A dynamically based method for estimating the Atlantic overturning circulation at 26° N from satellite altimetry

Abstract. The large-scale system of ocean currents that transport warm surface (1000 m) waters northward and return cooler waters southward is known as the Atlantic meridional overturning circulation (AMOC). Variations in the AMOC have significant repercussions for the climate system, hence there is a need for long term monitoring of AMOC fluctuations. Currently the longest record of continuous directly measured AMOC changes is from the RAPID-MOCHA-WBTS programme, initiated in 2004. The RAPID programme, and other mooring programmes, have revolutionised our understanding of large-scale circulation, however, by design they are constrained to measurements at a single latitude. High global coverage of surface ocean data from satellite altimetry is available since the launch of TOPEX/Poseidon satellite in 1992 and has been shown to provide reliable estimates of surface ocean transports on interannual time scales. Here we show that a direct calculation of ocean circulation from satellite altimetry compares well with transport estimates from the 26° N RAPID array on low frequency (18-month) time scales for the upper mid-ocean transport (UMO; r = 0.75), the Gulf Stream transport through the Florida Straits (r = 0.70), and the AMOC (r = 0.83). The vertical structure of the circulation is also investigated, and it is found that the first baroclinic mode accounts for 83 % of the interior geostrophic variability, while remaining variability is explained by the barotropic mode. Finally, the UMO and the AMOC are estimated from historical altimetry data (1993 to 2018) using a dynamically based method that incorporates the vertical structure of the flow. The effective implementation of satellite-based method for monitoring the AMOC at 26° N lays down the starting point for monitoring large-scale circulation at all latitudes.

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A dynamically based method for estimating the Atlantic overturning circulation at 26°N from satellite altimetry

10.5194/egusphere-egu21-15886 ◽

2021 ◽

Author(s):

Alejandra Sanchez-Franks ◽

Eleanor Frajka-Williams ◽

Ben Moat ◽

David Smeed

Keyword(s):

Time Scales ◽

Ocean Circulation ◽

Satellite Altimetry ◽

Large Scale ◽

Vertical Structure ◽

Meridional Overturning Circulation ◽

Overturning Circulation ◽

Large Scale Circulation ◽

Surface Ocean ◽

Starting Point

<p>The large-scale system of ocean currents that transport warm surface (1000 m) waters northward and return cooler waters southward is known as the Atlantic meridional overturning circulation (AMOC). Variations in the AMOC have significant repercussions for the climate system, hence there is a need for long term monitoring of AMOC fluctuations. Currently the longest record of continuous directly measured AMOC changes is from the RAPID-MOCHA-WBTS programme, initiated in 2004. The RAPID programme, and other mooring programmes, have revolutionised our understanding of large-scale circulation, however, by design they are constrained to measurements at a single latitude.</p><p>High global coverage of surface ocean data from satellite altimetry is available since the launch of TOPEX/Poseidon satellite in 1992 and has been shown to provide reliable estimates of surface ocean transports on interannual time scales. Here we show that a direct calculation of ocean circulation from satellite altimetry compares well with transport estimates from the 26&#176;N RAPID array on low frequency (18-month) time scales for the upper mid-ocean transport (UMO; r = 0.75), the Gulf Stream transport through the Florida Straits (r = 0.70), and the AMOC (r = 0.83). The vertical structure of the circulation is also investigated, and it is found that the first baroclinic mode accounts for 83% of the interior geostrophic variability, while remaining variability is explained by the barotropic mode. Finally, the UMO and the AMOC are estimated from historical altimetry data (1993 to 2018) using a dynamically based method that incorporates the vertical structure of the flow. The effective implementation of satellite-based method for monitoring the AMOC at 26&#176;N lays down the starting point for monitoring large-scale circulation at all latitudes.</p>

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A dynamically based method for estimating the Atlantic meridional overturning circulation at 26° N from satellite altimetry

Ocean Science ◽

10.5194/os-17-1321-2021 ◽

2021 ◽

Vol 17 (5) ◽

pp. 1321-1340

Author(s):

Alejandra Sanchez-Franks ◽

Eleanor Frajka-Williams ◽

Ben I. Moat ◽

David A. Smeed

Keyword(s):

