocean heat transport divergence

Figure 1 – (left) RAPID and OSNAP lines in the North Atlantic (red lines) and time-mean sea-surface height (colors). Figure adapted from Lozier et al. (2019). (right) Simplified box representation of the mid-latitude North Atlantic heat budget with ocean heat fluxes into the box at RAPID, out of the box at OSNAP, and a net surface heat flux out of the ocean.

To close the heat budget with observations, we need reliable measurements of all three terms. There are very reliable reconstructions of ocean temperature variability (from satellites and Argo floats), decent guesses of the surface fluxes (primarily derived from satellites on these scales), but very poor estimates of the ocean heat transports. The processes that govern ocean heat transport operate on such small scales that they are difficult to measure in the absence of dedicated in situ arrays. Consequently, what is often done in the literature is the ocean heat transport term is inferred from the difference between the other two terms, and the heat budget is assumed to close. But this is not completely satisfactory because the surface heat fluxes typically have significant uncertainties (more on that later), so relying on them as the “known” component in a heat budget doesn’t inspire confidence in the result.

This is where the OSNAP and RAPID lines come in – they offer an unprecedented opportunity to bound the ocean heat transports over a large region. Never before has a region of this size been this densely sampled. This means that we no longer have to rely on the surface fluxes and conservation laws to close the budget. By knowing all three terms of the heat budget, we can assess how closely we can close the budget… essentially how well do our measurements from independent platforms agree with one another?

There is still one hurdle to overcome in this problem, and that is the uncertainty in surface heat fluxes. This is not a new problem to the field, it has plagued both oceanographic and atmospheric studies for decades. There are two well-known unknowns in surface heat fluxes: (1) the time variability between different surface fluxes data sets do not agree with one another and (2) the global surface fluxes averaged over time do not integrate to what we would expect from observed ocean warming rates. I recently ran into the former concern in a recent paper (Foukal and Lozier, 2018) where we looked at the heat budget for the eastern North Atlantic subpolar gyre in two models, and the end result of our study depended on which surface flux data set we used. With respect to the latter concern, we know that from rates of global ocean warming, the global net surface heat flux must be around 0.5-1 W/m2, yet some surface heat flux products sum to almost 25 W/m2globally (Yu, 2019, Cronin et al., 2019). So there is good reason to doubt both the mean and the variability in surface fluxes, which is not encouraging.

As a taste of this uncertainty, I compiled time series of the surface fluxes over the region bounded by RAPID and OSNAP for three different surface flux products (Fig. 2). To give a rough idea of what we should expect from the surface fluxes, the oceanic heat flux into the box through RAPID from 2004-2007 was 1.33 +/- 0.40 PW (Johns et al., 2011), and the oceanic heat flux out of the box through OSNAP from 2014-2016 was 0.45 +/- 0.02 PW (Lozier et al., 2019). If we assume that these two time periods are representative of the long-term mean, and that the ocean is in steady-state (i.e. the temperature variability is zero when time-averaged), then we should expect the surface heat fluxes to equal the difference between the two oceanic heat fluxes, or 0.88 PW out of the box. Instead, the mean surface heat fluxes are 0.43 PW (ERA5), 0.11 PW (OAFlux), and 0.11 PW (NOCS), all directed out of the box. While it is encouraging that the sign of the fluxes is correct in all three products and that two of the products agree with one another, it also means that somewhere between 0.45-0.77 PW is missing in our heat budget. To put this another way, at least an entire OSNAP of heat transport is missing from this budget, and maybe more. Furthermore, the only statistically-significant correlation between the time series is a relatively weak (r=0.56) connection between the annually-averaged ERA5 and OAFlux. NOCS had no significant correlations to either of the other two. So overall, the spread between the three data products, their lack of coherent variability, and their disagreement in the mean with the net ocean heat divergence does not inspire confidence that we can close a heat budget for the mid-latitude North Atlantic.

Figure 2. Surface heat flux variability integrated over the region between the RAPID and OSNAP arrays in three surface flux products (positive downward; units are petawatts = 1015 W). The thin lines are at monthly resolution, and the thick lines are annually-averaged. The seasonal cycles are removed from the monthly data to consider the non-seasonal variability. The ERA5 (Copernicus Climate Change Service) reanalysis is a ¼° product covering 1979-2018. The OAFlux (Yu et al., 2008) data set covers 1984-2009 at 1° resolution. The NOCS (Berry and Kent, 2011) surface heat fluxes are produced at 1° resolution for the period 1973-2014. The RAPID and OSNAP time periods are shown in the bottom right.

