In the remote reaches of the northern Atlantic, a major ocean current brings warm surface water from the tropics toward the Arctic and returns cold deep water toward the equator. This flow of warm water, known as the Atlantic Meridional Overturning Circulation (AMOC), has played a fundamental role in maintaining the mild climate of central Europe and Scandinavia as we know it today.
We also know that changes in its strength seem to have contributed to well-known climate events in recent millennia, and it continues to modulate global climate today. For example, a weakened AMOC may have played a role in causing almost 600 years’ worth of frigid winters in Europe and North America. This period, called the Little Ice Age, lasted roughly from 1300 until 1870 and came on the heels of the Medieval Warm Period (circa 950–1250), when temperatures in the Northern Hemisphere were unusually warm.
Nearly half of the AMOC’s poleward flow of warm, salty waters enters the Nordic Seas—comprising the Greenland, Iceland, and Norwegian Seas. Here the water cools and pools north of the undersea Greenland-Scotland Ridge (GSR), which spans from southeastern Greenland across Iceland and the Faroe Islands to northern Scotland, before spilling back into the deep North Atlantic (Figure 1). A host of important questions remains about the dynamics of the ocean near the GSR and the effects of these dynamics on regulating climate. The more we know about the variability and driving mechanisms of the exchange of waters across the GSR, the better we can explain and predict future changes in this system.
Here we discuss two recent studies we conducted to address these important questions, and we highlight surprises encountered along the way that point to the importance of continuing research and improved ocean monitoring.
Reconstructing AMOC’s History
Scientists have come to realize that the AMOC has two pathways of overturning circulation. One is open ocean convection in the Irminger and Labrador Seas (Figure 1) that produces the upper layer of North Atlantic Deep Water (NADW). The second involves progressive cooling of warm, salty water from the Atlantic in the Nordic Seas. This cooling results in dense water spilling over the GSR back into the North Atlantic—mainly through two passages, the Denmark Strait between Greenland and Iceland and the Faroe Bank Channel south of the Faroes—and forming a lower layer of NADW. Both pathways play important roles in climate variability over a wide range of timescales, although the interplay between the two is not well understood.
Both regions depend upon heat loss to produce water of greater density, but it appears that huge heat losses from the Nordic Seas and the concomitant production and pooling of very dense water behind the GSR are fundamental to maintaining a mild climate in northern Europe. This heat loss produces a healthy supply of NADW that spills back into the global abyss and enables warm, salty water to feed the Nordic Seas [Chafik and Rossby, 2019]. Exploring the intricate processes and routes by which warm, saline North Atlantic water is gradually transformed into this cold, dense water motivates our research.
The strength of the AMOC at the GSR since the mid-1990s has been a major focus of many institutional and international research initiatives, which, among other findings, have helped to reveal the stability of the water mass exchanges (warm inflows and cold deep flows or overflows) across the GSR [Østerhus et al., 2019]. But how did this inflow to the Nordic Seas vary before the 1990s? And how well do we know the deep pathways that the densest water takes before exiting the Faroe Bank Channel to the North Atlantic Ocean?
To investigate the warm upper-ocean flow at the GSR, we used historic hydrographic data dating back to the early 1900s to gain insight into how the Nordic Seas inflow varied since the beginning of modern oceanography [Rossby et al., 2020]. To do this, we constructed a time series of dynamic height difference (which essentially represents the pressure gradient in the upper ocean that determines water flow) between areas north and south of the GSR to measure transport of almost all water entering the Nordic Seas.
This transport time series showed evidence of strong variability in Nordic Seas inflow on multidecadal timescales. We found that the volume of and heat transported in this poleward flow, as measured at the GSR, are strongly coupled to the Atlantic multidecadal variability (AMV), which describes natural patterns of sea surface temperature variability in the North Atlantic that influence climate globally [Zhang et al., 2019].
The AMV affects Nordic Seas inflow because deep convection in the northeast Atlantic translates the surface temperature variations down into the upper layers of the ocean, and these variations shape the ocean’s dynamic height field. Coupling between the inflow of Atlantic water into the Nordic Seas and the AMV was so tight that we could find no evidence for long-term or secular weakening or strengthening of this poleward flow (related to anthropogenic warming, for example). In short, the inflow of warm water to the Nordic Seas has been quite stable over the past century since the start of modern oceanography.
