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Ocean Sciences

Ocean Sciences

Why do mesoscale eddies disappear at ocean western boundaries?

Why do mesoscale eddies disappear at ocean western boundaries?

In the ocean, mesoscale eddies are everywhere. In fact, about 90% of the ocean’s kinetic energy is contained within these ubiquitous, ~100 km swirling vortices. We know how these eddies form, through a process known as baroclinic instability, where density surfaces are tilted too steeply and become unstable. What remains unclear however, is what happens to this energy when eddies die. This knowledge gap is a key uncertainty in ocean/climate models that parameterise ocean eddies, like the state-of-the-art CMIP6 models used in the latest IPCC report.

What we do know however, is where these eddies typically go to die: ocean western boundaries. Using satellite-based altimetry we can observe westward propagating mesoscale eddies as they reach ocean western boundaries, like the east coast of the United States, and disappear. What happens to this eddy energy at western boundaries then becomes a question of the direction of the energy cascade. An eddy may become entrained within the larger scale flow features of the region in what is known as an inverse or reverse energy cascade (from mesoscale to large scale). Alternatively, an eddy may interact with the topography along the ocean boundary, forming smaller scale features that eventually dissipate as turbulence. This is what we call a direct or forward energy cascade.

The MeRMEED study region offshore of Great Abaco. The grey lines show location of our survey and the white markers show the location of existing moorings. (Fig. 2; credit: Gwyn Evans)

During my post-doc at the University of Southampton, I was part of the MeRMEED (MEchanisms Responsible for Mesoscale Eddy Energy Dissipation) project led by Eleanor Frajka-Williams. We set out to observe and quantify the strength of the direct energy cascade in mesoscale eddies at an ocean western boundary by measuring the turbulence generated as eddy flow interacts with topography. Our chosen study region was just offshore of Great Abaco, Bahamas, where we already have observations of possible eddy-topography interactions from mooring-based measurements and satellite altimetry (Fig. 2). Over the course of two years, the plan was to use a relatively small vessel to perform a short but intensive survey of the flow and turbulence along the steep and rough slope offshore of Great Abaco during several different eddies.

To understand how eddy flow-topography interactions can lead to an increase in turbulence and an eventual dissipation of eddy energy, we required detailed measurements of two key variables: the sub-surface velocity and the turbulent dissipation. For sub-surface velocity measurements, we used an Acoustic Doppler Current Profiler or an ADCP. Our chosen vessel, the R/V F.G. Walton Smith of the University of Miami (Fig. 1), came equipped with an ADCP mounted to the hull of the vessel, allowing us to perform a comprehensive survey of the eddy flow offshore of Great Abaco. To complement these measurements, we used a Vertical Microstructure Profiler or VMP to estimate turbulent dissipation (Fig. 3). The VMP is specialist piece of equipment that measures centimetre to millimetre scale changes in vertical gradients of velocity and temperature. By examining the spectra of these small-scale changes, we can then make estimates of the turbulent dissipation to tell us how much of the eddy flow’s kinetic energy is dissipated along the western boundary offshore of Great Abaco.

The VMP in action during a recover on the aft deck of the R/V F.G. Walton Smith handled expertly by colleagues from the National Marine Facilities group of the National Oceanography Centre. (Fig. 3; credit: Eleanor Frajka-Williams)

Our measurements across three different mesoscale eddies revealed a whole host of eddy flow-topography interactions that led to increased turbulence and dissipation of the eddy flow’s kinetic energy. For example, we observed several regions of strong vertical and horizontal gradients in velocity linked to interactions between the eddy flow and topography where we also observed increased turbulence. Turbulence was also stronger where the eddy flow is forced over bumps in the topography including the generation of internal gravity waves, which are linked to the presence of a hydraulic jump. Our work has therefore shown us that the interaction between eddies and the steep and rough slope at an ocean western boundary can lead to turbulent energy dissipation within the eddy flow. However, does the energy dissipation observed during our surveys explain the decay of eddy energy observed at western boundaries via satellite altimetry?

