Upper ocean manifestations of a reducing meridional overturning circulation
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Most climate models predict a slowing down of the Atlantic Meridional Overturning Circulation during the 21st century. Using a 100 year climate change integration of a high resolution coupled climate model, we show that a 5.3 Sv reduction in the deep southward transport in the subtropical North Atlantic is balanced solely by a weakening of the northward surface western boundary current, and not by an increase in the southward transport integrated across the interior ocean away from the western boundary. This is consistent with Sverdrup balance holding to a good approximation outside of the western boundary region on decadal time scales, and may help to spatially constrain past and future change in the overturning circulation. The subtropical gyre weakens by 3.4 Sv over the same period due to a weakened wind stress curl. These changes combine to give a net 8.7 Sv reduction in upper western boundary transport.Keywords:
Boundary current
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Gulf Stream
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Abstract. The circulation in the Atlantic Ocean is marked by the complex system of pathways of the Atlantic Meridional Overturning Circulation (AMOC). These currents change meridionally due to the interaction with nearby water masses. Hydrographic data provide the opportunity to characterize these currents for the whole water column with high-resolution data over the last 30 years. Moreover, inverse methods enable the quantification of absolute zonal transports across these sections, determining the strength of each current at a certain latitude in terms of mass, heat, and freshwater, as well as their transport-weighted temperature and salinity. Generally, no changes can be found among decades for each of the currents in terms of transport or their properties. In the South Atlantic, the circulation describes the subtropical gyre affected by several recirculations. There are nearly 61 Sv entering from the Southern and Indian oceans at 45∘ S. The South Atlantic subtropical gyre exports 17.0 ± 1.2 Sv and around 1 PW northward via the North Brazil Current, as well as −55 Sv southward at 45∘ S into the Antarctic Circumpolar Current. In the North Atlantic, most of the transport is advected northward via the western boundary currents, which reduce their strength as they take part in convection processes in the subpolar North Atlantic, also reflected in the northward progress of mass and heat transport. Deep layers carry waters southward along the western boundary, maintaining similar values of mass and heat transport until the separation into an eastern branch crossing the mid-Atlantic Ridge in the South Atlantic. Abyssal waters originating in the Southern Ocean are distributed along the South Atlantic mainly through its western subbasin, flowing northward up to 24.5∘ N, subjected to an increasing trend in their temperature with time.
Boundary current
Gulf Stream
Antarctic Intermediate Water
Mid-Atlantic Ridge
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Abstract. The circulation in the Atlantic Ocean is marked by the complex system of pathways of the Atlantic Meridional Overturning Circulation (AMOC). These currents change meridionally due to the interaction with nearby water masses. Hydrographic data provide the opportunity to characterize these currents for the whole water column with high-resolution data over the last thirty years. Moreover, inverse methods enable the quantification of absolute zonal transports across these sections, determining the strength of each current at a certain latitude in terms of mass, heat and freshwater, as well as their transport-weighted temperature and salinity. Generally, no changes can be found among decades for each of the currents in terms of transport or their properties. In the South Atlantic, the circulation describes the subtropical gyre affected by several recirculations. There are nearly 61 Sv entering from the Southern and Indian Oceans at 45° S. The South Atlantic subtropical gyre exports northward 17.0 ± 1.2 Sv and around 1 PW via the North Brazil Current and −55 Sv southward at 45° S into the Antarctic Circumpolar Current. In the north Atlantic, most of the transport is advected northward via the western boundary currents, which reduce in strength as they take part in convection processes in the subpolar North Atlantic, reflected also in the northward progress of mass and heat transport. Deep layers carry waters southward along the western boundary, maintaining similar values of mass and heat transport until the separation into an eastern branch crossing the mid-Atlantic ridge in the south Atlantic. Abyssal waters originating in the Southern Ocean distribute along the South Atlantic mainly through its western subbasin, flowing northward up to 24.5° N, subjected to an increasing trend in their temperature with time.
