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Climate Change

OSNAP! This is what we know about global ocean circulation

ArticleLozier, M.S. et al. A sea change in our view of overturning in the subpolar North Atlantic. 2019. Science. Vol. 363, Issue 6426, pp. 516-521. DOI: 10.1126/science.aau6592.

For decades, ocean scientists have been trying to get to the root of the Earth system responsible for regulating our climate: the global ocean circulation. The meridional overturning circulation (MOC) refers to a system of ocean currents that flows throughout the world’s oceans, redistributing heat, nutrients, and moisture from the equator to the poles. Although it takes approximately one-thousand years to complete its full circuit from the surface through the deep ocean, its rapid communication with the atmosphere influences the local climate and weather patterns we experience from year to year.

The response of such a slow ocean system to the fast pace of global warming is a key question at the heart of MOC research. Now, with the first results from an unprecedented international ocean observing system called OSNAP (Overturning in the Subpolar North Atlantic Program), we may have reached a turning point in our understanding of it.

The OSNAP mission

Figure 1: The OSNAP array located between Newfoundland and Greenland, and between Greenland and Scotland. Warm surface currents are shown in red, and cold deep currents are shown in blue. (Image credit: OSNAP)

At the beginning of February, scientists leading OSNAP published a glimpse at their long-awaited first results of MOC behavior in the subpolar North Atlantic (SPNA). The SPNA is located near sites of deep water formation (DWF), where surface currents sink when they become cold and dense after giving off heat to the atmosphere – an overturning (vertical) flow unique to high latitudes that provides an important pathway for heat, carbon, oxygen, and nutrients to get from the surface to the deep ocean. These scientists planned the OSNAP array to find out not only where this sunken water comes from and where it’s going, but whether it’s slowing down as the ocean begins to lose less heat to the atmosphere in a warming climate.

Figure 2: The OSNAP set-up of moorings in the eastern part of the array from Greenland to Scotland, which includes the Irminger Sea and Iceland Sea. (Image credit: OSNAP)

In 2014, the OSNAP array was deployed to collect data along two transects between Newfoundland  and Greenland and between Greenland and Scotland (Figure 1) using a complex suite of ocean sensors. Moorings were anchored to the sea floor to collect data on ocean temperature, salinity, and velocity along land boundaries (Figure 2). Subsurface floats collected similar data as they drifted along individual flow pathways. Where the seafloor was rough, instruments called gliders followed planned routes to capture the ocean’s variability there. After 21 months, the first set of data was recovered from this array and supplemented with additional  datasets available from the worldwide Argo float program and satellite sensors. By observing how ocean characteristics changed along these transects in just under 2 years, the scientists hoped to obtain a more complete understanding of what influences MOC variability.

A long road to OSNAP

The OSNAP array was a long time in the making for its lead scientist, Dr. Susan Lozier of Duke University, who helped conceive of the program in 2007. The idea came three years after a crucial ocean array called RAPID-MOCHA was deployed in 2004, spanning from the east coast of Florida to the west coast of Africa. Located along 26.5N, the array was used to observe the Atlantic MOC (AMOC) at a low latitude far from the overturning of the subpolar and polar regions.

The results of RAPID-MOCHA revealed rapid variations in a complex system of ocean currents that was once perceived to be much more stable. They also showed little coherence between the subtropics and subpolar latitudes, meaning that the nature of global ocean circulation did not resemble a globally linked “great ocean conveyor belt” as previously envisioned. The OSNAP effort aimed to close the gap in knowledge that the RAPID-MOCHA program exposed by looking strictly at ocean currents in the vicinity of overturning DWF near the poles. By focusing on the SPNA region, they could get a better idea of how the ocean behaves immediately before and after overturning – that is, how it connects the surface and deep ocean.

More data lead to more important questions

Just like RAPID-MOCHA, the preliminary results of OSNAP published this month have baffled oceanographers. By analyzing east and west sections of the array separately, they observed key differences in the role each region played to facilitate the overturning that connects the surface and deep ocean.

Figure 3: NASA satellite image of sea ice in the Labrador Sea off the coast of Newfoundland and Labrador. (Image credit: NASA)

The most groundbreaking finding of this study is that the flow in and out of the eastern section between Greenland and Scotland accounts for most of the surface-to-deep communication in the entirety of the SPNA; that is, the DWF in the east is most significant to the overturning of the AMOC. By pinpointing this region as the specific birthplace of deep water in the North Atlantic, we can now focus on how the overturning there is driving climate in the Northern hemisphere, and altering the efficiency with which carbon is removed from the atmosphere and oxygen and nutrients are provided to ocean ecosystems.

These results shocked the OSNAP team because their attention had been focused mainly on the western section of the array – the Labrador Sea (Figure 3) – where water also gets heavy and sinks as it cools, forming a mass of water known as the Labrador Sea Water (LSW). The LSW is known to spread throughout the subpolar region, leaving a unique imprint on both the eastern and western SPNA that can change as sea ice melts and freshens the Labrador Sea. While this region is intricately connected to the AMOC, the study showed for the first time that – at least during this observation period – it was not a primary point of communication between the surface and the deep.

What next?

Although these unexpected results might raise more questions than they answer, they will likely be the basis for many future observational and modeling studies seeking to improve our understanding of the MOC and its relationship to climate. With 21 months of successful ocean observing under their belts, OSNAP scientists emphasize the need for continued observations to understand what’s happening in the SPNA over much longer time scales. Even while new research is underway, the OSNAP array will continue to build on what is so far only a scratch at the surface of the nature of global ocean circulation.

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