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Physical oceanography

Antarctica’s bottom waters freshen up

Source: Menezes, Viviane V., Alison M. Macdonald, and Courtney Schatzman. “Accelerated freshening of Antarctic Bottom Water over the last decade in the Southern Indian Ocean.” Science Advances 3.1 (2017): e1601426.

The deep ocean conveyer

The global ocean conveyor belt acts like a superhighway, carrying heat from one part of the Earth to another, controlling our climate. One key part of the conveyer belt is the formation of very salty, dense water around the coasts of Antarctica. This is called Antarctic Bottom Water, or AABW for short, and forms when sea ice freezes on the ocean surface, leaving behind very dense brine. Dense, almost freezing water then sinks to the seafloor and spreads northward from Antarctica, filling the bottom of the world’s oceans.

It is extremely challenging for oceanographers to measure how AABW changes over time because, currently, the only way to measure the properties of water that deep (more than 3000 m below the sea surface) is with instruments deployed over the side of a ship. Despite this challenge, scientists have measured striking changes in AABW over the past two decades. A set of repeated ship transects across the world’s oceans are repeated every 10 years so we can track how the oceans are changing globally on large scales. Looking at the difference in deep water between the 1990’s and the 2000’s revealed a striking pattern of warming of AABW extending northward from Antarctica. In addition, the AABW also became slightly less salty, so the density decreased overall. This signal was so striking and robust that it was included in the IPCC report. The question is, with only two decades to compare, how would the warming and freshening trend in AABW hold up?

Figure 1 from Menezes 2017. Map of the seafloor (Australia in the top right corner) showing the location of the repeated ship line. Black dotted lines show the pathways of AABW flow, and the colored contours show regions with high eddy activity.

Figure 1 from Menezes 2017. Map of the seafloor (Australia in the top right corner) showing the location of the repeated ship line. Black dotted lines show the pathways of AABW flow, and the colored contours show regions with high eddy activity.

New lines of evidence

Last year, one of the repeat ship lines in the Australian Antarctic Basin, which is south of Australia, was repeated, and another set of measurements was collected to compare to the previous ship measurements in 1994 and 2007. A group of researchers led by WHOI oceanographers Viviane Menezes and Alison Macdonald and Scripps researcher Courtney Schatzman, analyzed the newly collected data to assess changes in the region compared with the previous two decades. They immediately found that AABW had continued to change, and some changes had accelerated unexpectedly. In particular, AABW had continued to get fresher, but at a much faster rate than in the previous decade. 92% of all the data they collected in 2016 were fresher compared to the same locations in 2007. These changes have caused the density of AABW to decrease, which could potentially disrupt the ocean circulation and contribute to sea level rise.

Figure 4 from Menezes et al. 2017. A and B show the distribution of changes in temperature in 2016 compared to 2007 (A) and 1994 (B), with red showing all the data that got warmer over the time period. C and D show the same for changes in salinity, with blue showing that most of the water became fresher.

Figure 4 from Menezes et al. 2017. A and B show the distribution of changes in temperature in 2016 compared to 2007 (A) and 1994 (B), with red showing all the data that got warmer over the time period. C and D show the same for changes in salinity, with blue showing that most of the water became fresher.

An icy suspect

The research team’s next challenge was to figure out why the AABW has freshened so much more in the last decade. There are two possible culprits: the amount of AABW formed in Antarctica could change or the temperature and salinity of the AABW could change, or a combination of both. Currently, with so few measurements of deep water properties spaced many years apart, it is impossible to pin down the exact cause. However, given the evidence available, the research team found a possible cause: an enormous iceberg that broke from Antarctica in 2010. The Mertz Glacier Tongue calved abruptly from the Antarctic Coast in February 2010 after another iceberg, B-09B, collided with it. When the iceberg melted, it added a large pulse of freshwater to the ocean, which could have been carried to the seafloor in AABW. Right now the iceberg is a hypothesis, so scientists have a new task ahead of them to use observations and ocean models to figure out if the iceberg could really be to blame.

Satellite image of the Mertz Glacier Tongue just after breaking off from the glacier. Image from NASA Earth Observatory.

Satellite image of the Mertz Glacier Tongue just after breaking off from the glacier. Image from NASA Earth Observatory.

The future of the deep ocean

This study shows that while we are successfully monitoring changes in our deep oceans, our current technology for observing them limits scientists’ ability to determine what drives these changes. The good news is that new ways of tracking AABW are rapidly being developed and deployed. NASA satellites measuring tiny changes in gravity are being used to estimate the strength of deep ocean currents, and a new generation of underwater ocean robots is being tested and deployed. These new models can dive to depths up to 6000 m, while the previous generation could only dive to 2000 m. These new technologies are giving scientists unprecedented access to the deep ocean, which will provide new insights into how the deep ocean both influences and responds to changes in our climate.

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