Satellite Altimetry ◽

Large Scale ◽

Vertical Structure ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

Baroclinic Mode ◽

Overturning Circulation ◽

Large Scale Circulation ◽

Surface Ocean ◽

Meridional Overturning

Abstract. The large-scale system of ocean currents that transport warm waters in the upper 1000 m northward and return deeper cooler waters southward is known as the Atlantic meridional overturning circulation (AMOC). Variations in the AMOC have significant repercussions for the climate system; hence, there is a need for long-term monitoring of AMOC fluctuations. Currently the longest record of continuous directly measured AMOC changes is from the RAPID-MOCHA-WBTS programme, initiated in 2004. The RAPID programme and other mooring programmes have revolutionised our understanding of large-scale circulation; however, by design they are constrained to measurements at a single latitude and cannot tell us anything pre-2004. Nearly global coverage of surface ocean data from satellite altimetry has been available since the launch of the TOPEX/Poseidon satellite in 1992 and has been shown to provide reliable estimates of surface ocean transports on interannual timescales including previous studies that have investigated empirical correlations between sea surface height variability and the overturning circulation. Here we show a direct calculation of ocean circulation from satellite altimetry of the upper mid-ocean transport (UMO), the Gulf Stream transport through the Florida Straits (GS), and the AMOC using a dynamically based method that combines geostrophy with a time mean of the vertical structure of the flow from the 26∘ N RAPID moorings. The satellite-based transport captures 56 %, 49 %, and 69 % of the UMO, GS, and AMOC transport variability, respectively, from the 26∘ N RAPID array on interannual (18-month) timescales. Further investigation into the vertical structure of the horizontal transport shows that the first baroclinic mode accounts for 83 % of the interior geostrophic variability, and the combined barotropic and first baroclinic mode representation of dynamic height accounts for 98 % of the variability. Finally, the methods developed here are used to reconstruct the UMO and the AMOC for the time period pre-dating RAPID, 1993 to 2003. The effective implementation of satellite-based method for monitoring the AMOC at 26∘ N lays down the starting point for monitoring large-scale circulation at all latitudes.

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Momentum Balance of the Wind-Driven and Meridional Overturning Circulation

Journal of Physical Oceanography ◽

10.1175/2011jpo4528.1 ◽

2011 ◽

Vol 41 (5) ◽

pp. 960-978 ◽

Cited By ~ 3

Author(s):

David P. Marshall ◽

Helen R. Pillar

Keyword(s):

Ocean Circulation ◽

Large Scale ◽

Boundary Layer Separation ◽

Momentum Equation ◽

Meridional Overturning Circulation ◽

Momentum Balance ◽

Overturning Circulation ◽

Potential Applications ◽

Rotational Momentum ◽

Meridional Overturning

Abstract When a force is applied to the ocean, fluid parcels are accelerated both locally, by the applied force, and nonlocally, by the pressure gradient forces established to maintain continuity and satisfy the kinematic boundary condition. The net acceleration can be represented through a “rotational force” in the rotational component of the momentum equation. This approach elucidates the correspondence between momentum and vorticity descriptions of the large-scale ocean circulation: if two terms balance pointwise in the rotational momentum equation, then the equivalent two terms balance pointwise in the vorticity equation. The utility of the approach is illustrated for three classical problems: barotropic Rossby waves, wind-driven circulation in a homogeneous basin, and the meridional overturning circulation in an interhemispheric basin. In the hydrostatic limit, it is shown that the rotational forces further decompose into depth-integrated forces that drive the wind-driven gyres and overturning forces that are confined to the basin boundaries and drive the overturning circulation. Potential applications of the approach to diagnosing the output of ocean circulation models, alternative and more accurate formulations of numerical ocean models, the dynamics of boundary layer separation, and eddy forcing of the large-scale ocean circulation are discussed.

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Pending recovery in the strength of the meridional overturning circulation at 26° N

10.5194/os-2019-134 ◽

2020 ◽

Author(s):

Ben I. Moat ◽

David A. Smeed ◽

Eleanor Frajka-Williams ◽

Damien G. Desbruyères ◽

Claudie Beaulieu ◽

...

Keyword(s):

