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Agulhas Current enigma: An oceanic gap in our climate understanding

Deploying a weather balloon

Deploying a weather balloon from the SA Agulhas II. Image by Petro Kotzé.

  • Comprehending the workings of western boundary ocean currents, like those of the Agulhas Current off the South African coast, may hold a key to Earth’s climate system. But understanding this particular current is hampered by a major lack of in-situ data. This gap leaves us in the dark about local, regional and global climate impacts.
  • The Agulhas Current, located in the Indian Ocean, is one of the most energetic ocean current systems in the world. Changes to it can impact local weather in South Africa and elsewhere in the Southern Hemisphere, and perhaps influence large-scale climatic changes in the Northern Hemisphere and globally as well.
  • However, it is not clear how and what these impacts may be, or when they may occur. With climate change escalating rapidly due to unabated human carbon emissions, it is now more important than ever that we understand the impacts of Southern Hemisphere ocean currents, and integrate their actions into climate models.
  • But attempts at long-term monitoring of the Agulhas Current System have not been fully successful. Accomplishments and failures to date have underscored significant local research capacity challenges, and differences in the approach to, and financing of, ocean science in the Global North as compared to the Global South.

THE AGULHAS CURRENT, Indian Ocean, Off the coast of South Africa — This past July, I boarded the South African research vessel SA Agulhas II with high hopes of journeying along with the scientists on board to study the mysterious Agulhas Current, and with plans to report their data and findings to Mongabay readers.

But the best laid plans … Just days into our voyage, we were suddenly forced to abandon the vessel’s important climate mission and veer away from our scheduled route northward along South Africa’s east coast, and steer southeast instead to evacuate two injured researchers from Marion Island, a remote nature reserve in the sub-Antarctic Indian Ocean.

In the rush to make that emergency sea rescue, months of planning by the scientists aboard South Africa’s sole Antarctic research vessel had to be shelved, leaving the riddles of the extraordinarily strong Agulhas Current, which barrels down the East Coast of South Africa, unsolved for at least another year.

Add to this the previous three-year delay due to COVID-19, during which the Agulhas System Climate Array (ASCA) transect hadn’t been monitored, and a bigger problem rears its head: If humanity is to fully understand how Earth’s climate system works, we must comprehend how boundary ocean currents function in the Global South just as thoroughly as we do in the Global North.

The SA Agulhas II detours to Marion Island through meters-high swells on an emergency rescue mission. Image by Petro Kotzé.

Shaped by coastlines, boundary currents are surface currents that flow along the eastern and western sides of oceanic basins. But while some of those boundary currents in the north — such as the well-known Gulf Stream — have been under scrutiny for many decades and sometimes centuries, those swirling in seas south of Earth’s equator suffer from a dearth of study, with current efforts underfunded and with scientific resources here often stretched thin.

“All of us involved in the Global Ocean Observing System” — a program led by the U.N.’s Intergovernmental Oceanographic Commission, or IOC — “recognize that the shortcoming of this system right now is a lack of measurements in boundary current systems,” such as the Agulhas Current, says Lisa Beal, professor of oceanography in the Ocean Sciences Department at the University of Miami’s Rosenstiel School.

The resulting lack of historical data, along with recent data gaps, is a concern echoed by researchers and policymakers. “Long-term monitoring is one of the most important aspects of science when attempting to understand the differences and impacts between natural variability and human-induced climate change,” explains Albi Modise, chief director of communications for South Africa’s Department of Forestry, Fisheries and the Environment.

The Greater Agulhas Current System, showing the source regions of mesoscale eddies from the Mozambique Channel and East Madagascar Current and the recirculation of Agulhas Return Current; the Agulhas Retroflection and shedding of rings and filaments into the South Atlantic and the relative position of the Subtropical Convergence in the Southern Ocean (after Lutjeharms 2001). Image courtesy of Morris et al. (2017).

Scientists strongly suspect the Agulhas Current System plays a role in global climate change, but it likely also has regional significance too: “Without long-term data time-series across the Earth System surrounding [South Africa], accurately determining localized climate change impacts and [climate] adaptation strategies become almost impossible,” notes Modise.

