Atlantic meridional overturning circulation
![Schematic of the thermohaline circulation][float-right] The Atlantic meridional overturning circulation (AMOC) constitutes the primary mechanism of large-scale, density-driven ocean circulation in the Atlantic basin, featuring northward advection of warm, saline surface waters—primarily via the Gulf Stream and North Atlantic Current—and their southward return as cold, dense North Atlantic Deep Water formed through convection in the subpolar gyre.[1][2][3] This system transports approximately 15–20 Sverdrups of water and over 1 petawatt of heat poleward, exerting a profound influence on hemispheric climate by moderating temperatures in northwestern Europe and modulating global precipitation patterns through its interaction with atmospheric circulation.[4][5] Direct measurements from the RAPID-MOCHA array at 26.5°N latitude reveal a multi-decadal weakening trend of about 3–4 Sverdrups since 2004, attributed to reduced deep water formation amid freshwater inputs from Arctic melting and increased precipitation, though this decline appears to have stabilized since the early 2010s.[6][4] Paleoclimate proxies indicate past AMOC collapses during Heinrich events and Dansgaard-Oeschger cycles, linked to massive ice discharges disrupting Nordic Sea convection, underscoring its potential for abrupt shifts under sufficient freshwater forcing.[7][8] Contemporary modeling ensembles, however, suggest substantial resilience to projected greenhouse gas emissions and even extreme freshwater perturbations, with no consensus on an imminent tipping point despite variability in sensitivity across simulations.[9][10] The AMOC's stability hinges on the balance between thermal and haline buoyancy gradients, where ongoing anthropogenic warming may counteract salinity-driven weakening through enhanced subtropical evaporation, though empirical monitoring remains essential to resolve discrepancies between observations and projections.[11][6]Definition and Components
Physical Structure and Mechanisms
The Atlantic Meridional Overturning Circulation (AMOC) comprises a northward flux of warm, saline waters in the upper ocean layers, balanced by a southward return of colder, denser deep waters. This structure is evident in meridional sections where the overturning streamfunction peaks at approximately 15–20 Sverdrups (Sv), with the upper branch occupying depths shallower than about 1000 meters and the lower branch extending to abyssal depths.[12][13] Key components include the northward-flowing Gulf Stream and North Atlantic Current in the upper limb, which transport heat and salt poleward, and the formation of North Atlantic Deep Water (NADW) through deep convection in the Nordic Seas and Labrador Sea. NADW, characterized by temperatures around 2–4°C and salinities of 34.9–35.0, constitutes the primary southward branch, while a shallower return flow of Antarctic Bottom Water (AABW) occupies the deep western boundary. The density-driven nature arises from cooling and increased salinity in the subpolar North Atlantic, where winter convection reaches depths of 2000 meters or more, enabling water mass transformation.[14][15][16] Mechanisms sustaining the AMOC involve buoyancy forcing from surface heat loss and evaporation minus precipitation, creating meridional density gradients that drive the sinking of dense waters. Unlike wind-driven gyres, the overturning is primarily thermohaline, with density differences (Δρ/ρ ≈ 10^{-3}) generating geostrophic flows that compensate the meridional pressure gradients. Observational estimates from programs like RAPID confirm the buoyancy component dominates the total overturning, with wind contributions modulating but not overturning the circulation.[17][18][14] The interplay of advection and diffusion maintains the required salinity contrast, as freshwater inputs in the south must be balanced by export northward; disruptions in this salt-advection feedback can alter stability, though empirical data indicate robustness under current forcings. Vertical shear in the flow arises from thermal wind balance, with warmer surface waters sloping equatorward in the deep return path.[15][8]Key Subsurface and Surface Elements
The surface branch of the Atlantic Meridional Overturning Circulation (AMOC) features northward transport of warm, saline water from subtropical to subpolar latitudes, primarily via the Gulf Stream and North Atlantic Current, with peak intensities of 13–20 Sverdrups (Sv).[15] The Gulf Stream separates from the North American coast at Cape Hatteras, continuing as the North Atlantic Current eastward of the Grand Banks.[15] This upper-ocean flow, extending to depths of roughly 1000 m, is driven by westerly winds inducing Ekman transport and buoyancy gradients from air-sea heat and freshwater fluxes.[18] In the subpolar North Atlantic, wintertime cooling and evaporation salinize surface waters, increasing density and triggering open-ocean convection that penetrates to depths exceeding 1500 m.[18] The subsurface branch comprises the southward return of cold, dense North Atlantic Deep Water (NADW) in the deep western boundary current, at rates of 13–17 Sv below 2000 m.[15] NADW formation totals 15–18 Sv, augmented by entrainment, with primary sources in the Nordic Seas and Labrador Sea.[18] In the Nordic Seas, deep convection reaches 3000 m, followed by dense overflows across the Greenland-Scotland Ridge: Denmark Strait Overflow Water (2.4–2.9 Sv) and Faroe Bank Channel overflow (2.4–2.7 Sv), contributing to lower NADW.[18] The Labrador Sea generates Labrador Sea Water (LSW) through convection to 1500–2200 m, yielding 2–4 Sv of relatively fresh (salinity <34.88), cold water that forms upper NADW after mixing with overflow components like Iceland-Scotland and Denmark Strait waters.[19][18] LSW acts as a salinity minimum at intermediate depths, ventilating the North Atlantic and influencing NADW properties through entrainment and spreading.[19] The deep southward flow balances surface convergence via diapycnal mixing and wind-driven upwelling, predominantly in the Southern Ocean.[18]