Category Archives: Student/Postdoc Blog

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.

References:

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.

by Tillys Petit

My last contribution on this blog reminded the importance of in-situ data measurements on the evaluation of numerical modeling used to predict climate. As part of my PhD thesis, I had the chance to record, process and analyze observations across and along the Reykjanes Ridge within the framework of the RREX project. It included an experience at sea during the RREX cruise in 2017, which was an amazing human and scientific experience. Now that my PhD ended, I would like to tell you about the scientific results that were obtained. 

During my PhD, I studied the connection between the Iceland Basin and the Irminger Sea through the Reykjanes Ridge. A main result was to describe and quantify the top-to-bottom transport of the subpolar gyre that crossed the Reykjanes Ridge during the summer 2015. These results highlighted interconnection between the two main along-ridge currents: the southwestward East Reykjanes Ridge Current (ERRC) in the Iceland Basin and the northeastward Irminger Current (IC) in the Irminger Sea. From about 56 to 63°N, the hydrological properties, structures and transports of the ERRC and IC consistently evolved as they flowed along the Reykjanes Ridge. During my PhD, I showed that these latitudinal evolutions were due to flows connecting the ERRC and IC at specific locations through the complex bathymetry of the ridge, but also to significant connections between these currents and the interiors of the basins. These results highlighted a more complex circulation in the vicinity of the Reykjanes Ridge than it was assumed.

From three different cruises and Argo floats, I also investigated the deep circulation and properties of overflow water through the deepest sills of the Bight Fracture Zone. At the end of my PhD, I showed the strong variability of its transport and property over time by comparing three successive years. Now, I think that it could be interesting to continue this study and to better understand the variability of overflow water at higher frequency. As a continuity of my PhD, I am thus exciting to investigate the variability and linkage between the overflow water transports and properties across the Iceland-Scotland Ridge and the Denmark Strait as part of my postdoctoral position. These inflows from the Nordic Seas feed the lower limb of the Meridional Overturning Circulation and are crucial to characterize the variability of the North-Atlantic subpolar gyre. I am excited to fulfil this study by using the OSNAP array that provides new and key measurements of the AMOC, and also to move in USA for a new beautiful and rewarding postdoctoral experience.

by Yavor Kostov

The end of the year is a time to reflect on the past and make long-term plans for our future. Some readers of this blog, especially our young audience, may be considering a career in oceanography or climate science. I will tell you my story: what motivated me to join this field and the factors that shaped my career path.

My first encounter with physical oceanography was 14 years ago, at an international summer school where I learned basic gravity wave dynamics. Fluid motion fascinated me and sparked a lasting interest in the field. The following year I was on my high school team for the International Young Physicists’ Tournament (IYPT). Within our team, I was responsible for problems related to fluid dynamics.

By the time I began my undergrad studies, I was already very interested in modeling the environment. I also realized that to do well in the natural sciences, I should expand my background in math. So I majored in Applied Mathematics, but I also took physics courses. As an undergrad, I did different research projects applying mathematical methods to study the environment. For example, my senior thesis was on modeling the El Niño / La Niña phenomenon.

Nine years ago, I decided to do a Ph.D. in climatology and oceanography. I became interested in the field because I wanted to do research in an area of science that is socially significant. Nature has direct impact on humankind.  At the same time, climate science and oceanography attracted me because many fundamental questions in our field remain unresolved.

My Ph.D. and postdoc research has explored the large-scale ocean circulation and its impact on global and regional climate. I have studied various parts of the World Ocean: the North Atlantic, the Arctic Ocean, and the Southern Ocean. My work involves coding algorithms and analyzing data from complex climate models and observations, but also developing simple conceptual models.

In my current OSNAP project, I examine how the ocean circulation in the subpolar North Atlantic responds to local and remote fluctuations in atmospheric conditions. I analyze the computer code of a global ocean model as if it were a system of math equations. One of the most interesting aspects of my work is trying to understand the ocean’s delayed response to past atmospheric changes that took place years ago.

