Everyone’s favourite autonomous submarine vehicle, Boaty McBoatface, has helped scientists decipher a mystery in the Southern Ocean…
Temperatures at the seafloors of the world's oceans have been rising over the past few decades, which has implications for rising sea levels. At the same time, wind strengths over the seas around Antarctica have been strengthening. Scientists had an inkling that these two phenomena might be connected, but they didn’t know how. Now Boaty McBoatface has helped to solve the mystery!
Data recorded by Boaty, along with other ocean measurements collected from research vessel RRS James Clark Ross, has helped scientists uncover a previously-unknown mechanism in the Southern Ocean. Increased wind speeds intensify turbulence deep within the Southern Ocean, causing warm water at mid depths to mix with cold, dense water from below. The water that sinks in the Antarctic then ripples around the world.
Professor Alberto Naveira Garabato from the University of Southampton sent Boaty on a mission to explore an underwater valley off the Antarctic coast. Boaty was able to take measurements of temperature, salinity and turbulence down to within 50 metres of the valley floor.
Garabato explains turbulence as “what you feel when you are in a plane flying over a mountain range and it gets all bumpy. The same kind of thing happens in the ocean as the water flows over submarine mountains.” The strong winds above the Southern Ocean accelerate the currents in the water and, as the faster currents flow over the submarine mountains and valleys, they generate more turbulence. This leads to mixing between the different layers of the ocean, which are naturally at different temperatures, just like when you stir cold milk into your hot cup of tea.
The winds blowing over the Southern Ocean have been getting stronger due to the hole in the ozone layer above Antarctica and increasing concentrations of greenhouse gases in the atmosphere. Both the ozone layer and greenhouse gases are able to trap solar warming. Atmospheric temperatures are rising in the northern hemisphere due to rising levels of greenhouse gases, but there is less solar warming above the Antarctic due to depleted levels of atmospheric ozone. Gas at different temperatures has different pressure, so there is lower pressure air above the Antarctic. This pressure gradient is driving the strong winds in Antarctica.
Consequently, temperatures at the bottom of the ocean have risen by between 0.03°C and 0.1°C across the world since the 1990s. While that might not sound like a lot, Garabato says that “because the volume of this body of water is very large, it actually represents a big chunk of heat. The latest estimates suggest that about 30% of all the ocean warming has happened in this very bottom layer.”
This research shows that, for scientists modeling the effect of greenhouse gas emissions on rising sea levels, it is not enough to only consider rising atmospheric temperatures. The added mechanism of increasing greenhouse emissions strengthening the winds above the Southern Ocean, leading to a global increase in temperatures at the sea bottom, means that sea levels could rise even faster than previously predicted.
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