Before we lose hope, it is worth revisiting some of our methods and assumptions: (1) can we really compare the RAPID meridional heat transport from 2004 to 2007 to the more recent RAPID data from 2014 to 2016? Are the heat transports at RAPID from 2014 to 2016 perhaps lower than 1.33 PW? Lozier et al. (2019) report a net heat transport divergence of 0.80 PW between RAPID and OSNAP for the 2014 to 2016 period, which accounts for 0.08 PW of the missing heat fluxes. (2) Can we prioritize the ERA5 time series because it is the highest resolution and the most recently-released of the three products? During the 21 months of published OSNAP data, the mean ERA5 heat flux was 0.57 PW, or 33% larger than the 1979-2018 mean. So if we believe ERA5 over OAFlux and NOCS, then we are only missing 0.23 PW (0.80 PW – 0.57 PW), or only half of an OSNAP! (3) Maybe the ocean was not in steady-state for the OSNAP period (2014-2016), and instead of zero temperature change, the ocean actually warmed considerably? This is a bit of a stretch, as 0.23 PW would be a lot of warming. But it would be worth considering how much the region did warm over this time period to see how it affects the heat budget. After all, each of these heat transports has associated error bars, so maybe we can get close enough so that the error bars explain the residual? I will leave this analysis to further work, as this blog post is getting closer and closer to possible publication material…

So where does this exercise leave us? Surface heat fluxes are certainly a wild-card, but recent improvements (ERA5) seem to be trending in the right direction. In the next few years, OAFlux will be updated with higher resolution, so it would be worth checking if that time series validates the higher surface flux values of ERA5. Finally, I am contractually obligated to mention that continuation of the OSNAP line in the coming years is absolutely critical to closing the heat budget for the mid-latitude North Atlantic. A longer time series would improve our assumption of steady-state in the temperature variability and provide a better understanding of the inherent time scales of the overturning in the subpolar North Atlantic.



Berry, D. I. and E. C. Kent (2011). Air-sea fluxes from ICOADS: the construction of a new gridded dataset with uncertainty estimates. International Journal of Climatology, 31, 987-1001.

Cronin, M. F. and 26 co-authors (2019). Air-Sea Fluxes With a Focus on Heat and Momentum, Frontiers in Marine Science, 6, 430, doi:10.3389/fmars.2019.00430.

Foukal, N. P. and M. S. Lozier (2018). Examining the origins of ocean heat content variability in the eastern North Atlantic subpolar gyre, Geophysical Research Letters, 45, 40, 11275-11283.

Johns, W. E., M. O. Baringer, L. M. Beal, S. A. Cunningham, T. Kanzow, H. L. Bryden, J. J. M. Hirschi, J. Martotzke, C. S. Meinen, B. Shaw, and R. Curry (2011). Continuous, Array-based estimates of Atlantic ocean heat transport at 26.5°N, Journal of Climate, 24, 2429-2449.

Lozier, M. S. and 37 co-authors (2019). A sea change in our view of overturning in the subpolar North Atlantic, Science, 363, 516-521.

Yu, L., X. Jin, and R. A. Weller (2008). Multidecade Global Flux Datasets from the Objectively Analyzed Air-se Fluxes (OAFlux) Project: Latent and sensible heat fluxes, ocean evaporation, and related surface meteorological variables. Woods Hole Oceanographic Institution, OAFlux Project Technical Report. OA-2008-01, 64pp. Woods Hole, Massachusetts.

Yu, L. (2019). Global Air-Sea Fluxes of Heat, Fresh water, and Momentum: Energy Budget Closure and Unanswered Questions, Annual Review of Marine Science, 11, 227-248.



OSNAP at Ocean Science Meeting 2020

Below is a list of Ocean Sciences 2020 special sessions that may be interested to the OSNAP community.  Abstracts are due September 11, 2019.


Ocean Sciences 2020 – OSNAP related abstract submission

We are want to draw your attention to the following session at the Ocean Sciences Meeting, 16-21 February 2020 in San Diego, CA. Abstracts can be submitted by visiting https://agu.confex.com/agu/osm20/prelim.cgi/Session/85893 until the submission deadline Wednesday, 11 September.