I am now looking forward to another productive year of research on the ocean circulation. Happy holidays to all readers of this blog and best wishes for the New Year!

By Charlène Feucher

The sea and the ocean have always held a fascination for me. I grew up in the bay of Saint-Malo (France) and the sea coast was my first playground (Figure 1). During my childhood, I was mostly interested in the processes involved in sandcastle destruction by waves, or in the tide processes that could not be ignored for safe crab fishing. When I was a little older (and braver), I started exploring the sea and left the beach and the rocks behind me. Sailing, surfing, windsurfing and kayaking became my favourite hobbies. Boat rides were always fun and exciting. Visiting traditional sailboats or big fishing vessels were captivating and it nourished my imagination and my dreams of sea adventures. I used to think: “One day, I will also be on a boat to explore the ocean and learn more about it!”. 

I would later discover oceanographic sciences: the ocean was not only my playground anymore, it could now be my field work too! I was very glad to start a master in physical oceanography at the University of Brest (France) and learn the secrets of the ocean (with an affinity for the Atlantic Ocean), how it works, and why its role is of paramount importance to the global climate. 

During my master studies, I got the opportunity to complete two projects that introduced me to physical oceanography research. My first internship was at IFREMER and IUEM (Brest, France) to evaluate realistic hydrodynamic simulations for biogeochemical applications. I developed inter- comparisons of several ocean simulations (differing in resolution and model parameters) and evaluated the simulated fields against relevant observation data sets, with a focus on mixed layer dynamics in the North Atlantic Ocean. I did a second internship at Woods Hole Oceanographic Institute. The project was to study the dissipation of the North Atlantic Subtropical Mode Water (also well known as Eighteen Degree Water) based on eddy-resolving ocean simulations. I examined the eddy covariance flux divergence of the North Atlantic Subtropical Mode Water thickness and potential vorticity to understand the spatial distribution and mechanisms of the destruction. 

After my master thesis, I came back to the University of Brest to complete my PhD. The objective of my PhD project was to evaluate the properties and variability of the stratification in the North Atlantic subtropical gyre. I developed a method to characterize the properties of the stratification of the ocean (permanent pycnocline and mode waters) in subtropical gyres. Focusing on the North Atlantic subtropical gyre and based on the use of Argo data. I have documented the properties of subtropical mode waters and permanent pycnoclines.

Right after PhD, I travelled to Canada to start a postdoc at the University of Alberta (Edmonton) where I am still working. My postdoc research focuses on the relationship between the meridional overturning circulation and the formation of the Labrador Sea Water. This study is based on the use of NEMO model outputs with an Arctic and Northern Hemisphere Atlantic configuration.

In Edmonton, I am living far from the ocean but I never forget about it. The sea is always calling me and when cruise opportunities are out, I am willing to embark when possible. My first experience at sea was in 2015 during my PhD. We took several measurements along and across the Reykjanes Ridge to study the ocean circulation there. This first research cruise was full of discoveries. I learnt what oceanographic field is all about, how to take measurements, and how life aboard a ship feels like. I was enchanted by the immensity of the ocean and the power of the winds and waves during strong storms (small storms according to the Captain but I was not convinced).

I got the chance to renew this sea experience in June 2018 on board of the RV Maria S. Marian. This cruise was quite epic! We flew to Cadix (Spain) where the boat departed. We crossed the whole Atlantic while taking measurements, we docked in St. John’s (Canada) for a day and then we crossed the whole Labrador Sea before coming back to St. John’s where our cruise ended. Seven intense weeks at sea! During this cruise, I was amazed to see icebergs for the first time, admiring them drifting off the Greenland coast under a beautiful sunset (Figure 2). Performing CTD casts with the Greenland coast in the horizon was also a very special moment (Figure 3). And more importantly, I was glad to take measurements in the Labrador Sea to observe deep convection and compared these observed results with what we simulated in our NEMO model.

