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ocean engineering

Making waves in the Southern Ocean

 

Thomson, J., and J. Girton. 2017. Sustained measurements of Southern Ocean air-sea coupling from a Wave Glider autonomous surface vehicle. Oceanography 30(2):104–109, https://doi.org/10.5670/oceanog.2017.228.

The Southern Ocean frontier

The Southern Ocean, which wraps around Antarctica, is one of the windiest places on earth. In the winter months,much of the Southern Ocean is covered with a layer of sea ice.Because of this, it is very challenging for oceanographers to collect measurements in the Southern Ocean, and most observations are collected during summer when the days are longer, winds are weaker, and there is less sea ice. As a result, there are a lot of basic things scientists still don’t know about the Southern Ocean. One particularly mysterious topic is air-sea interaction. Transfers of heat and gases are really important for understanding how the atmosphere and ocean interact and how the ocean modulates our climate. The Antarctic Circumpolar Current (known as the ACC), which wraps around the Southern Ocean, has many strong ocean fronts and swirling eddies, which lead to large changes in ocean properties and exchanges of heat and carbon over small distances. Measuring how air-sea interaction changes across these small spatial scales is particularly challenging.

 

Observing the interface

The two most common ways scientists directly collect observations at the air-sea interface is using research ships, which can collect measurements over a specific area for a short time period, and moorings, which collect continuous measurements over time at a single location. These two platforms for measurements each have different strengths and weaknesses, and alone are not able to give a full picture of how air-sea interaction changes in both space and time. Deploying and maintaining moorings are very expensive and time consuming. For perspective, on one cruise I was on in the Southern Ocean where we replaced a large mooring, we had 30 days of  ship time to do 15 days of work because we expected the weather to be too rough to work on half the days. In the end we got lucky, and managed to get all the work done with a couple of days to spare, but had multiple stretches where all we could do was sit inside and watch the 30 ft waves hit the ship’s bow while we waited out a storm.

Scientists Matt Boyer (left) and Jim Thomson from the Applied Physics Laboratory recover the Wave Glider in March off the coast of Argentina. From the University of Washington.

As newer technologies become available, scientists are looking to autonomous platforms as more adaptable and cost effective ways to observe air-sea interaction. One of these is called a Wave Glider, a small surfboard like platform that is attached to wings hanging below the surface and that harnesses wave motion to propel itself. As each wave passes, the wings allow the the sub to move forward but not backward, pulling the surface platform along with it. The surface platform also has solar panels and a battery pack to power all of the instruments attached to it. Many different measurements can be made using instruments attached to the Wave Glider including wind speed, wave height, humidity, solar radiation, surface ocean currents and temperature of the air and sea surface. Wave Gliders have been used successfully in many parts of the world’s oceans and recently one successfully collected data during the passage of a tropical cyclone. Given its success so far, a team of scientists at the Applied Physics Laboratory in Seattle decided to test how well a Wave Glider would fare in the extreme conditions of the Southern Ocean for the first time.

Surfing for science

The research team deployed the Wave Glider in Drake Passage, where the ACC squeezes through the gap between the tip of South America and the Antarctic Peninsula, creating many strong fronts and eddies. They released the Wave Glider from a ship near Antarctica in December, the peak of summer in the Southern Hemisphere. Pilots sent signals to the glider from onshore and navigated it northward across Drake Passage. Using information about the position of ocean fronts from satellite data, the pilots zig-zagged the Wave Glider across a front to see how the air-sea interaction changed on each side. By late summer though, the Wave Glider ran into trouble: as the days got shorter it could no longer generate enough energy from the solar panels to recharge the batteries and keep collecting data. Luckily, the scientists on shore still had enough power to navigate the Wave Glider and a research ship recovered it near Argentina in March.

Figure 2 from Thomson and Girton 2017 showing the location of the Wave Glider track in Drake Passage between the Antarctic Peninsula and South America. COlor shows the sea surface temperature on January 21st 2017 during the Wave Glider deployment. Magenta dots show an example of when the Wave Glider crossed a strong ocean front.

Now that the data is gathered, the next step for the research team is to process it. The data collected have more detailed spatial coverage than a mooring and have longer coverage in time than most ship observations, making this a unique dataset. With these detailed observations scientists can then estimate fluxes of heat and momentum between the atmosphere and ocean, and compare the results to other estimates. The researchers can also use data collected on wave height to better understand wave patterns and how these contribute to ocean mixing on small scales.

Figure 4 from Thomson and Girton 2017 showing the zig-zag track of the Wave Glider across a front in Drake Passage. The middle panel shows velocities from the glider (red arrows) and sea surface height contours from satellite data (blue) and the lower panel shows sea surface temperature from satellite data and from the Wave Glider.

This experiment is an excellent demonstration of how new technology is rapidly changing the ways we can observe and understand what is happening in our atmosphere and oceans. The first deployment of a Wave Glider in the Southern Ocean showed that it is possible to measure air-sea interaction in extreme, high latitude conditions and this could potentially fill a gap in the current observations collected from moorings and ships. However, there are still challenges with storing and managing enough power for Wave Gliders to work year-round in the Southern Ocean and there is a need for different tools for accessing the ice-covered regions surrounding Antarctica. Future plans for the research team are to see if they can do more experiments with the Wave Glider to figure out how this new tool could best be applied as part of a long-term strategy for observing air-sea interaction in the Southern Ocean.

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