Atlantic Ocean Variability in A Changing Climate: Observations, Modeling, and Theories


By redistributing a large amount of heat and salt, the Atlantic Ocean significantly impacts regional and global climate over a wide range of time scales. In particular, the Atlantic has seen strong variations in the ocean heat and freshwater content over the past couple of decades, as well as in the uptake and storage of anthropogenic carbon, which has been attributed to changes in the ocean circulation, e.g., those related to the Atlantic Meridional Overturning Circulation (AMOC). However, the mechanisms through which the ocean circulation changes (e.g., in the mean state and variability) and impacts the climate system (e.g., via a series of modes of variability such as the Atlantic Multidecadal Variability, the North Atlantic Oscillation), as well as the feedback, remain poorly understood. This session invites submissions that advance our understanding of the Atlantic Ocean variability, the role it plays in the atmosphere–ocean–sea-ice system, and its impact on the future climate. It aims to bring together recent progress in understanding the circulation and climate variability in the Atlantic sector from paleoclimate, historical and future perspectives. Studies utilizing observational, modeling and/or theoretical frameworks are all welcome.


We very much hope to see you in San Diego.


Best regards,

Feili Li (Duke University)

Rohit Ghosh (Max Planck Institute for Meteorology)

Laifang Li (Duke University)

Dian Putrasahan (Max Planck Institute for Meterology)


Postdoctoral Researcher – Georgia Tech

Postdoctoral Researcher in Physical Oceanography at Georgia Tech

A postdoctoral position in physical oceanography is available at the School of Earth and Atmospheric Sciences at Georgia Tech. The postdoctoral researcher will work on a recently funded NSF project, Leveraging the AMOC arrays and models to understand heat and freshwater transports in the North Atlantic. The main goal of this study is to place observations from the Atlantic Meridional Overturning Circulation (AMOC) observing system network into a basin-wide context in order to uncover the mechanisms governing heat and freshwater transports throughout the basin and into the Arctic. The study also aims to quantify, via observing system simulation experiments and optimal observing network design, the degree to which information contained in the AMOC arrays can constrain unobserved climate indices. 

The initial term of the appointment will be 12 months, with the possibility of extension for another two years based on performance.

Job Responsibility:  The individual will be expected to conduct independent, high-quality research in physical oceanography; publish papers; and present work at national and international conferences.  The postdoctoral researcher will also work collaboratively with oceanographers at Georgia Tech and at the University of Texas-Austin on projects related to this grant.  Opportunities for cruise participation as part of the Overturning in the Subpolar North Atlantic Program (www.o-snap.org) will be available.

Qualifications: A PhD in physical oceanography or a related field is required by the time of appointment. A background in large-scale oceanography is preferable and experience with observing system simulations experiments and large observational and model data sets is desirable.  Strong oral and written communication skills are expected.

Start date: On or about December 1, 2019.

Salary: Commensurate with the individual’s experience and education.

Application Procedure/To apply: Interested individuals should send a CV, a one-page statement of research interest, and the names and contact information of at least 3 references to susan.lozier@gatech.edu.  Review of applications will begin immediately and continue until the position is filled.

Research Scientist – Georgia Tech

Research Scientist in Physical Oceanography at Georgia Tech

A research scientist position in physical oceanography is available at the School of Earth and Atmospheric Sciences at Georgia Tech. The research scientist will work collaboratively on a number of projects focused on the Atlantic Meridional Overturning Circulation (AMOC).  Specifically, the research scientist will work closely with Dr. Susan Lozier, the international lead for the Overturning in the Subpolar North Atlantic Program (OSNAP; (www.o-snap.org), on the study of: mechanisms responsible for heat and freshwater fluxes in the North Atlantic; deep water pathways throughout the Atlantic basin; and the impact of subpolar dynamics on the downstream signature of the meridional overturning circulation.  Additionally, the research scientist will work closely with postdoctoral researchers and graduate students in Dr. Lozier’s lab and coordinate communication among the OSNAP international and national partners. 

Job Responsibility:  The individual will be expected to conduct high-quality research in physical oceanography; publish papers; and present work at national and international conferences.  The researcher will also work collaboratively with oceanographers at Georgia Tech and elsewhere on projects related to OSNAP.  Opportunities for cruise participation as part OSNAP will be available.

Qualifications: A PhD in physical oceanography or a related field is required, as is at least two years of post-PhD research experience. A background in large-scale oceanography is preferable and experience with observational and model data sets is desirable.  Excellent organizational and collaborative skills are required and strong oral and written communication skills are expected.