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Antarctic Intermediate Water
Gulf Stream
Circumpolar star
Temperature salinity diagrams
Circulation (fluid dynamics)
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Abstract. The circulation in the Atlantic Ocean is marked by the complex system of pathways of the Atlantic Meridional Overturning Circulation (AMOC). These currents change meridionally due to the interaction with nearby water masses. Hydrographic data provide the opportunity to characterize these currents for the whole water column with high-resolution data over the last thirty years. Moreover, inverse methods enable the quantification of absolute zonal transports across these sections, determining the strength of each current at a certain latitude in terms of mass, heat and freshwater, as well as their transport-weighted temperature and salinity. Generally, no changes can be found among decades for each of the currents in terms of transport or their properties. In the South Atlantic, the circulation describes the subtropical gyre affected by several recirculations. There are nearly 61 Sv entering from the Southern and Indian Oceans at 45° S. The South Atlantic subtropical gyre exports northward 17.0 ± 1.2 Sv and around 1 PW via the North Brazil Current and −55 Sv southward at 45° S into the Antarctic Circumpolar Current. In the north Atlantic, most of the transport is advected northward via the western boundary currents, which reduce in strength as they take part in convection processes in the subpolar North Atlantic, reflected also in the northward progress of mass and heat transport. Deep layers carry waters southward along the western boundary, maintaining similar values of mass and heat transport until the separation into an eastern branch crossing the mid-Atlantic ridge in the south Atlantic. Abyssal waters originating in the Southern Ocean distribute along the South Atlantic mainly through its western subbasin, flowing northward up to 24.5° N, subjected to an increasing trend in their temperature with time.
Boundary current
Antarctic Intermediate Water
Gulf Stream
Temperature salinity diagrams
Circumpolar deep water
Circumpolar star
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This chapter contains sections titled: Introduction The Mean Circulation of the Subpolar North Atlantic Estimates of the Mean Meridional Overturning Circulation Labrador Sea Convection Variability Water Mass and Circulation Variability of the Subpolar Gyre MOC Variability at the Exit of the Subpolar North Atlantic Summary and Outlook Appendix List of Acronyms
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Walker circulation
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Abstract. The circulation in the Atlantic Ocean is marked by the complex system of pathways of the Atlantic Meridional Overturning Circulation (AMOC). These currents change meridionally due to the interaction with nearby water masses. Hydrographic data provide the opportunity to characterize these currents for the whole water column with high-resolution data over the last thirty years. Moreover, inverse methods enable the quantification of absolute zonal transports across these sections, determining the strength of each current at a certain latitude in terms of mass, heat and freshwater, as well as their transport-weighted temperature and salinity. Generally, no changes can be found among decades for each of the currents in terms of transport or their properties. In the South Atlantic, the circulation describes the subtropical gyre affected by several recirculations. There are nearly 61 Sv entering from the Southern and Indian Oceans at 45° S. The South Atlantic subtropical gyre exports northward 17.0 ± 1.2 Sv and around 1 PW via the North Brazil Current and −55 Sv southward at 45° S into the Antarctic Circumpolar Current. In the north Atlantic, most of the transport is advected northward via the western boundary currents, which reduce in strength as they take part in convection processes in the subpolar North Atlantic, reflected also in the northward progress of mass and heat transport. Deep layers carry waters southward along the western boundary, maintaining similar values of mass and heat transport until the separation into an eastern branch crossing the mid-Atlantic ridge in the south Atlantic. Abyssal waters originating in the Southern Ocean distribute along the South Atlantic mainly through its western subbasin, flowing northward up to 24.5° N, subjected to an increasing trend in their temperature with time.