Ocean Circulation ◽

North Atlantic ◽

Large Scale ◽

Meridional Overturning Circulation ◽

Overturning Circulation ◽

Ocean Reanalysis ◽

Buoyancy Forcing ◽

Modelling Studies ◽

Meridional Overturning ◽

Lower Magnitude

Abstract. The strength of the Atlantic meridional overturning circulation (AMOC) at 26° N has now been continuously measured by the RAPID array over the period Apr 2004–Sept 2018. This record provides unique insight into the variability of the large-scale ocean circulation, previously only measured by sporadic snapshots of basin-wide transports from hydrographic sections. The continuous measurements have unveiled striking variability on timescales of days to a decade, driven largely by wind-forcing, contrasting with previous expectations about a slowly-varying, buoyancy forced large-scale ocean circulation. However, these measurements were primarily observed during a warm state of the Atlantic Multidecadal Variability (AMV) which has been steadily declining since a peak in 2008–2010. In 2013–2015, a period of strong buoyancy-forcing by the atmosphere drove intense watermass transformation in the subpolar North Atlantic and provides a unique opportunity to investigate the response of the large-scale ocean circulation to buoyancy forcing. Modelling studies suggest that the AMOC in the subtropics responds to such events with an increase in overturning transport, after a lag of 3–9 years. At 45° N, observations suggest that the AMOC my already be increasing. We have therefore examined the record of transports at 26° N to see whether the AMOC in the subtropical North Atlantic is now recovering from a previously reported low period commencing in 2009. Comparing the two latitudes, the AMOC at 26° N is higher than its previous low. Extending the record at 26° N with ocean reanalysis from GloSea5, the transport fluctuations follow those at 45° N by 0–2 years, albeit with lower magnitude. Given the short span of time and anticipated delays in the signal from the subpolar to subtropical gyres, it is not yet possible to determine whether the subtropical AMOC strength is recovering.

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Latitudinal Structure of the Meridional Overturning Circulation Variability on Interannual to Decadal Time Scales in the North Atlantic Ocean

Journal of Climate ◽

10.1175/jcli-d-19-0215.1 ◽

2020 ◽

Vol 33 (9) ◽

pp. 3845-3862 ◽

Cited By ~ 2

Author(s):

Sijia Zou ◽

M. Susan Lozier ◽

Xiaobiao Xu

Keyword(s):

Time Scales ◽

Ocean Circulation ◽

North Atlantic ◽

Meridional Overturning Circulation ◽

Ekman Transport ◽

Overturning Circulation ◽

Coherent Component ◽

The North ◽

Meridional Overturning ◽

The North Atlantic

AbstractThe latitudinal structure of the Atlantic meridional overturning circulation (AMOC) variability in the North Atlantic is investigated using numerical results from three ocean circulation simulations over the past four to five decades. We show that AMOC variability south of the Labrador Sea (53°N) to 25°N can be decomposed into a latitudinally coherent component and a gyre-opposing component. The latitudinally coherent component contains both decadal and interannual variabilities. The coherent decadal AMOC variability originates in the subpolar region and is reflected by the zonal density gradient in that basin. It is further shown to be linked to persistent North Atlantic Oscillation (NAO) conditions in all three models. The interannual AMOC variability contained in the latitudinally coherent component is shown to be driven by westerlies in the transition region between the subpolar and the subtropical gyre (40°–50°N), through significant responses in Ekman transport. Finally, the gyre-opposing component principally varies on interannual time scales and responds to local wind variability related to the annual NAO. The contribution of these components to the total AMOC variability is latitude-dependent: 1) in the subpolar region, all models show that the latitudinally coherent component dominates AMOC variability on interannual to decadal time scales, with little contribution from the gyre-opposing component, and 2) in the subtropical region, the gyre-opposing component explains a majority of the interannual AMOC variability in two models, while in the other model, the contributions from the coherent and the gyre-opposing components are comparable. These results provide a quantitative decomposition of AMOC variability across latitudes and shed light on the linkage between different AMOC variability components and atmospheric forcing mechanisms.

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Pending recovery in the strength of the meridional overturning circulation at 26° N

Ocean Science ◽

10.5194/os-16-863-2020 ◽

2020 ◽

Vol 16 (4) ◽

pp. 863-874 ◽

Cited By ~ 2

Author(s):

Ben I. Moat ◽

David A. Smeed ◽

Eleanor Frajka-Williams ◽

Damien G. Desbruyères ◽

Claudie Beaulieu ◽

...

Keyword(s):