Start date: On or about December 1, 2019.

Salary: Commensurate with the individual’s experience and education.

Application Procedure/To apply: Interested individuals should send a CV, a one-page statement of research interest, and the names and contact information of at least 3 references to susan.lozier@gatech.edu.  Review of applications will begin immediately and continue until the position is filled.

Update on OSNAP Floats

by Amy bower

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 Oeanography 146, 142-158

Modelling in the Labrador Sea

by Clark Pennelly

The Labrador Sea in the northwestern Atlantic Ocean is a bit like salad dressing: the multiple water layers present will not happily mingle with others nearby, just like oil and vinegar. Of course, this all changes when the salad bowl that is the Labrador Sea gets stirred up! Strong winter storms pass through the Labrador Sea, cooling the surface water and making it more dense. Should this cooling process continue, the density will increase until the surface layer will have the same density as the subsurface layer, allowing them to efficiently mix together – just like shaking a bottle of dressing. We call this ‘shaking’ deep convection.

However, the Labrador Sea is not simply a 2-ingredient salad dressing mixture. A fresh and cold water cap is at the surface; this water originates from the Arctic and could be from melting of Greenland glaciers, sea-ice from the Beaufort Sea, or even runoff from one of the many rivers which discharge into the Arctic Ocean from Canada, northern Europe, and Russia. A warm and salty layer of water from the Atlantic Ocean exists below this. Further below is yet another water mass which is cold and salty, a product of mixing the two layers above it during the cold winter; we call this layer Labrador Sea Water. There are even more layers beneath, but I won’t get into them for now. These top three layers are important to the deep convection and water mass formation that previous blog posts have discussed.

My PhD thesis revolves around the ocean layers within the Labrador Sea and how they are modified by a variety of sources, such as an input of freshwater, heat, or perhaps changes in the weather. As this region is notorious for its rough seas, particularly during the stormy winter, research cruises (see https://www.o-snap.org/fleur-de-sel-life/ ) tend to occur/gather data after the winter has passed when seas are calmer and the 12-point Beaufort sea state scale (see https://en.wikipedia.org/wiki/Beaufort_scale) goes from 10’s and 11’s down to more manageable single digits. Unfortunately, research cruises may therefore miss clues about the mixing and formation of water layers that happen during the convective season. I use computer models to investigate what may be occurring when research cruises are not around to gather data. While they are not without their own set of problems, models allow us a peek into the unknown from the comfort of a Beaufort 0 sea-state office.

One portion of my research explores rotating circular features, which we call eddies, that form off the west coast of Greenland. These eddies are produced via changes in the seafloor nearby. As the West Greenland Current moves alongside the west coast of Greenland, it carries both fresh and cold water at the surface as well as salty and warm water below. These eddies carry both water layers away from the coast and into the heart of the Labrador Sea. Both of these water masses impact the Labrador Sea in a different way, so these eddies are rather interesting and important features. However, these small events are notoriously difficult to represent within numerical models, especially multi-decade simulations. We had to get a little creative in order to make sure we would resolve these eddies as modelling such eddies requires a lot of computing resources. We set up a complex simulation which has a resolution of 1/60 of a degree, meaning this simulation should be able to resolve features where are larger than a few kilometers. The simulation requires over 600 CPUs and about 72 hours of real time to produce 40 days of simulation time. To put that into perspective, our regular simulations may use 60 CPUs and finish 2 years of simulation time in 72 real hours; quite the difference! We are interested in running this expensive simulation across 17 years, from 2002 to 2018, so we can capture variability across many years, though some quick math says that this simulation will take about a year to finish. But what do we gain by resolving the Labrador Sea at high resolution?

Well-defined eddies is what we get! You can see those individual eddies within the red circle. From running this model we’ve so far learned that those eddies can last up to two years, meaning that as they travel southwest from the coast of Greenland they may encounter multiple winters and therefore be involved in multiple rounds of deep convection. I’m currently exploring how these eddies evolve through their lifetime in the Labrador Sea. We know they bring substantial heating into the region, limiting deep-water formation. Thus being able to detect them with our model is crucial in order to achieve an accurate spatial representation of convection. Lower resolution simulations may fail to resolve these features and thus their importance. Since they persist for multiple convective years, they may have a more complicated story than previously thought which I hope my research will help uncover. Numerical modelling allows for the scrutiny of such events as observations within the Labrador Sea may not provide the full information we seek.