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.

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

Description

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.

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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)

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 21^st century. Progress in Oeanography 146, 142-158

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.

by Helen Johnson

with input from Sam Cornish and Yavor Kostov

The first results from the OSNAP array, published this month in Science (https://science.sciencemag.org/content/363/6426/516), are incredibly exciting! Over the 21 months of data collected so far, it has been the conversion of warm, salty, shallow Atlantic water into colder, fresher, deep water east of Greenland that has dominated the overturning circulation and its variability.  This challenges the prevailing view that deep water formation in the Labrador Sea to the west of Greenland is the major player in determining overturning variability.  And it gives us some clues about how changes further north, in the Arctic Ocean, might affect things.

A major motivation for measuring the overturning circulation with the OSNAP and other observational arrays arises from the expectation that the overturning will change as a result of human-induced climate change. The overturning circulation is predicted to weaken over the coming century, due to a warming and freshening of the high latitude North Atlantic. Part of the freshening expected in these critical deep water formation regions is due to changes in the amount of freshwater exported from the Arctic to the Atlantic, on both sides of Greenland. The Arctic Ocean has recently accumulated a large amount of freshwater, but we do not know if or when it will be exported to the Atlantic, or at what rate.

My group in Oxford have been investigating changes in the amount of freshwater stored in the Arctic, and the reasons for them, in the hope that this will teach us something about the changes we might expect in freshwater export (https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2017GL076870). We have deduced the relationship between winds over the Arctic and total Arctic freshwater storage in a climate model. We now know that if the winds over the Arctic change, it takes the system at least a decade to come into a new equilibrium, with a different amount of freshwater stored.  Based on the relationship we deduce between winds and freshwater content, we estimate changes in Arctic freshwater content over the last century – and our time series agrees well with the limited observational data available (see figure), giving us confidence that the relationship is a useful description of the real world!

Our results suggest that the large increase in Arctic freshwater content since 1992 can largely be explained by historical changes in the winds driving the Arctic Ocean circulation (although we can’t rule out a smaller contribution from sea-ice melt).  What’s more, this increase doesn’t seem exceptional compared to variability in our time series over the rest of the century, suggesting that it may simply be natural variability.

Of course, changes in freshwater exported from the Arctic, natural or not, still have the potential to affect the overturning circulation! The long memory of atmospheric conditions revealed by our results is important because, provided we know what the winds have done, we can potentially predict changes in Arctic freshwater content a few years ahead. Based on our work so far, we expect Arctic freshwater content to decrease over the coming decade, and freshwater export to the Atlantic to increase.

The OSNAP array is ideally-placed to detect any impact of this change on the high-latitude overturning circulation. Based on the OSNAP results so far, we might expect that Arctic freshwater export through Fram Strait to the Nordic Seas (rather than through the Canadian Archipelago to the Labrador Sea) will have the most impact. So we have several hypotheses that our continued OSNAP observations and modelling efforts will put to the test in the coming years!