If global and regional climate modeling forecasts are to be greatly enhanced, the international scientific community must better address the needs, resources, financing and cultural disparities between the Global North and South, Juliet Hermes says. Hermes is manager of the South African Environmental Observation Network (SAEON) Egagasini Node, and is also acting manager of the country’s polar research infrastructure.

Escalating anthropogenic climate change is fast highlighting major gaps in our knowledge, especially regarding oceans. Western boundary currents and their extensions have become the fastest-warming oceanic regions over the last 100 years. The reasons for this, and its implications, are still murky. Some scientists speculate that the high rate of warming might be because of changing winds and a poleward shift of the currents, which might impact the ability of the ocean to absorb anthropogenic carbon dioxide over these regions.

With the various currents playing a key role in the operation of Earth’s climate system, researchers fear that local ocean effects may ripple outward regionally, then globally.

Researchers with sediment samples taken during the July voyage aboard the SA Agulhas II. Image by Petro Kotzé.

Western boundary currents and climate: What we know

The first challenge to understanding oceans is size. They represent a vast, interconnected, complex, study area. Set in continuous motion by the rotation of the globe and the sun’s radiative energy, their waters whirl in massive, circular vortices called gyres, bounded by powerful boundary currents that churn up, and suck down, water and heat around the globe.

Subtropical western boundary currents like the Agulhas are strong, narrow, deep and warm. Just like the Gulf Stream up north, it moves heat from the warm tropics to colder latitudes, moderating Earth’s climate and shaping weather patterns on sea and land.

Boundary currents bordering industrialized countries including the U.S., Australia and Japan are closely scrutinized, with long-term, sustained observation programs in place. That’s not the case for the Agulhas, says Tamaryn Morris, marine unit senior scientist at the South African Weather Service (SAWS) and a co-principal investigator of the ASCA project. Though the Agulhas Current has been studied extensively from local shores, Morris notes that “long-term, sustainable monitoring of the current is not presently taking place.”

Tamaryn Morris, co-principal investigator of the ASCA project, aboard the SA Agulhas II. Image by Petro Kotzé.

The so-called “screaming” Agulhas is one of the most energetic current systems on Earth. The current shoots southwest along the continental slope of South Africa at more than 75 million cubic meters (2.65 billion cubic feet) per second — about 350 times the flow rate of the Amazon River.

In the 1400s, Portuguese explorer Vasco de Gama reported the current to be so strong it forced his flotilla of sailing ships steadily back for three days. “It’s just a beast,” Hermes says, adding that the current’s Herculean strength contributes to the challenges of research on the Agulhas. Even on the July voyage — usually a calmer time of the year — data-gathering equipment was swept away by the current.

At the southern tip of the Agulhas Bank, a broad and shallow part of Africa’s continental shelf, the current is diverted eastward by westerly winds then loops counterclockwise back into the Indian Ocean, basically dividing it from the Southern Ocean.

In this retro flow, Agulhas waters leak westward around the tip of South Africa into the South Atlantic Ocean in smaller circular currents that churn almost like whirlpools called eddies and in filaments of warm and salty water.

Here, in the so-called “Cape cauldron,” warm and saline Indian Ocean waters mix with cooler, fresher Atlantic water, flowing north and influencing the Atlantic Meridional Overturning Circulation (AMOC). The AMOC is a system of ocean currents (driven by differences in temperature and salt) that moves heat across the equator from the Southern to the Northern Hemisphere.

The AMOC works like a conveyor belt. Warm water flows north from the tropics and subtropics until it cools and sinks in the subarctic south of Greenland, then flows in the deep Atlantic, returning southward. The potential impact of the Agulhas on the ever-changing AMOC has driven international attention to understand the southern current.

Filtering water samples in one of the SA Agulhas II laboratories. Image by Petro Kotzé.

 AMOC modeling mysteries

Climate models suggest that the AMOC will weaken as atmospheric greenhouse gases increase and as the atmosphere and ocean surface warm. The escalated melting of the Greenland ice sheet in recent years appears to be reinforcing this weakening by flooding the sub-Arctic Atlantic with freshwater. Some research suggests there may be a threshold for freshwater input, beyond which the AMOC could collapse and cease flowing.

The climate impacts of such an event would be catastrophic for humanity and biodiversity.