To answer this question, we again used satellite altimetry to track every eddy that entered our study region and “died”. We estimated the energy of these eddies and monitored the decay of their energy over time within our study region. Comparing this satellite-based estimate of eddy decay rate to the turbulent dissipation measured during our surveys, we find that these separate estimates agree very closely. This suggests that offshore of Great Abaco, eddy energy is lost through a forward cascade of energy, as the mesoscale eddy flow interacts with the steep and rough slope, forming smaller scale features that eventually lead to the generation of turbulence and an increase in the dissipation rate.

The MeRMEED project has shown us that the death of mesoscale eddies at ocean western boundaries can in some cases be explained through a forward cascade of energy, where eddy-topography interactions lead to smaller scale flow features that eventually dissipate as turbulence. The question remains, however: are our results relevant for other ocean western boundaries beyond this particular region offshore of Great Abaco? And further, how should these findings inform eddy parameterisations in the next generation of ocean/climate models? These remain open questions that will no doubt guide the future research within this field.

OceanTalk with Frédéric Le Moigne

OceanTalk with Frédéric Le Moigne

Frédéric Le Moigne has been awarded the 2020 Outstanding Early Career Scientist Award for the Division of Ocean Sciences and he agreed to be interviewed for our first blog post on the new Ocean Sciences blog.

Frédéric can you tell us about your background and education?

I was born and raised on the Atlantic coast, near the bay of Brest in Brittany, western France. In Brittany the ocean is an important part of life. I’ve always seen myself doing something related to the ocean. This is why I sought a degree in marine biology at the University of Western Brittany in Brest. I then crossed the Channel to the UK (only a few hundred kilometers away!) to do a PhD at the National Oceanography Centre, Southampton. This is where it all started, I really enjoyed my PhD, both from a personal and a professional perspective. My research was funded by the European community and as a result I travelled all over Europe. After a brief time in Germany at GEOMAR-Kiel, I returned to France to start a position as a tenured researcher. I am glad my scientific career has allowed me to discover various countries. Europe is a fantastic place for this, my generation is probably one of the first that considers Europe as a country thanks to the Schengen Agreement. I’m deeply attached to the idea that we can cross between the borders of European countries without showing ID.

Your research brings you to remote regions. What is the most challenging thing for you while working in remote places?

Working as an oceanographer brings you to amazing places all around the world that are not typical holiday destinations. Polar regions are a good example for instance. The downside of so much travel is that you are often far away from family, relatives and friends for long periods of time. We have all got used to this separation but it still hard at times. Last year’s Christmas and New Year’s Eve I spend on the RRS Discovery in the Southern Ocean, a time of the year that is normally dedicated to family and friends.

Frédéric Le Moigne taking samples from the CTD on board the RRS DISCOVERY December 2019. (Credit: Frédéric Le Moigne)

A part of your current research focuses on polar oceanography. Can you briefly explain how icebergs can influence marine primary production and how this might be affected by climate change?

In remote regions like the Southern Ocean the growth of small marine algae called phytoplankton is limited by the availability of specific nutrients needed in tiny amounts so called micronutrients. Icebergs act as gigantic delivery trucks full of micronutrients that could potentially fertilise the Southern Ocean. Further, global warming may increase the number of icebergs that are shed to the ocean. More icebergs could potentially increase phytoplankton activity in the Southern Ocean and thus increase carbon draw down from the atmosphere into the ocean referred to as carbon sequestration. However, a recent study (Hopwood et al., 2019, Nature Communications) showed that not all icebergs are created equal. Some tend to deliver most of their micronutrients in coastal waters, rather than in offshore waters as they drift and melt. This happens because in most icebergs, micronutrients are concentrated on the edges contrary to what was previously thought. Therefore, taking into consideration iceberg types is very important if one wants to assess the effect of future iceberg melting on marine productivity.

 

 

What challenges do you have to address when investigating the transport of organic matter from the surface water into the deep ocean in different oceanic regions?

Mainly logistical I would say. Doing any kind of fieldwork at sea on research vessels involves an incredible amount of planning before, during and after expeditions. Sea going expeditions always involve a lot of people, ranging from the ship’s engineers to the science support team. It’s like a small village going to sea where everyone has a role. Science would not be possible without these people. Investigating export fluxes has its own challenges. We have the habit of deploying drifting instruments in the ocean to catch flakes of “marine snow” made of ageing plankton falling from the surface ocean to the deep. Locating and recovering these drifting instruments is sometimes complicated.