Physical oceanography research work gives me the opportunity to work in different places and and meet many great people. This is full of very enriching experiences, professionally but also personally. I hope I can continue this oceanographic adventure in the years to come.

As this is the last blog post of 2018, it is time to leave you wishing everyone a wonderful Christmas season from a freezing cold and white Edmonton!

by Roos Bol

Now that temperatures outside are dropping and storms are raging over the subpolar gyre, it is clear that the OSNAP field season had ended. Many blogposts have been written about the exciting adventures at sea last summer. This time however, I would like to tell you a bit about the – slightly more boring – work that happens after. When the exhausted but ultimately satisfied scientist returns home, with a hard drive full of newly recovered data; that’s when our real work begins. Before the new data can be used to answer actual science questions, they are in dire need of some cleaning.

Why is that necessary? Instruments on a mooring (a cable anchored to the sea floor) are impacted by currents, storms, tides and sometimes fishing activities. There is the risk of colonization by omnipresent sea creatures. In the vast open space of the ocean, an instrument can provide a welcome place for shelter. Instruments deployed in the sunlit surface layer are all overgrown with algae upon recovery. But deeper instruments can also host interesting inhabitants, such as the anemone in the picture below. Last but not least, the ocean is salty, wet and under high pressure. A challenging environment for an electronic instrument, especially with our long deployment period of two years! 

Figure 1: Some examples of sea life on the moorings upon recovery: a slimy creature on a Microcat, an anemone growing on a ring of the release that was at almost 2000m depth, and a shallow UK buoy overgrown with algae.

Data quality sometimes suffers from all these environmental impacts. This is where my work of the past few weeks comes in: checking, cleaning and processing the raw data records. Below are examples of Microcats on one of our moorings, called IC2, during the last deployment. Unlucky for us, the anchor of this mooring ended up shallower than planned, on top of a small seamount. A storm resulted in the exposed top buoys breaking down, making the shallow instruments sink to the deep. The top Microcat, originally at 25m, was instead dangling loose at around 700m depth… it is a small miracle it was still attached to the cable when the mooring was recovered! 

(For those who are wandering; yes, luckily there was another buoy at 350m, keeping the deeper part of the cable standing up straight!)

Severe storms impact the mooring even deep down in the water column. The pressure record of the Microcat at 352m depth (figure 2) still shows many big and small ‘blowdown’ events. During such an event, strong currents push against the mooring cable, blowing it down at an angle and pushing the instruments deeper into the water. When the storm ceases, the buoys on the cable pull the mooring cable back to its original straight position.

Figure 2: Pressure time series of the Microcat at 352m in mooring IC2 for the last deployment (August 2016 to July 2018)

Microcats also measure temperature and salinity. Figure 3 shows the raw salinity record from the deepest Microcat on IC2, close to the bottom at almost 1900m depth. As you can see, salinity differences in the deep ocean are generally very small. However, salinity measurements are notoriously noisy, so we need to perform some filtering of data spikes. But which spikes are bad data, and which represent real variability? Then there is a suspicious drop at the start of the record; perhaps a small animal or algae was temporarily covering the sensor?

Figure 3: Raw salinity record from the deepest Microcat in IC2, at 1892m

Lastly, and perhaps most importantly, we need to perform a calibration for all sensors. From the ship, we dip the instrument into the ocean and test its readings against a calibrated reference sensor to determine any offsets, both before and after the deployment. This is the only way to check whether an instrument gives accurate readings.

