Book Review Physical oceanography

Earth’s strongest current even stronger than previously thought

Source: Donohue, K. A., Tracey, K. L., Watts, D. R., Chidichimo, M. P., & Chereskin, T. K. Mean Antarctic Circumpolar Current Transport Measured in Drake Passage. Geophysical Research Letters.

The greatest current on Earth

The first time I crossed the Antarctic Circumpolar Current on board a ship, I remember watching the ship’s instruments intently as the sea surface temperature dropped suddenly, and the surface currents spiked as we crossed a front. Looking out the porthole, the surface of the water looked the same, but our measurements showed the ocean we were in was very different. For the next few days, the ship was battered by storms and howling winds until we crossed the southern boundary of the current and entered the relative calm of the Ross Sea near Antarctica, where the sun shone on icebergs drifting in the gentle waves.

The surface speed of the ocean showing the strong Antarctic Circumpolar Current circling the Antarctic continent. From Los Alamos National Laboratory.

The Antarctic Circumpolar Current (ACC) is the largest ocean current on Earth. Strong winds drive its flow around the Antarctic Continent, providing a big ocean conveyer belt that where water from all of the world’s major oceans comes together and mixes in the vigorous jets and eddies of the ACC. The ACC has the power to influence Earth’s climate, by carrying heat, nutrients and carbon along its path. As our climate is changing, tracking changes in the strength of the ACC is essential to understand how the Oceans are changing.


Going with the flow

Despite the enormous scale and power of the ACC, or perhaps because of it, scientists have struggled for decades to measure the flow of the ACC. To measure the volume of water transported by the ACC means collecting frequent measurements that cover a wide band of ocean, withstanding strong velocities and braving extreme storms from the atmosphere.

Scientists put in a large effort in the 1970’s to measure the ACC strength with current meter and bottom pressure measurements combined with shipboard measurements. They found the mean ACC transport 134 Sverdrups (Sv), where 1 Sv is 1 million cubic meters per second. To put that in perspective, if you combine the flow from all the rivers in the world into the ocean, they add up to about 1.2 Sv. The 134 Sv estimate was a great start, but the measurements didn’t have ideal coverage in space and time, so there were large uncertainties of 27 Sv associated this number. Since then, scientists have used satellite sensing of the sea surface to estimate the ACC strength, but again there are still large uncertainties. Getting an accurate estimate of the ACC flow from observations is important for climate studies, because the strength of the ACC is frequently used to check the validity of ocean and climate models.


A new era of current tracking

A team of scientists from the Rhode Island, Argentina and San Diego ventured to the Southern Ocean to get a more accurate estimate of the ACC strength. The team planned their experiment in Drake Passage, the gap between the tip of South America and the tip of the Antarctic Peninsula where the ACC is squeezed through at its narrowest point. The researchers deployed 22 instruments on the seafloor from 2007 to 2011, measuring bottom pressure, bottom currents and ocean properties every hour. With this new data, the researchers were able to come up with a new estimate of the ACC strength, with narrower uncertainty. They came up with 173.3 ± 10.7 Sv, which is 30% larger than the commonly used estimate of 134 Sv. This new estimate is likely larger because more closely spaced instruments were able to capture the many narrow, strong flows at the seafloor that might have been missed in earlier measurements.

Figure 1 from Donohue et al. showing the location of the instruments used to measure the Antarctic Circumpolar Current strength in Drake Passage from 2007 to 2011.

Full speed ahead

This research team was not alone at trying to improve on the early estimates of ACC strength, and they compared their result with other recent estimates from different data, and found their number agreed well with 2 out of 3 recent estimates. This means we now have a more accurate estimate of ACC strength to use as a baseline to compare the quality of climate models. However, the experiment only ran for 4 years because of the large cost of deploying and maintaining instruments, so now the challenge is to come up with a way to measure the ACC continuously, to assess how it is changing over many years, decades and centuries. It will take ingenuity from the oceanography community, and ongoing funding to continue to monitor changes in the ACC. And hopefully the next generation of oceanography PhD students will cross the ACC aboard a ship just like I did, but with much more understanding of the mysteries of the current flowing beneath them.

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