Deploying a drifting mooring line onboard the RV METEOR offshore Peru in 2017. This mooring line is specifically designed to catch flakes of marine snow that fall from the surface ocean. (Credit: Jon Roa)

Your research spans across a wide range of topics of ecology, biogeochemistry and oceanography. How did you expand your knowledge and expertise so widely?

It starts with a solid university training in oceanography, all aspects of it. Understanding how ocean biology works requires a good knowledge of ocean physics and chemistry first. In addition, working in some of the largest ocean research institutes worldwide gives you the opportunity to hear about a very wide range of ocean research programs and to work with people from very different science backgrounds. My knowledge and expertise expand as a function of this. It helps to tackle science questions from different angles.

You will be recognized by the Ocean Sciences Division as an Outstanding Early Career Scientist in 2020. What personal and professional factors do you think led to this great recognition?

It’s easy to get somewhat discouraged by various aspects of this job. So, I think being persistent is crucial. Also, it’s important to always present your ideas to different people with different scientific backgrounds. This gives you a sense of how important and relevant your science questions are. This also helps to refine and improve your ideas. As Early Career scientists, we are often scrutinized. It’s important to listen to criticism and to make your scientific demonstration better, stronger and more understandable to all.

Deploying water pumps, called Stand Alone Pumps (or SAPS) onboard the RRS DISCOVERY in the Southern Ocean, 2020. The SAPS usually filter around 1500 L of seawater during just one hour of pumping. Thanks to these pumps, we get detailed information about the chemistry of marine snow. (Credit: Sophia Alexou)

What do you think are your major challenges as an early career scientist, and how are you tackling or preparing for them?

Giving yourself time to think about science is key. It’s easy to get involved in too many things. The scientific process takes time to mature and refine. Early careers scientists are somehow pushed towards publishing data too quickly without giving time to interpretation to fully develop. I recently secured a permanent CNRS researcher position (Centre National de la Recherche Scientifique) hosted at the Mediterranean Institute of Oceanography in Marseille, southern France. Such a position comes with a lot of administrative, organizational and representation tasks. But it also gives me a lot of freedom to think about what science should be done. This is crucial for conceiving and delivering high impact science.

Thank you, Frédéric, for the interview while you were at sea and for your advice to other Early Career Scientists!

Interviewed and edited by Meriel J. Bittner & Gwyn Evans

Reference:
Hopwood, M.J., Carroll, D., Höfer, J. et al. Highly variable iron content modulates iceberg-ocean fertilisation and potential carbon export. Nat Commun 10, 5261 (2019) doi:10.1038/s41467-019-13231-0

 

Welcome to the new Ocean Sciences Division blog!

Welcome to the new Ocean Sciences Division blog!

Hello and welcome to the blog of the EGU Ocean Sciences Division!

We are very excited to launch this new blog and finally join the EGU Blogsphere.
Here we provide content of interest to the division and especially to the Early Career Scientists. This blog aims to spread our fascination for the ocean and share different aspects of ocean research.

We will feature different types of blog articles, including general News and OS Research where scientists write about their own research or about a recent publication. Behind the Research articles will cover fieldwork stories and with OceanTalk we will be interviewing interesting ocean scientists.

The OS blog will be updated regularly and is managed by the OS Early Career Scientist Representative, as well as a blog team, which is currently being established. We encourage students, early career and senior scientists in the wider ocean sciences community to write blog posts and share their stories. If you would like to write a blog entry or get involved with the blog please get in touch with the editorial team (ecs-os@egu.eu)!

Meriel on the RV Marion Dufresne in the Southern Ocean (Credit: Meriel Bittner)

Once again, welcome to the OS blog and we hope you will enjoy our first OceanTalk with Frédéric Le Moinge, this year’s OS Outstanding ECS awardee.

Stay tuned for the next upcoming blog posts!

Meriel Bittner & Gwyn Evans