Why Does the Deep Western Boundary Current “Leak” around Flemish Cap?

AbstractThe southward-flowing deep limb of the Atlantic meridional overturning circulation is composed of both the deep western boundary current (DWBC) and interior pathways. The latter are fed by “leakiness” from the DWBC in the Newfoundland Basin. However, the cause of this leakiness has not yet been explored mechanistically. Here the statistics and dynamics of the DWBC leakiness in the Newfoundland Basin are explored using two float datasets and a high-resolution numerical model. The float leakiness around Flemish Cap is found to be concentrated in several areas (hot spots) that are collocated with bathymetric curvature and steepening. Numerical particle advection experiments reveal that the Lagrangian mean velocity is offshore at these hot spots, while Lagrangian variability is minimal locally. Furthermore, model Eulerian mean streamlines separate from the DWBC to the interior at the leakiness hot spots. This suggests that the leakiness of Lagrangian particles is primarily accomplished by an Eulerian mean flow across isobaths, though eddies serve to transfer around 50% of the Lagrangian particles to the leakiness hot spots via chaotic advection, and rectified eddy transport accounts for around 50% of the offshore flow along the southern face of Flemish Cap. Analysis of the model’s energy and potential vorticity budgets suggests that the flow is baroclinically unstable after separation, but that the resulting eddies induce modest modifications of the mean potential vorticity along streamlines. These results suggest that mean uncompensated leakiness occurs mostly through inertial separation, for which a scaling analysis is presented. Implications for leakiness of other major boundary current systems are discussed.

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Characteristics and causes of Deep Western Boundary Current transport variability at 34.5° S during 2009–2014

10.5194/os-2016-76 ◽

2016 ◽

Author(s):

Christopher S. Meinen ◽

Silvia L. Garzoli ◽

Renellys C. Perez ◽

Edmo Campos ◽

Alberto R. Piola ◽

...

Keyword(s):

Volume Transport ◽

Horizontal Resolution ◽

Western Boundary Current ◽

Meridional Overturning Circulation ◽

Time Variability ◽

Western Boundary ◽

Boundary Current ◽

Meridional Heat Transport ◽

Data Set ◽

Deep Western Boundary Current

Abstract. The Deep Western Boundary Current (DWBC) at 34.5° S in the South Atlantic carries a significant fraction of the cold deep limb of the Meridional Overturning Circulation (MOC), and therefore its variability affects both the meridional heat transport and the regional and global climate. Nearly six years of observations from a line of pressure-equipped inverted echo sounders (PIES) have yielded an unprecedented data set for studying the characteristics of the time-varying DWBC volume transport at 34.5° S. Furthermore, the horizontal resolution of the observing array was greatly improved in December 2012 with the addition of two current-and-pressure-equipped inverted echo sounders (CPIES) at the midpoints of three of the existing sites. Regular hydrographic sections along the PIES/CPIES line confirm the presence of recently-ventilated North Atlantic Deep Water carried by the DWBC. The time-mean absolute geostrophic transport integrated within the DWBC layer, defined between 800–4800 dbar, and within longitude bounds of 51.5° W to 44.5° W is −15 Sv (1 Sv = 106 m3 s−1; negative indicates southward flow). The observed peak-to-peak range in volume transport using these integration limits is from −89 Sv to &plus;50 Sv, and the temporal standard deviation is 23 Sv. Testing different vertical integration limits based on time-mean water-mass property levels yields small changes to these values, but no significant alteration to the character of the transport time series. The time-mean southward DWBC flow at this latitude is confined west of 49.5° W, with recirculations dominating the flow further offshore. As with other latitudes where the DWBC has been observed for multiple years, the time variability greatly exceeds the time-mean, suggesting the presence of strong coherent vortices and/or Rossby Wave-like signals propagating to the boundary from the interior.

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How Does Labrador Sea Water Enter the Deep Western Boundary Current?

Journal of Physical Oceanography ◽

10.1175/2007jpo3807.1 ◽

2008 ◽

Vol 38 (5) ◽

pp. 968-983 ◽

Cited By ~ 18

Author(s):

Jaime B. Palter ◽

M. Susan Lozier ◽

Kara L. Lavender

Keyword(s):

Heat Flux ◽

Water Mass ◽

Sea Water ◽

Western Boundary Current ◽

Meridional Overturning Circulation ◽

Labrador Sea ◽

Western Boundary ◽

Boundary Current ◽

Deep Western Boundary Current ◽

Labrador Sea Water

Abstract Labrador Sea Water (LSW), a dense water mass formed by convection in the subpolar North Atlantic, is an important constituent of the meridional overturning circulation. Understanding how the water mass enters the deep western boundary current (DWBC), one of the primary pathways by which it exits the subpolar gyre, can shed light on the continuity between climate conditions in the formation region and their downstream signal. Using the trajectories of (profiling) autonomous Lagrangian circulation explorer [(P)ALACE] floats, operating between 1996 and 2002, three processes are evaluated for their role in the entry of Labrador Sea Water in the DWBC: 1) LSW is formed directly in the DWBC, 2) eddies flux LSW laterally from the interior Labrador Sea to the DWBC, and 3) a horizontally divergent mean flow advects LSW from the interior to the DWBC. A comparison of the heat flux associated with each of these three mechanisms suggests that all three contribute to the transformation of the boundary current as it transits the Labrador Sea. The formation of LSW directly in the DWBC and the eddy heat flux between the interior Labrador Sea and the DWBC may play leading roles in setting the interannual variability of the exported water mass.

