Extended OSNAP overturning time series for 2014-2018

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.



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).

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Transiting to the Irminger Sea

by Sarah Nickford, URI

Greetings from the R/V Neil Armstrong! I am a graduate student studying physical oceanography at the University of Rhode Island’s Graduate School of Oceanography. Originally, I planned on participating in the OSNAP-23 cruise to help calibrate and deploy oxygen sensors on some of the OSNAP moorings in the Labrador Sea as part of the newly funded Gases in the Overturning and Horizontal circulation of the Subpolar North Atlantic Program (GOHSNAP) awarded to my advisor, Dr. Jaime Palter, and her collaborators.

In the midst of the pandemic, I suddenly became an alternate for that cruise as science parties were slashed in half. After completing the 14-day self-isolation as an alternate, the entire science party tested negative for COVID-19 (phew) and the R/V Neil Armstrong began its voyage to the Labrador Sea. I resumed my “normal” pandemic day-to-day life until one morning, not long after, I was asked if I would be willing to complete another self-isolation period to be able to go to the Irminger Sea as a Hydrographer for the next OSNAP cruise. This cruise was a joint venture between OOI and OSNAP, with both OOI Irminger Node and OSNAP mooring turn-arounds. I had about a day to make a decision. Fond memories of my month aboard the SSV Corwith Cramer crossing the North Atlantic with Sea Education Association (SEA) helped me make up my mind. Ever since that voyage, I’ve been drawn to going to sea, where there is no land in sight. So, when the opportunity presented itself, I was eager to help out. I immediately was trained in Woods Hole and after the weekend, I entered my second self-isolation period of the summer. As I learned more about the cruise activities, I became increasingly excited. I suddenly had the opportunity to learn how to collect water samples, watch mooring turn-around operations and help document them, watch glider deployments, and help deploy an Argo float.

After the science party completed their 15-day self-isolation period, we finally arrived and settled into the R/V Neil Armstrong. We left Woods Hole, MA on Saturday August 8th at 0815 and the science team has been busy planning mooring operations, CTD deployments, and water sampling. Today is the fifth transit day and we are just past Canada, making a sharp turn north and heading to the Irminger Sea. So far, we’ve had incredibly clear skies in the evenings, creating the perfect conditions for beautiful sunsets and vibrant stargazing (great for viewing the Milky Way and the Perseid meteor shower!).

The first bit of science was on Saturday afternoon when we did a shallow water CTD cast to test how the CTD and associated machinery were working and so that the OSNAP Chief Scientist, Heather Furey, and I could practice water sampling. We have been orienting ourselves in the wetlab onboard and developing a sampling plan that fits the needs of both the OSNAP project and the OOI Irminger Array. After a CTD comes back on deck, we will be collecting water samples for the following parameters: dissolved oxygen, salt, DIC/TA, pH, nutrients, and chlorophyll. Each sample is taken at a specified depth, which can change depending on the water mass characteristics that we are sampling. We plan to do a CTD cast near each of the OSNAP moorings. This is useful for the calibration of the instruments on the new moorings and for what is called a “post-recovery caldip,” where the instruments from the recovered mooring are strapped to the CTD frame for one final dive to detect possible sensor drift over the two years they lived underwater collecting data. As we continue our transit, we are looking forward to the excitement that is soon to come.

A CTD just below the surface of the water. Before sending the CTD to the depths of the ocean, it must sit at the surface so that the instruments equilibrate with the surrounding water. After reaching its target depth, on its way back up to the surface, scientists monitoring the CTD close individual bottles at specified depths, capturing a sample of water from different water masses.

Reflections of the dwindling daylight on the surface of the ocean after departing Woods Hole.

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MSM94 expedition on the RV Maria S. Merian

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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

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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

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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


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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.



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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.


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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)


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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.

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