Atlantic meridional overturning circulation

The Atlantic meridional overturning circulation (AMOC) is the zonally-integrated component of surface and deep currents in the Atlantic Ocean. It is characterized by a northward flow of warm, salty water in the upper layers of the Atlantic, and a southward flow of colder, deep waters that are part of the thermohaline circulation. These "limbs" are linked by regions of overturning in the Nordic and Labrador Seas and the Southern Ocean. The AMOC is an important component of the Earth's climate system, and is a result of both atmospheric and thermohaline drivers.

Topographic map of the Nordic Seas and subpolar basins with schematic circulation of surface currents (solid curves) and deep currents (dashed curves) that form a portion of the Atlantic meridional overturning circulation. Colors of curves indicate approximate temperatures.

General

Northward surface flow transports a substantial amount of heat energy from the tropics and Southern Hemisphere toward the North Atlantic, where the heat is lost to the atmosphere due to the strong temperature gradient. Upon losing its heat, the water becomes denser and sinks. This densification links the warm, surface limb with the cold, deep return limb at regions of convection in the Nordic and Labrador Seas. The limbs are also linked in regions of upwelling, where Ekman pumping causes a divergence of surface waters and an upward flux of deep water.

AMOC consists of upper and lower cells. The upper cell consists of northward surface flow as well as southward return flow of North Atlantic Deep Water (NADW). The lower cell represents northward flow of dense Antarctic Bottom Water (AABW) – this bathes the abyssal ocean.[1]

AMOC exerts a major control on North Atlantic sea level, particularly along the Northeast Coast of North America. Exceptional AMOC weakening during the winter of 2009–10 has been implicated in a damaging 13 cm sea level rise along the New York coastline.[2]

AMOC and climate

The net northward heat transport in the Atlantic is unique among global oceans, and is responsible for the relative warmth of the Northern Hemisphere.[1] AMOC carries up to 25% of the northward global atmosphere-ocean heat transport in the northern hemisphere.[3] This is generally thought to ameliorate the climate of Northwest Europe, although this effect is the subject of debate.[4][5][6]

As well as acting as a heat pump and high-latitude heat sink,[7][8] AMOC is the largest carbon sink in the Northern Hemisphere, sequestering ∼0.7 PgC/year.[9] This sequestration has significant implications for evolution of anthropogenic global warming – especially with respect to the recent and projected future decline in AMOC vigour.

Recent decline

AMOC has undergone exceptional weakening in the last 150 years compared to the previous 1500 years,[10] as well as a weakening of around 15% since the mid-twentieth century.[11] Direct observations of the strength of the AMOC have only been available since 2004 from the in situ mooring array at 26°N in the Atlantic.[12] While climate models predict a weakening of AMOC under global warming scenarios, the magnitude of observed and reconstructed weakening is out of step with model predictions. Observed decline in the period 2004–2014 was a factor of 10 higher than that predicted by climate models participating in Phase 5 of the Coupled Model Intercomparison Project (CMIP5).[13][14] While observations of Labrador Sea outflow showed no negative trend from 1997–2009, this period is likely an atypical and weakened state.[15] As well as an underestimation of the magnitude of decline, grain size analysis has revealed a discrepancy in the modelled timing of AMOC decline after the Little Ice Age.[10]

Regions of overturning

Convection and return flow in the Nordic Seas

Low air temperatures at high latitudes cause substantial sea-air heat flux, driving a density increase and convection in the water column. Open-ocean convection occurs in deep plumes and is particularly strong in winter when the sea-air temperature difference is largest.[16] Of the 6 sverdrup (Sv) of dense water that flows southward over the GSR, 3 Sv does so via the Denmark Strait forming Denmark Strait Overflow Water (DSOW). 0.5-1 Sv flows over the Iceland-Faroe ridge and the remaining 2–2.5 Sv returns through the Faroe-Shetland Channel; these two flows form Iceland Scotland Overflow Water (ISOW). The majority of flow over the Faroe-Shetland ridge flows through the Faroe-Bank channel and soon joins that which flowed over the Iceland-Faroe ridge, to flow southward at depth along the Eastern flank of the Reykjanes Ridge. As ISOW overflows the GSR, it turbulently entrains intermediate density waters such as Sub-Polar Mode water and Labrador Sea Water. This grouping of water-masses then moves geostrophically southward along the East flank of Reykjanes Ridge, through the Charlie Gibbs Fracture Zone and then northward to join DSOW. These waters are sometimes referred to as Nordic Seas Overflow Water (NSOW). NSOW flows cyclonically following the surface route of the SPG around the Labrador Sea and further entrains LSW.

Convection is known to be suppressed at these high latitudes by sea-ice cover. Floating sea ice "caps" the surface, reducing the ability for heat to move from the sea to the air. This in turn reduces convection and deep return flow from the region. The summer Arctic sea ice cover has undergone dramatic retreat since satellite records began in 1979, amounting to a loss of almost 30% of the September ice cover in 39 years. Climate model simulations suggest that rapid and sustained September Arctic ice loss is likely in future 21st century climate projections.

