Atlantic water carried north sinks in an unexpected place

Climate models predict that the ocean overturning circulation should slow down over the next few decades. Surface water that moves from the tropical Atlantic Ocean to higher latitudes loses heat to the atmosphere. Once the cold, salty, and consequently dense water reaches the northern Atlantic and Arctic, it sinks, or overturns, and becomes what’s known as deep water, which travels southward back to the tropics (see the article by J. Robert Toggweiler, Physics Today, November 1994, page 45 ). Previous efforts to observe the so-called meridional overturning circulation (MOC) have focused on the subtropical Atlantic, where the ocean circulation is simpler. (The international observational system, RAPID-MOCHA, can be seen at 26.5˚ N on the map.) Now, Susan Lozier of Duke University and her colleagues have analyzed data from a new array in the subpolar North Atlantic to better understand where the northward-moving water sinks before it travels southward.

Deployed in 2014, the Overturning in the Subpolar North Atlantic Program (OSNAP) array was designed to provide ground observations for climate models that oceanographers have relied on to understand the MOC in that region. The array consists of two sets of moorings that measure temperature, salinity, and velocity fields. The first, known as OSNAP West, crosses the Labrador Sea between northeastern Canada and Greenland; the second, OSNAP East, stretches across the Irminger and Iceland basins before terminating at the Scottish coast.

To their surprise, the researchers learned that during 2014–16, the amount of overturning across the OSNAP East array was seven times as much as that across OSNAP West. Such a result clashes with most climate models, which predict substantial deepwater formation in the Labrador Sea. The OSNAP result was even more startling because the Labrador Sea experienced exceptionally strong convection in the winters of 2014 and 2015, which would suggest more overturning. The researchers are now trying to determine what mechanisms could explain the results.

With only 21 months of data so far, OSNAP is still in its early days. Figuring out whether the MOC has strengthened or weakened because of climate change will require a longer time series. For now, at least, the initial results will help climate modelers develop more realistic simulations of the overturning circulation. (M. S. Lozier et al., Science 363, 516, 2019 .)