Overall, it is probably clear that there is a lot of effort involved in scrutinizing the records and ensuring data quality control is performed correctly. But although it may sound a bit tedious, the quality control step is extremely important. It ensures that we have accurate, reliable records, on which we can build for further analysis – to eventually formulate valid answers to scientific questions!

by Leah McRaven

Physical Oceanography
Woods Hole Oceanographic Institution

When you decide to study the currents that whip past the continent of Greenland and that transform the waters in the Irminger and Labrador Seas, an oceanographer must be willing to make peace with an ocean that isn’t entirely liquid. The extreme elements that shape the rocks along the Greenland coast also actively chisel away at the hundreds of glacial termini that meet the ocean edge. This chiseling leads to a constant flux of icebergs, small icebergs called bergy bits, and even smaller ice chunks called growlers. With ice in its various sizes and jagged shapes breaking away from the entire continent, the currents in the OSNAP region transport and mix more than just water.

Logistically, the OSNAP study region is one of the hardest places in the world ocean to successfully execute fieldwork. To start, the East Greenland Coastal Current, the East Greenland Current, and the Irminger Current can flow at speeds well over 1 knot as they round the tip of Greenland. In addition to strong currents, the area is home to a record: the windiest place in the world ocean. Simple ship maneuvering tasks, such as holding station while collecting data or recovering moorings (Figure 1), become challenging for the mates on the bridge as unforgiving winds build up rough seas that are already swiftly flowing. Floating ice is quite literally the icing on the OSNAP cake.

Ice adds a whole new dimension, and phase of matter, to navigation and operations at sea. From a distance, it can be nearly impossible to decipher ice chunks from whitecaps and sea spray. Large icebergs can be easy to see if they express above the surface of the ocean, but the majority of an iceberg’s mass lies below the sea surface and is difficult to see. Ice can also block access to nearby fjords used for shelter in severe weather. These navigation dangers keep the R/V Armstrong mates on high alert at all times as they steer through storms, darkness, and thick fog.

In order to help navigation efforts, WHOI researchers employed the help of the Danish Meteorological Institute (DMI), which specializes in satellite sea ice imagery. DMI is able to provide the ship with updates on the location of ice based on satellites that take Infrared (to see through clouds) and visible images of the Earth’s surface. Not only is this information extremely helpful, the maps can be stunning. Ice information from our current OSNAP cruise (on September 9th) is shown in Figure 2. This satellite image from the southern tip of Greenland is an example of how satellite-detectible ice features disperse from their mother fjords into the surrounding ocean.

Ice can also run into the OSNAP moorings, pushing instruments out of the way, or even snapping them off their lines making it impossible to recover them and their precious data. Our six shallow moorings on the continental shelf were, in fact, designed with drifting ice in mind. Equipped with a tripod-like structure at their base, these moorings have most of their instrumentation mounted near the sea floor. In an attempt to capture shallow data, the moorings also have special tethers extending up from the tripod base with weak links to top flotation. These weak links are designed to break easily should an iceberg snag the line, with the break point located strategically below the flotation and above an instrument. In the event of tether breakage, the instrument sinks to the bottom, but remains attached to the rest of the mooring so that it can still be recovered. Figure 3 shows a depiction of this breaking process. Of the recovered moorings from this year’s cruise, three of the six shallow moorings had their top floats ripped off within less than a year!

With all of the challenges handed to us from ocean elements, our crew has excelled in accepting the challenges brought on by ice. Of all 16 moorings that we aimed to recover on this cruise, all have successfully come back. In the face of extreme weather and rough seas, we have completed over 240 profile measurements of ocean temperature, salinity, and velocity thus far. And through all of these challenges, no one can deny how much they still enjoy seeing the Greenland coast in full panache with its towering and craggy icebergs.

Figure 1. An iceberg off the stern of R/V Armstrong during a tripod mooring recovery during the current OSNAP cruise.

Figure 2. Denoted infrared satellite imagery courtesy of the Danish Meteorological Institute. Pink triangle indicate the position of satellite-identified icebergs throughout the southern Greenland region.

Figure 3. Illustration of the OSNAP tripod moorings with weak links to top flotation. The left mooring demonstrates a normal deployment, while the right mooring shows a deployment with interference from an iceberg. In the event of an iceberg snagging the upper mooring tether, the top floats are released and the shallowest instrument falls, while still remaining connected to the mooring.