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Wind-Forced Variability of the Remote Meridional Overturning Circulation

Journal of Physical Oceanography ◽

10.1175/jpo-d-19-0190.1 ◽

2020 ◽

Vol 50 (2) ◽

pp. 455-469 ◽

Cited By ~ 1

Author(s):

Michael A. Spall ◽

David Nieves

Keyword(s):

Wind Stress ◽

Rossby Wave ◽

Western Boundary Current ◽

Meridional Overturning Circulation ◽

Western Boundary ◽

Boundary Current ◽

Overturning Circulation ◽

Deep Western Boundary Current ◽

Wave Basin ◽

Meridional Overturning

AbstractThe mechanisms by which time-dependent wind stress anomalies at midlatitudes can force variability in the meridional overturning circulation at low latitudes are explored. It is shown that winds are effective at forcing remote variability in the overturning circulation when forcing periods are near the midlatitude baroclinic Rossby wave basin-crossing time. Remote overturning is required by an imbalance in the midlatitude mass storage and release resulting from the dependence of the Rossby wave phase speed on latitude. A heuristic theory is developed that predicts the strength and frequency dependence of the remote overturning well when compared to a two-layer numerical model. The theory indicates that the variable overturning strength, relative to the anomalous Ekman transport, depends primarily on the ratio of the meridional spatial scale of the anomalous wind stress curl to its latitude. For strongly forced systems, a mean deep western boundary current can also significantly enhance the overturning variability at all latitudes. For sufficiently large thermocline displacements, the deep western boundary current alternates between interior and near-boundary pathways in response to fluctuations in the wind, leading to large anomalies in the volume of North Atlantic Deep Water stored at midlatitudes and in the downstream deep western boundary current transport.

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Characteristics and causes of Deep Western Boundary Current transport variability at 34.5° S during 2009–2014

Ocean Science ◽

10.5194/os-13-175-2017 ◽

2017 ◽

Vol 13 (1) ◽

pp. 175-194 ◽

Cited By ~ 14

Author(s):

Christopher S. Meinen ◽

Silvia L. Garzoli ◽

Renellys C. Perez ◽

Edmo Campos ◽

Alberto R. Piola ◽

...

Keyword(s):

Volume Transport ◽

Horizontal Resolution ◽

Western Boundary Current ◽

Meridional Overturning Circulation ◽

Time Variability ◽

Western Boundary ◽

Boundary Current ◽

Meridional Heat Transport ◽

Data Set ◽

Deep Western Boundary Current

Abstract. The Deep Western Boundary Current (DWBC) at 34.5° S in the South Atlantic carries a significant fraction of the cold deep limb of the Meridional Overturning Circulation (MOC), and therefore its variability affects the meridional heat transport and consequently the regional and global climate. Nearly 6 years of observations from a line of pressure-equipped inverted echo sounders (PIESs) have yielded an unprecedented data set for studying the characteristics of the time-varying DWBC volume transport at 34.5° S. Furthermore, the horizontal resolution of the observing array was greatly improved in December 2012 with the addition of two current-and-pressure-equipped inverted echo sounders (CPIESs) at the midpoints of the two westernmost pairs of PIES moorings. Regular hydrographic sections along the PIES/CPIES line confirm the presence of recently ventilated North Atlantic Deep Water carried by the DWBC. The time-mean absolute geostrophic transport integrated within the DWBC layer, defined between 800–4800 dbar and within longitude bounds of 51.5 to 44.5° W, is −15 Sv (1 Sv  =  106 m3 s−1; negative indicates southward flow). The observed peak-to-peak range in volume transport using these integration limits is from −89 to +50 Sv, and the temporal standard deviation is 23 Sv. Testing different vertical integration limits based on time-mean water-mass property levels yields small changes to these values, but no significant alteration to the character of the transport time series. The time-mean southward DWBC flow at this latitude is confined west of 49.5° W, with recirculations dominating the flow further offshore. As with other latitudes where the DWBC has been observed for multiple years, the time variability greatly exceeds the time mean, suggesting the presence of strong coherent vortices and/or Rossby Wave-like signals propagating to the boundary from the interior.