According to climate models, an AMOC shutdown could cool the Northern Hemisphere to ice age temperatures, reducing precipitation over Europe and North America, bringing permanent drought conditions in West Africa, and shifting monsoons in South America and Africa southward. More severe winter storms and floods in Northern Europe and more summer rain in the Mediterranean might occur. Studies indicate that subsequent sea-level rise could permanently flood cities along the United States eastern seaboard.

But even as AMOC changes get underway up north, the leakage of warm and salty Agulhas waters into the Atlantic may be increasing — and that could change the AMOC’s modeled behavior.

The crew deploys a dredge to gather sediment samples. Image by Petro Kotzé.

“It’s really all about the strength of the Westerly winds,” Beal explains. “The amount of Indian Ocean water leaking into the Atlantic Ocean has increased because the Westerlies around Antarctica have intensified as an effect of anthropogenic climate change.”  In the past, a strengthening of the Agulhas leakage has been linked to a strengthening of the AMOC.

Scientists are asking whether these salty Indian Ocean waters might eventually balance out and cancel the oceanic impact of glacial melt on the AMOC. The answer: We don’t know for sure.

Climate models so far have not captured the characteristics of the Greater Agulhas Current System or its leakage realistically, because the current system is so turbulent. To increase accuracy, scientists need detailed, ongoing, onsite Agulhas data they can plug into their models.

Monitoring data at sea. Image by Petro Kotzé.

Reading the Agulhas Current System

Even satellite monitoring can only read the top “skin” of the ocean, Morris explains.

“The near-only way to study the Greater Agulhas Current System to its full depth is with long-term sustained measurements from moored instrumental arrays and repeat-hydrographic observations from ships,” researchers wrote in a 2017 paper on the importance of monitoring the current and its inter-ocean exchanges. “Only then will the scientific community begin to understand and predict changes in the Greater Agulhas Current System and [recognize] its feedbacks on regional and global climate change, thus providing societal and economic benefits to the general public.”

A first attempt to monitor the current at sea was launched in 2010, with Beal as project leader. The three-year initiative required three consecutive scientific cruises aboard U.S. research vessels and the maintenance of an array of instruments across the Agulhas Current and along a satellite ground track.

Researchers combined this invaluable in situ shipboard data with more than 20 years of satellite data to estimate how the Agulhas Current has been changing since the early 1990s.

An instrument called a CTD, which measures conductivity, temperature and depth, is lowered into sea. Image by Petro Kotzé.

The project’s findings challenged scientists’ understanding of western boundary currents and the waters’ response to climate change. Contrary to expectations based on increased global warming and intensifying winds, particularly in the Southern Hemisphere, the strength of the Agulhas Current has not increased over the past two decades. Instead, the current appears to have become more variable, Beal reveals. But what exactly that greater variability portends is uncertain.

Following this first project, ASCA was launched in 2016 — an ambitious collaboration between U.S. and South African scientists to maintain the first long-term observations of Agulhas Current volume, heat, and salt transport using South African vessels. “Measuring heat transport instead of just water transport is integral to climate studies,” Beal explains.

But, as was the case this year, things didn’t go according to plan. The moored component of ASCA was “paused” after only two deployments due to difficulties with vessel capabilities. The transect was then visited annually by the SA Agulhas II, until the COVID-19 pandemic, which created a big data gap.

However, Hermes says, all was not in vain. “We still did amazing work.” The 26 months of data gathered have provided new insights into the current, including the first estimates of its heat and salt transports.

“From a relatively small data set, we’ve been able to see quite a lot about what’s happening in the Agulhas Current [seasonally],” Laura Braby, a postdoctoral research fellow at SAEON, says. Researchers learned that deep down, nutrient-rich water located far offshore migrates up onto the continental shelf in summer when the current is stronger. In winter when the Agulhas is weaker and slower than in summer, North Atlantic Deep Water (water from the North Atlantic deeper than 2,000 meters) flows around the point of Africa underneath the Agulhas current, and northward to the equator.

Keeping an eye on cold fronts moving in during the July voyage. Image by Petro Kotzé.
Laura Braby, a postdoctoral research fellow at SAEON. Image by Petro Kotzé.

Questions far and near

Still, the ongoing three-year research gap did result in a lack of continuous in-situ data, leaving serious questions remaining as to how the Agulhas Current System changes the AMOC and influences global climate, with other questions raised closer to home. “It’s not just the Global North that is affected [by the Agulhas], but the Global South too,” Morris says.

