Mechanisms Responsible for Mesoscale Eddy Energy Dissipation (MeRMEED)

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.

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.