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Deep Western Boundary Current transport variability in the South Atlantic: preliminary results from a pilot array at 34.5° S

Ocean Science Discussions ◽

10.5194/osd-9-977-2012 ◽

2012 ◽

Vol 9 (2) ◽

pp. 977-1008 ◽

Cited By ~ 2

Author(s):

C. S. Meinen ◽

A. R. Piola ◽

R. C. Perez ◽

S. L. Garzoli

Keyword(s):

Annual Cycle ◽

Statistical Significance ◽

South Atlantic ◽

Western Boundary Current ◽

Meridional Overturning Circulation ◽

Western Boundary ◽

Model Output ◽

Boundary Current ◽

The South ◽

Deep Western Boundary Current

Abstract. The first direct estimates of the temporal variability of the absolute transport of the Deep Western Boundary Current (DWBC) at 34.5° S in the South Atlantic Ocean are obtained using just under one year of data from a line of four pressure-equipped inverted echo sounders. Hydrographic sections collected in 2009 and 2010 confirm the presence of the DWBC, one of the main deep pathways of the Meridional Overturning Circulation, based on neutral density, temperature, salinity, and oxygen values. Both observations confirm that the DWBC reconstitutes itself after breaking into eddies in the western sub-tropical Atlantic near 8° S. The amplitude and spectral character of the DWBC transport variability are comparable with those observed at 26.5° N, where longer records exist, with the DWBC at 34.5° S exhibiting a transport standard deviation of 25 Sv and variations of ~40 Sv occurring within periods as short as a few days. There is little indication of an annual cycle in the DWBC transports, although the observation record is too short to be definitive, and the dominant time scale during the first year of the experiment was about 9–10 days. A "Monte Carlo-style" analysis using 27 yr of model output from the same location as the observations indicates that another 48–60 months of data will be required to encompass a fairly complete span of deep transport variability. The model suggests the presence of an annual cycle in DWBC transport, however the statistical significance of the annual cycle with even 27 yr of model output is low, suggesting that annual period variations in the model are weak as well.

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Deep Western Boundary Current transport variability in the South Atlantic: preliminary results from a pilot array at 34.5° S

Ocean Science ◽

10.5194/os-8-1041-2012 ◽

2012 ◽

Vol 8 (6) ◽

pp. 1041-1054 ◽

Cited By ~ 11

Author(s):

C. S. Meinen ◽

A. R. Piola ◽

R. C. Perez ◽

S. L. Garzoli

Keyword(s):

Annual Cycle ◽

Statistical Significance ◽

South Atlantic ◽

Western Boundary Current ◽

Meridional Overturning Circulation ◽

Western Boundary ◽

Model Output ◽

Boundary Current ◽

The South ◽

Deep Western Boundary Current

Abstract. The first direct estimates of the temporal variability of the absolute transport in the Deep Western Boundary Current (DWBC) at 34.5° S in the South Atlantic Ocean are obtained using just under one year of data from a line of four pressure-equipped inverted echo sounders. Hydrographic sections collected in 2009 and 2010 confirm, based on neutral density, temperature, salinity, and oxygen values, the presence of the DWBC, one of the main deep pathways of the Meridional Overturning Circulation. Both data sets indicate that the DWBC reconstitutes itself after breaking into eddies in the western sub-tropical Atlantic near 8° S. The amplitude and spectral character of the DWBC transport variability are comparable with those observed in the North Atlantic, where longer records exist, with the DWBC at 34.5° S exhibiting a transport standard deviation of 25 Sv and variations of ∼ 40 Sv occurring within periods as short as a few days. There is little indication of an annual cycle in the DWBC transports, although the observational records are too short to be definitive. A Monte Carlo-style analysis using 27 yr of model output from the same location as the observations indicates that about 48–60 months of data will be required to fully assess the deep transport variability. The model suggests the presence of an annual cycle in DWBC transport, however its statistical significance with even 27 yr of model output is low, suggesting that seasonal variations in the model are weak.

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Midlatitude–Equatorial Dynamics of a Grounded Deep Western Boundary Current. Part I: Midlatitude Flow and the Transition to the Equatorial Region

Journal of Physical Oceanography ◽

10.1175/jpo-d-14-0207.1 ◽

2015 ◽

Vol 45 (10) ◽

pp. 2457-2469 ◽

Cited By ~ 4

Author(s):

Gordon E. Swaters

Keyword(s):

Bottom Topography ◽

Zonal Flow ◽

Ocean Basin ◽

Equatorial Region ◽

Western Boundary Current ◽

Western Boundary ◽

Boundary Current ◽

Qualitative Properties ◽

Deep Western Boundary Current ◽

The Tropics

AbstractA comprehensive theoretical study of the nonlinear hemispheric-scale midlatitude and cross-equatorial steady-state dynamics of a grounded deep western boundary current is given. The domain considered is an idealized differentially rotating, meridionally aligned basin with zonally varying parabolic bottom topography so that the model ocean shallows on both the western and eastern sides of the basin. Away from the equator, the flow is governed by nonlinear planetary geostrophic dynamics on sloping topography in which the potential vorticity equation can be explicitly solved. As the flow enters the equatorial region, it speeds up and becomes increasingly nonlinear and passes through two distinguished inertial layers referred to as the “intermediate” and “inner” inertial equatorial boundary layers, respectively. The flow in the intermediate equatorial region is shown to accelerate and turn eastward, forming a narrow equatorial jet. The qualitative properties of the solution presented are consistent with the known dynamical characteristics of the deep western boundary currents as they flow from the midlatitudes into the tropics. The predominately zonal flow across the ocean basin in the inner equatorial region (and its exit from the equatorial region) is determined in Part II of this study.

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