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Gulf Stream
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Abstract In an attempt to elucidate the role of atmospheric and oceanic processes in setting a vigorous ocean overturning circulation in the North Atlantic but not in the North Pacific, a comparison of the observed atmospheric circulation and net surface freshwater fluxes over the North Atlantic and Pacific basins is conducted. It is proposed that the more erratic meridional displacements of the atmospheric jet stream over the North Atlantic sector is instrumental in maintaining high surface salinities in its subpolar gyre. In addition, it is suggested that the spatial pattern of the net freshwater flux at the sea surface favors higher subpolar Atlantic salinity, because the geographical line separating net precipitation from net evaporation is found well south of the time-mean gyre separation in the North Pacific, whereas the two lines tend to coincide in the North Atlantic. Numerical experiments with an idealized two-gyre system confirm that these differences impact the salinity budget of the subpolar gyre. Further analysis of a coupled climate model in which the Atlantic meridional overturning cell has been artificially weakened suggests that the more erratic jet fluctuations in the Atlantic and the shift of the zero [net evaporation minus precipitation (E − P)] line are likely explained by features independent of the state of the thermohaline circulation. It is thus proposed that the atmospheric circulation helps “locking” high surface salinities and an active coupling between upper and deep ocean layers in the North Atlantic rather than in the North Pacific basin.
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Atmospheric Circulation
Gulf Stream
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Abstract. The circulation in the Atlantic Ocean is marked by the complex system of pathways of the Atlantic Meridional Overturning Circulation (AMOC). These currents change meridionally due to the interaction with nearby water masses. Hydrographic data provide the opportunity to characterize these currents for the whole water column with high-resolution data over the last thirty years. Moreover, inverse methods enable the quantification of absolute zonal transports across these sections, determining the strength of each current at a certain latitude in terms of mass, heat and freshwater, as well as their transport-weighted temperature and salinity. Generally, no changes can be found among decades for each of the currents in terms of transport or their properties. In the South Atlantic, the circulation describes the subtropical gyre affected by several recirculations. There are nearly 61 Sv entering from the Southern and Indian Oceans at 45° S. The South Atlantic subtropical gyre exports northward 17.0 ± 1.2 Sv and around 1 PW via the North Brazil Current and −55 Sv southward at 45° S into the Antarctic Circumpolar Current. In the north Atlantic, most of the transport is advected northward via the western boundary currents, which reduce in strength as they take part in convection processes in the subpolar North Atlantic, reflected also in the northward progress of mass and heat transport. Deep layers carry waters southward along the western boundary, maintaining similar values of mass and heat transport until the separation into an eastern branch crossing the mid-Atlantic ridge in the south Atlantic. Abyssal waters originating in the Southern Ocean distribute along the South Atlantic mainly through its western subbasin, flowing northward up to 24.5° N, subjected to an increasing trend in their temperature with time.
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Gulf Stream
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<p>The Antilles Current is a narrow, northward flowing boundary current in the western Atlantic just east of the Bahamas. Its role in the larger scale circulation has been debated: alternately thought to be part of the western boundary closure of the gyre circulation or the northward flowing limb of the meridional overturning circulation (MOC). From 19 years of moored current meter observations (1987--1991, 2004--2018), we define the strength of the Antilles Current by the net transport between the Bahamas and 76.5&#176;W (spanning about 45 km zonally) and in the thermocline (0&#8211;1000 m). We find a mean northward transport of 3.5 Sv, substantial interannual variability, and no discernable trend since 1987. The interannual variability of the AC transport is independent of the variability of the Florida Current (the Gulf Stream through the Florida Straits). Instead, the Antilles Current contributes to the interannual variability of the MOC at 26&#176;N, while the trend in the strength of the gyre circulation (defined as the transbasin thermocline transport minus the AC) is responsible for the trend in the MOC. In particular, the 2009/10 slowdown of the MOC resulted from a weaker northward AC transport, rather than an intensified gyre transport. Using the recent 14 years of in situ transport records, we compare the interannual variability of the gyre circulation to that of wind stress curl forcing via a Sverdrup transport calculation, identifying a potential role for wind stress curl (WSC) forcing at 26&#176;N with a ~2 year lag until 2016.<span>&#160; </span>From 2016, the predicted gyre circulation using WSC diverges from the measured gyre strength.</p>
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Ekman transport
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