Ocean Circulation ◽

Large Scale ◽

Meridional Overturning Circulation ◽

Overturning Circulation ◽

Ocean Reanalysis ◽

Buoyancy Forcing ◽

Modelling Studies ◽

Meridional Overturning ◽

Lower Magnitude

Abstract. The strength of the Atlantic meridional overturning circulation (AMOC) at 26∘ N has now been continuously measured by the RAPID array over the period April 2004–September 2018. This record provides unique insight into the variability of the large-scale ocean circulation, previously only measured by sporadic snapshots of basin-wide transport from hydrographic sections. The continuous measurements have unveiled striking variability on timescales of days to a decade, driven largely by wind forcing, contrasting with previous expectations about a slowly varying buoyancy-forced large-scale ocean circulation. However, these measurements were primarily observed during a warm state of the Atlantic multidecadal variability (AMV) which has been steadily declining since a peak in 2008–2010. In 2013–2015, a period of strong buoyancy forcing by the atmosphere drove intense water-mass transformation in the subpolar North Atlantic and provides a unique opportunity to investigate the response of the large-scale ocean circulation to buoyancy forcing. Modelling studies suggest that the AMOC in the subtropics responds to such events with an increase in overturning transport, after a lag of 3–9 years. At 45∘ N, observations suggest that the AMOC may already be increasing. Examining 26∘ N, we find that the AMOC is no longer weakening, though the recent transport is not above the long-term mean. Extending the record backwards in time at 26∘ N with ocean reanalysis from GloSea5, the transport fluctuations at 26∘ N are consistent with a 0- to 2-year lag from those at 45∘ N, albeit with lower magnitude. Given the short span of time and anticipated delays in the signal from the subpolar to subtropical gyres, it is not yet possible to determine whether the subtropical AMOC strength is recovering nor how the AMOC at 26∘ N responds to intense buoyancy forcing.

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Structure and Forcing of Observed Exchanges across the Greenland–Scotland Ridge

Journal of Climate ◽

10.1175/jcli-d-17-0889.1 ◽

2018 ◽

Vol 31 (24) ◽

pp. 9881-9901 ◽

Cited By ~ 10

Author(s):

Carina Bringedal ◽

Tor Eldevik ◽

Øystein Skagseth ◽

Michael A. Spall ◽

Svein Østerhus

Keyword(s):

Time Scales ◽

Large Scale ◽

Atlantic Water ◽

Cyclonic Circulation ◽

Water Masses ◽

Meridional Overturning Circulation ◽

Wind Forcing ◽

Overturning Circulation ◽

Poleward Heat Transport ◽

Denmark Strait

The Atlantic meridional overturning circulation and associated poleward heat transport are balanced by northern heat loss to the atmosphere and corresponding water-mass transformation. The circulation of northward-flowing Atlantic Water at the surface and returning overflow water at depth is particularly manifested—and observed—at the Greenland–Scotland Ridge where the water masses are guided through narrow straits. There is, however, a rich variability in the exchange of water masses across the ridge on all time scales. Focusing on seasonal and interannual time scales, and particularly the gateways of the Denmark Strait and between the Faroe Islands and Shetland, we specifically assess to what extent the exchanges of water masses across the Greenland–Scotland Ridge relate to wind forcing. On seasonal time scales, the variance explained of the observed exchanges can largely be related to large-scale wind patterns, and a conceptual model shows how this wind forcing can manifest via a barotropic, cyclonic circulation. On interannual time scales, the wind stress impact is less direct as baroclinic mechanisms gain importance and observations indicate a shift in the overflows from being more barotropically to more baroclinically forced during the observation period. Overall, the observed Greenland–Scotland Ridge exchanges reflect a horizontal (cyclonic) circulation on seasonal time scales, while the interannual variability more represents an overturning circulation.

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Can satellites replace mooring arrays? Satellite altimetry transport estimates of the Atlantic overturning meridional circulation along the RAPID 26°N mooring array

10.5194/egusphere-egu2020-11549 ◽

2020 ◽

Author(s):

Alejandra Sanchez-Franks ◽

Eleanor Frajka-Williams ◽

Ben Moat

Keyword(s):

Surface Layer ◽

Ocean Circulation ◽

Satellite Altimetry ◽

Large Scale ◽

Function Analysis ◽

Spatial Scales ◽

Meridional Overturning Circulation ◽

Return Flow ◽

Western Boundary ◽

Oceanic Circulation

<p>The Atlantic meridional overturning circulation (AMOC) is a large-scale oceanic circulation comprising a 2-layer flow: the net northward flow in the upper 1000 m of the Atlantic and net southward flow below. Variations in the AMOC have significant repercussions for the climate system hence there is a need for proxies that can measure changes in the AMOC on larger spatial scales. Here we show a direct calculation of ocean circulation at 26&#176;N from satellites compares well with transport estimates from the RAPID mooring array. In the surface layer (1000 m), transport is estimated from satellite altimetry and has a correlation of r=0.79 (significant at 95% level) with the MOC transport estimates from RAPID. We find that the relationship between sea level anomaly and dynamic height from the western boundary RAPID moorings is robust in the surface layer, with poor agreement occurring largely below 1000 m. Below 1000 m, the return flow of the AMOC is estimated using ocean bottom pressure from satellite gravimetry. This has a correlation of r=0.75 (significant at the 95% level) when compared to the deeper (1000-5000 m) RAPID transports. Combining the results from satellite altimetry and gravimetry, estimates of full-depth 2-layer circulation at 26&#176;N are demonstrated. Finally, empirical orthogonal function analysis reveals that the barotropic and baroclinic streamfunctions are linked to wind stress curl and buoyancy forcing, respectively.</p>

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Pending recovery in the strength of the meridional overturning circulation at 26°N

10.5194/egusphere-egu2020-5785 ◽

2020 ◽

Author(s):

Ben Moat ◽

David Smeed ◽

Eleanor Frajka-Williams ◽

Damien Desbruyeres ◽

Claudie Beaulieu ◽

...

