Mechanisms Responsible for Mesoscale Eddy Energy Dissipation (MeRMEED)

Lead Research Organisation: National Oceanography Centre
Department Name: Science and Technology

Abstract

Over the last decades, oceanographers have been searching for the missing mixing in the ocean to complete the ocean energy budget. Answering questions of where energy is added to the oceans, and where it is removed, helps us to understand the drivers of ocean circulation. With the advent of high-resolution satellite measurements of surface currents in the 1990s, scientists could see that the oceans were filled with swirling vortices of water called mesoscale eddies. While eddies are present in all ocean basins, with currents inside the eddies sometimes exceeding 1 m/s, they disappear from satellite measurements preferentially at western boundaries. There are several possibilities for why eddies disappear at western boundaries: they may radiate energy away, contribute energy to large scale ocean circulation, or lose energy locally through turbulence and dissipation. Of these candidate terms, previous work has suggested that local dissipation is strong enough to explain a substantial part of the eddy disappearance. Our aim is to determine how and why eddies are losing energy at the western boundaries. These results and our measurements will then be made available to scientists involved in numerical simulations of the ocean. As a longer-term goal, the results of our research may help guide how eddies are represented in ocean models, which is one of the critical areas needing improvement in climate simulations. However, due to the fledgling nature of the science in this field, that eventual goal is still several steps away.

Fundamental physics dictate that most eddies move slowly westward, and these eddies are visible in satellite measurements of sea surface height a few months before they arrive at the boundary. In the project MeRMEED, we will watch the eddies in near real-time satellite data, and when an appropriate eddy approaches the east coast of North America, we will deploy a small team of researchers, with advanced instruments, to meet the eddy upon arrival. There, we will survey the eddy using high-resolution profilers deployed from small boats and autonomous underwater vehicles called Seagliders. After the ship-based survey is completed, the gliders will continue to observe the eddies for several months, as the eddies are slowly disappearing. These gliders transmit their measurements via satellite communications back to our base station in England. We also plan to use the existing observations from the joint UK/US-funded RAPID programme, measuring ocean circulation at 26N. We will install additional high-resolution velocity and temperature meters on one of these moorings, to make continuous observations of the eddies over 18 months. From the survey, glider and moored measurements, we will be able to assess how important local dissipation is to the disappearance of eddies. We will use our findings to shed light on the processes responsible for eddy disappearance from the oceans, and how those processes change in time.

Planned Impact

The aim of this project is to provide the first observations of dissipation in mesoscale eddies---swirling vortices of water 100--200 km across---as they encounter the western boundary of ocean basins. Eddies are found in all ocean basins, where the tend to propagate westward. Upon reaching the sidewall of the ocean, they then disappear from satellite observations. How and why they they lose energy there is important to our understanding of large-scale ocean dynamics. This work will benefit two communities:
1. the community of ocean modellers who will have observations of the drain of energy from eddies at boundaries to compare against their numerical simulations; and
2. the wider public, as we engage with them to inspire enthusiasm for science.

(1) Ocean modellers. While the work we do will not immediately contribute to changing how eddies are represented in ocean models, it provides a valuable and previously non-existent set of observations of eddies and their dissipation at ocean boundaries. By shedding light on the mechanisms responsible for eddy energy loss, we will increase our knowledge of what processes are important to represent in climate models that may not yet be represented. Again, as the work is several steps from changing eddy representation, we intend to engage with numerical modellers who use more idealised models to investigate particular processes, rather than simulate the global oceans. In this way, our work will be immediately available to the modelling community.

(2) Wider public. The field work associated with this project---watching eddies in real-time altimetry and then flying out the research team to meet them with high tech instrumentation including gliders---lends itself to outreach with the general public. And so we have developed a suite of educational activities, embedded within existing university and county-wide efforts. In particular, we plan to create a teaching package for the Hampshire-area STEMNet ambassadors programme, and an interactive component in person, with area schools, as well as online during the fieldwork season. We will also contribute to Ocean and Earth Day activities at the National Oceanography Centre. These efforts will cultivate enthusiasm for, and understanding of marine and climate science issues.

Publications

10 25 50
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Evans D. G. (2019) Annual cycle of turbulent dissipation estimated from Seagliders in Geophysical Research Letters

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Meinen C (2019) Structure and Variability of the Antilles Current at 26.5°N in Journal of Geophysical Research: Oceans

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Testor P (2019) OceanGliders: A Component of the Integrated GOOS in Frontiers in Marine Science

Related Projects

Project Reference Relationship Related To Start End Award Value
NE/N001745/1 01/09/2015 14/08/2018 £867,385
NE/N001745/2 Transfer NE/N001745/1 15/08/2018 31/10/2019 £269,783
NE/N001745/3 Transfer NE/N001745/2 01/11/2019 28/02/2020 £134,891
 
Description For MerMEED, we have developed 2 key impacts:
(1) A novel method using autonomous underwater gliders to estimate how quickly the ocean is mixing. Measuring mixing in the ocean usually involves highly specialised equipment to measure centimetre-scale fluctuations in temperature and ocean currents, which requires continual presence of a large research vessel. With this new method it is possible to estimate mixing from autonomous gliders profiling for up to 6 months at a time.
(2) Applying this method to a 5-month glider mission carried out during MerMEED, it was possible to determine that one mechanism responsible for the decay of mesoscale eddies (swirling vortices of water which are ubiquitous in the world's oceans and approximately 100 km across) is the interaction between eddies and near inertial waves. This is a mechanism that had not previously been observed.
Exploitation Route The method developed (number 1 above) can be applied to past and future glider missions, effectively multiplying the number of measurements of mixing in the ocean at no additional cost (or the cost of further processing and analysis).
Sectors Environment

 
Description ALADDIN - Assuring Autonomy International Programme
Amount £165,000 (GBP)
Funding ID ALADDIN 
Organisation Lloyd's Register 
Sector Charity/Non Profit
Country United Kingdom
Start 02/2020 
End 08/2021
 
Description Climate Process Team: IMPROVING REPRESENTATIONS OF INTERNAL-WAVE DRIVEN MIXING IN GLOBAL OCEAN MODELS 
Organisation University of California, San Diego (UCSD)
Department Scripps Institution of Oceanography
Country United States 
Sector Academic/University 
PI Contribution Invited participant in the Climate Process Team meeting at the Scripps Institution of Oceanography, contributing to the session on Mesoscale Loss of Energy.
Collaborator Contribution Travel funding received to attend the workshop in San Diego, California.
Impact Outputs included a publication detailing advances in internal waves and mixing, though I was not a co-author.
Start Year 2015
 
Description Soapbox Art & Science, Bournemouth 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Participating in the 2018 Soapbox Art & Science in Bournemouth, UK as one of 12 female scientists.
Year(s) Of Engagement Activity 2018
URL http://soapboxscience.org/
 
Description Springhill Primary School visit to the NOC 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Schools
Results and Impact Students from Year 5 at Springhill Primary Catholic School in Southampton visited the NOC over two days in September. They went on a trip into the Solent on the R/V Callista, a small research vessel, then for a tour of the aquarium and then I told them about research with marine robots and we visited the Marine Autonomous and Robotic Systems facility at the NOC where they got to 'meet' Boaty McBoatface and some autonomous underwater gliders.
Year(s) Of Engagement Activity 2019