The Agulhas system influences regional storm development and can wreak havoc in the form of extreme rainfall events and tornadoes over Southern Africa. Warming of the Agulhas Current System since the 1980s may have increased the sensitivity of the African hydrological cycle to the effects of the El Niño/Southern Oscillation, but researchers aren’t sure.

“It’s likely that the changing current will impact rainfall, which in turn will affect multiple integral industries like agriculture,” Morris explains. She says current changes might also be detrimental to the prolific fish nurseries of the Agulhas Bank, which are key to the future of rich fishing grounds up the coast.

“But then again, it might not,” she says. “We simply do not know.”

Zooplankton and phytoplankton samples. Image by Petro Kotzé.

An uncertain future 

“I cannot overstate the importance of having continuous measurements of the Agulhas system,” Beal says, “And, there’s nobody else that can do it other than South Africa.”

But hurdles loom — especially money.

Challenges include finding funding to procure research infrastructure to measure in-situ parameters across the Agulhas Current in multiple, strategic locations; while also obtaining sufficient instrumentation for redundancy; and having a team of technically trained people to support this massive undertaking across all locations, Modise says.

“Once you have both the human capacity and equipment infrastructure,” he concludes, “the budget and availability of a suitable research ship is needed to deploy, maintain and retrieve equipment at sea.”

Recent research has also spotlighted other chronic concerns, including South Africa’s wide-ranging socioeconomic priorities that compete for scant state funding; a lack of strategic integration and cooperation across governmental departments; equipment not always up to international standards; lack of long-term opportunities to retain and nurture experts; and the overcommitment of technical staff.

Still, Hermes says, the future looks exciting and bright. Research projects need to focus on building on existing capacity in Africa, instead of building new capacity in Africa, she says. Modise points out that South Africa has successfully run internationally recognized oceanography projects. “The department has been monitoring the heat and salt exchange between the Indian Ocean basin and the South Atlantic as well as intrusions from the Southern Ocean as a larger climate study with international partners,” he says

Crew and researchers at work with bongo nets aboard the SA Agulhas II. Image by Petro Kotzé.

Meanwhile, back on the SA Agulhas II in July, Morris wrote up the cruise report as the vessel headed home to Cape Town, documenting how, instead of achieving the ASCA transect, onboard researchers studied a cyclonic eddy that formed in a bight on the continental shelf along their adjusted route. Maybe it wasn’t the data they sought, but maybe it would go some way toward solving the Agulhas’s many puzzles.

Hermes, who was not on board, isn’t surprised. “In Africa, you maak ‘n plan — make a plan,” she says. “We don’t have the numbers of necessary experts, and we don’t have the money, but we make the most of what we’ve got.”

Banner image: Deploying a weather balloon from the SA Agulhas II. Image by Petro Kotzé.

Citations:

Beal, L. M., & Elipot, S. (2016). Broadening not strengthening of the Agulhas Current since the early 1990s. Nature, 540, 570–573. doi:10.1038/nature19853

Beal, L. M., De Ruijter, W. P., Biastoch, A., Zahn, R., & SCOR/WCRP/IAPSO Working Group 136. (2011). On the role of the Agulhas system in ocean circulation and climate. Nature, 472, 429–436. doi:10.1038/nature09983

Morris, T., Hermes, J., Beal, L., Du Plessis, M., Duncombe Rae, C., Gulekana, M., … Ansorge, I. J. (2017). The importance of monitoring the Greater Agulhas Current and its inter-ocean exchanges using large mooring arrays. South African Journal of Science, 113(7-8). doi:10.17159/sajs.2017/20160330

McMonigal, K., Beal, L. M., Elipot, S., Gunn, K. L., Hermes, J., Morris, T., & Houk, A. (2020). The impact of meanders, deepening and broadening, and seasonality on Agulhas Current temperature variability. Journal of Physical Oceanography50(12), 3529-3544. doi:10.1175/JPO-D-20-0018.1

Gunn, K. L., Beal, L. M., Elipot, S., McMonigal, K., & Houk, A. (2020). Mixing of subtropical, central, and intermediate waters driven by shifting and pulsing of the Agulhas Current. Journal of Physical Oceanography50(12), 3545-3560. doi:10.1175/JPO-D-20-0093.1

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