Keyword(s):

Ocean Circulation ◽

North Atlantic ◽

Large Scale ◽

Meridional Overturning Circulation ◽

Overturning Circulation ◽

Ocean Reanalysis ◽

Buoyancy Forcing ◽

Modelling Studies ◽

Meridional Overturning ◽

Lower Magnitude

<div> <div> <div> <p>The strength of the Atlantic meridional overturning circulation (AMOC) at 26&#176;N has now been continuously measured by the RAPID array over the period April 2004 - Sept 2018. This record provides unique insight into the variability of the large-scale ocean circulation, previously only measured by sporadic snapshots of basin-wide transports from hydrographic sections. The continuous measurements have unveiled striking variability on timescales of days to a decade, driven largely by wind-forcing, contrasting with previous expectations about a slowly-varying, buoyancy forced large-scale ocean circulation. However, these measurements were primarily observed during a warm state of the Atlantic Multidecadal Variability (AMV) which has been steadily declining since a peak in 2008-2010. In 2013-2015, a period of strong buoyancy- forcing by the atmosphere drove intense watermass transformation in the subpolar North Atlantic and provides a unique opportunity to investigate the response of the large-scale ocean circulation to buoyancy forcing.</p> <p>Modelling studies suggest that the AMOC in the subtropics responds to such events with an increase in overturning transport, after a lag of 3-9 years. At 45&#176;N, observations suggest that the AMOC my already be increasing. We have therefore examined the record of transports at 26&#176;N to see whether the AMOC in the subtropical North Atlantic is now recovering from a previously reported low period commencing in 2009. Comparing the two latitudes, the AMOC at 26&#176;N is higher than its previous low. Extending the record at 26&#176;N with ocean reanalysis from GloSea5, the transport fluctuations follow those at 45&#176;N by 0-2 years, albeit with lower magnitude. Given the short span of time and anticipated delays in the signal from the subpolar to subtropical gyres, it is not yet possible to determine whether the subtropical AMOC strength is recovering.</p> </div> </div> </div>

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Labrador Sea sub-surface density as a precursor of multi-decadal variability in the North Atlantic: a multi-model study

10.5194/esd-2020-83 ◽

2020 ◽

Author(s):

Pablo Ortega ◽

Jon I. Robson ◽

Matthew Menary ◽

Rowan T. Sutton ◽

Adam Blaker ◽

...

Keyword(s):

Ocean Circulation ◽

North Atlantic ◽

Large Scale ◽

Climate Model ◽

Decadal Variability ◽

Meridional Overturning Circulation ◽

Labrador Sea ◽

Western Boundary ◽

Overturning Circulation ◽

Boundary Density

Abstract. The Subpolar North Atlantic (SPNA) is a region with prominent decadal variability that has experienced remarkable warming and cooling trends in the last few decades. These observed trends have been preceded by slow-paced increases and decreases in the Labrador Sea density (LSD), which are thought to be a precursor of large scale ocean circulation changes. This article analyses the inter-relationships between the LSD and the wider North Atlantic across an ensemble of coupled climate model simulations. In particular, it analyses the link between subsurface density and the deep boundary density, the Atlantic Meridional Overturning Circulation (AMOC), the Subpolar Gyre (SPG) circulation, and the upper ocean temperature in the eastern SPNA. All simulations exhibit considerable multidecadal variability in the LSD and the ocean circulation indices, which are found to be interrelated. LSD is strongly linked with the strength of subpolar AMOC and gyre circulation, and is also linked with the subtropical AMOC, although the strength of this relationship is model dependent. The connectivity of LSD with the subtropics is found to be sensitive to different model features, including: the mean density stratification in the Labrador Sea; the strength and depth of the AMOC; and the depth at which the LSD propagates southward along the western boundary. Several of these quantities can also be computed from observations, and comparison with these observation-based quantities suggests that models representing a weaker link with the subtropical AMOC may be more realistic. This would imply that RAPID AMOC measurements might not be adequate to represent decadal to multidecadal changes in the subpolar overturning circulation.

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