This finding is consistent with a previous study that reported stability of the inflow over the past 2 decades [Østerhus et al., 2019] and supports the conclusion that Nordic Seas overturning circulation has been stable over the past 100 years. This stability is surprising given the extraordinary warming presently underway in the Nordic Seas and Arctic Ocean. Understanding the reasons for this apparent disconnect is important and points to the need for improved and even expanded ocean monitoring because the continued stability of this vital ocean circulation system is not guaranteed in the future. It is also unclear how future change may manifest or which early-warning indicators should be relied upon to forecast change [Østerhus et al., 2019].
Discovering a New Flow Route
Another example of where we have had to revise our thinking regarding the pathways of the deep currents in the Nordic Seas came with the recent discovery of an unknown route by which cold water courses its way through the Norwegian Sea. We identified that this new route directs cold deep flows north of the Faroe Islands to the Norwegian slope before turning them south through the Faroe-Shetland Channel and into the deep North Atlantic (Figure 1) [Chafik et al., 2020]. Previously, only direct communication of water from north of the GSR to the Faroe-Shetland Channel was known [Søiland et al., 2008].
We found that which route water takes north of the GSR and how much is funneled each way depend on the prevailing winds [Chafik et al., 2020]. Under weak westerly wind conditions in the Nordic Seas, the densest water that feeds the Faroe Bank Channel comes primarily from north of Iceland. During strong westerly wind conditions, however, more water seems to originate from along the Jan Mayen Ridge, which is located farther north of Iceland and more in the middle of the Nordic Seas. This wind dependence is curious, considering the strong control that bathymetry can exert on the circulation, and requires more study—it hints at deep ocean variability we had not previously appreciated or recognized.
In the same study, we reported on a previously undiscovered deep rapid flow, or deep jet, called the Faroe-Shetland Channel Jet. Remarkably, this jet flows south along the eastern slope of the channel rather than along the western side as has long been assumed. The deep jet is found to be the main current branch in terms of transport that delivers the densest water to the North Atlantic Ocean via the Faroe Bank Channel. This surprising finding, which countered past observations and thinking, required that we carefully recheck our data and analyses, but ultimately, we decided that this deep jet was a real feature. This view thus alters our previous understanding of the deep circulation in the region and suggests that we do not yet have a firm grasp of the deep circulation of the Nordic Seas and how it varies over time.
Monitoring for Change
In the past few years, several workshops have been held to review and identify gaps in our knowledge of exchange between the Atlantic and the Nordic Seas and to advance our understanding of ocean circulation in this region.
All available observational evidence so far indicates that there is no long-term trend in the Nordic Seas meridional overturning circulation to date [Østerhus et al., 2019; Rossby et al., 2020]. Yet meeting attendees agreed that given the substantial ocean warming and freshening (from water runoff from Greenland and precipitation) taking place at higher latitudes, it is essential to continue monitoring the overturning to assess its role in ongoing and future climate change.
Several existing techniques will be useful for this monitoring effort. Satellite altimetry can be used to study flows at the surface and in the upper layers of the ocean. Vessel-mounted acoustic Doppler current profilers can also probe these flows in the upper ocean. Meanwhile, moored sensor arrays track the variability of deep currents. In addition, Lagrangian techniques, specifically using acoustically tracked subsurface floats that drift with ocean currents, have proven very effective at elucidating pathways [e.g., Søiland et al., 2008] and timescales along which subsurface waters flow and gradually disperse or mix with surrounding waters.
Floats in particular could help address one area of considerable interest, namely, the degree to which fresh water from the Arctic and Greenland Sea can mix with and dilute warm, saline water from the Atlantic. Such dilution could suppress deep temperature- and density-driven convection, thus weakening or shutting down the overturning in the Nordic Seas and, by extension, the deepest component of the AMOC.
However, most scientists no longer think such a shutdown scenario is likely because observations to date indicate that Arctic and Greenland waters tend to remain trapped around and south of Greenland rather than mixing and diluting the Atlantic water flowing north in the Nordic Seas [Intergovernmental Panel on Climate Change, 2019]. Nonetheless, there is broad agreement that the climatic consequences of a potential shutdown of this vital ocean circulation are so enormous that they obligate us to improve our understanding of the Nordic Seas (and generally about the overturning at higher latitudes) rather than to presume we know enough already about the inner workings of the ocean in this region. These concerns help to explain the rapidly growing interest in the dynamics of the Atlantic’s remote northern reaches.
L.C. acknowledges support from the Swedish National Space Agency through the Fingerprints of North Atlantic-Nordic Seas Exchanges from Space across Scales (FiNNESS) project (Dnr: 133/17).