Convection and entrainment in the Labrador Sea

Characteristically fresh LSW is formed at intermediate depths by deep convection in the central Labrador Sea, particularly during winter storms.[16] This convection is not deep enough to penetrate into the NSOW layer which forms the deep waters of the Labrador Sea. LSW joins NSOW to move southward out of the Labrador Sea: while NSOW easily passes under the NAC at the North-West Corner, some LSW is retained. This diversion and retention by the SPG explains its presence and entrainment near the GSR overflows. Most of the diverted LSW however splits off before the CGFZ and remains in the western SPG. LSW production is highly dependent on sea-air heat flux and yearly production typically ranges from 3–9 Sv.[17][18] ISOW is produced in proportion to the density gradient across the Iceland-Scotland Ridge and as such is sensitive to LSW production which affects the downstream density [19][20] More indirectly, increased LSW production is associated with a strengthened SPG and hypothesised to be anticorrelated with ISOW [21][22][23] This interplay confounds any simple extension of a reduction in individual overflow waters to a reduction in AMOC. LSW production is understood to have been minimal prior to the 8.2 ka event,[24] with the SPG thought to have existed before in a weakened, non-convective state.[25]

Atlantic upwelling

For reasons of conservation of mass, the global ocean system must upwell an equal volume of water to that downwelled. Upwelling in the Atlantic itself occurs mostly due to coastal and equatorial upwelling mechanisms.

Coastal upwelling occurs as a result of Ekman transport along the interface between land and a wind-driven current. In the Atlantic, this particularly occurs around the Canary Current and Benguela Current. Upwelling in these two regions has been modelled to be in antiphase, an effect known as "upwelling see-saw".[26]

Equatorial upwelling generally occurs due to atmospheric forcing and divergence due to the opposing direction of the Coriolis force either side of the equator. The Atlantic features more complex mechanisms such as migration of the thermocline, particularly in the Eastern Atlantic.[27]

Southern Ocean upwelling

North Atlantic Deep Water is primarily upwelled at the southern end of the Atlantic transect, in the Southern Ocean.[8] This upwelling comprises the majority of upwelling normally associated with AMOC, and links it with the global circulation.[1] On a global scale, observations suggest 80% of deepwater upwells in the Southern Ocean.[28]

This upwelling supplies large quantities of nutrients to the surface, which supports biological activity. Surface supply of nutrients is critical to the ocean's functioning as a carbon sink on long timescales. Furthermore, upwelled water has low concentrations of dissolved carbon, as the water is typically 1000 years old and has not been sensitive to anthropogenic CO2 increases in the atmosphere.[29] Because of its low carbon concentration, this upwelling functions as a carbon sink. Variability in the carbon sink over the observational period has been closely studied and debated.[30] The size of the sink is understood to have decreased until 2002, and then increased until 2012.[31]

After upwelling, the water is understood to take one of two pathways: water surfacing near to sea-ice generally forms dense bottomwater and is committed to AMOC's lower cell; water surfacing at lower latitudes moves further northward due to Ekman transport and is committed to the upper cell.[8][32]

AMOC stability

Atlantic overturning is not a static feature of global circulation, but rather a sensitive function of temperature and salinity distributions as well as atmospheric forcings. Paleoceanographic reconstructions of AMOC vigour and configuration have revealed significant variations over geologic time [33][34] complementing variation observed on shorter scales.[35][13]

Reconstructions of a “shutdown” or “Heinrich” mode of the North Atlantic have fuelled concerns of a future collapse of the overturning circulation due to global climate change. While this possibility is described by the IPCC as “unlikely” for the 21st century, a one-word verdict conceals significant debate and uncertainty about the prospect.[36] The physics of a shutdown would be underpinned by the Stommel Bifurcation, where increased freshwater forcing or warmer surface waters would lead to a sudden reduction in overturning from which the forcing must be substantially reduced before restart is possible.[37]

An AMOC shutdown would be fuelled by two positive feedbacks, the accumulation of both freshwater and heat in areas on downwelling. AMOC exports freshwater from the North Atlantic, and a reduction in overturning would freshen waters and inhibit downwelling.[38] Similar to its export of freshwater, AMOC also partitions heat in the deep-ocean in a global warming regime – it is possible that a weakened AMOC would lead to increasing global temperatures and further stratification and slowdown.[7] However, this effect would be tempered by a concomitant reduction in warm water transport to the North Atlantic under a weakened AMOC, a negative feedback on the system.

As well as paleoceanographic reconstruction, the mechanism and likelihood of collapse has been investigated using climate models. Earth Models of Intermediate Complexity (EMICs) have historically predicted a modern AMOC to have multiple equilibria, characterised as warm, cold and shutdown modes.[39] This is in contrast to more comprehensive models, which bias towards a stable AMOC characterised by a single equilibrium. However, doubt is cast upon this stability by a modelled northward freshwater flux which is at odds with observations.[13][40] An unphysical northward flux in models acts as a negative feedback on overturning and falsely-biases towards stability.[36]

To complicate the issue of positive and negative feedbacks on temperature and salinity, the wind-driven component of AMOC is still not fully constrained. A relatively larger role of atmospheric forcing would lead to less dependent on the thermohaline factors listed above, and would render AMOC less vulnerable to temperature and salinity changes under global warming.[41]

While a shutdown is deemed “unlikely” by the IPCC, a weakening over the 21st century is assessed as “very likely” and previous weakenings have been observed in several records. The cause of future weakening in models is a combination of surface freshening due to changing precipitation patterns in the North Atlantic and glacial melt, and greenhouse-gas induced warming from increased radiative forcing.

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See also

References
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