The Agulhas Bank Circulation

Western Boundary ◽

Oceanic Circulation ◽

Boundary Current ◽

Benguela Current ◽

Ocean Region ◽

Meridional Overturning ◽

Shelf Region

<p>The southern tip of Africa is the gateway between the Indian and Atlantic oceans, one of the most widely recognized chokepoints of the meridional overturning circulation (MOC). The oceanic circulation in this region connects a western boundary current, the Agulhas Current, to an eastern boundary current, the Benguela Current, a connection not replicated elsewhere and quite important, not only because of its peculiarity, but also because of its role in the MOC. During the last few decades numerous international research programs have collected large amounts of oceanographic data of this region. All these efforts, however, have been largely focused on the deep-ocean, leaving the coastal region practically unattended. In this presentation we will use the results of a suite of process-oriented numerical experiments to discuss the circulation along the Agulhas Bank (AB)&#8212;the shelf region sandwiched between the eastern and western margins of the African continent; in particular to illustrate its connections and interactions with the deep-ocean region. As we shall show these shelf/deep-ocean interactions, are not only important to the shelf but also to the Indian/Atlantic interoceanic exchange.</p>

Sensitivity of Atlantic Meridional Overturning Circulation Variability to Parameterized Nordic Sea Overflows in CCSM4

Journal of Climate ◽

10.1175/jcli-d-11-00149.1 ◽

2012 ◽

Vol 25 (6) ◽

pp. 2077-2103 ◽

Cited By ~ 42

Author(s):

Stephen Yeager ◽

Gokhan Danabasoglu

Keyword(s):

Deep Ocean ◽

Order Effect ◽

Western Boundary ◽

Boundary Current ◽

Overturning Circulation ◽

Climate System Model ◽

Frequency Space ◽

Abstract The inclusion of parameterized Nordic Sea overflows in the ocean component of the Community Climate System Model version 4 (CCSM4) results in a much improved representation of the North Atlantic tracer and velocity distributions compared to a control CCSM4 simulation without this parameterization. As a consequence, the variability of the Atlantic meridional overturning circulation (AMOC) on decadal and longer time scales is generally lower, but the reduction is not uniform in latitude, depth, or frequency–space. While there is dramatically less variance in the overall AMOC maximum (at about 35°N), the reduction in AMOC variance at higher latitudes is more modest. Also, it is somewhat enhanced in the deep ocean and at low latitudes (south of about 30°N). The complexity of overturning response to overflows is related to the fact that, in both simulations, the AMOC spectrum varies substantially with latitude and depth, reflecting a variety of driving mechanisms that are impacted in different ways by the overflows. The usefulness of reducing AMOC to a single index is thus called into question. This study identifies two main improvements in the ocean mean state associated with the overflow parameterization that tend to damp AMOC variability: enhanced stratification in the Labrador Sea due to the injection of dense overflow waters and a deepening of the deep western boundary current. Direct driving of deep AMOC variance by overflow transport variations is found to be a second-order effect.

Variability of Shallow and Deep Western Boundary Currents off the Bahamas during 2004–05: Results from the 26°N RAPID–MOC Array

Journal of Physical Oceanography ◽

10.1175/2007jpo3791.1 ◽

2008 ◽

Vol 38 (3) ◽

pp. 605-623 ◽

Cited By ~ 76

Author(s):

W. E. Johns ◽

L. M. Beal ◽

M. O. Baringer ◽

J. R. Molina ◽

S. A. Cunningham ◽

...

Keyword(s):

Boundary Layer ◽

North Atlantic ◽

Deep Ocean ◽

Western Boundary ◽

Boundary Current ◽

Overturning Circulation ◽

The Bahamas ◽

Meridional Overturning ◽

Baroclinic Transport

Abstract Data from an array of six moorings deployed east of Abaco, Bahamas, along 26.5°N during March 2004–May 2005 are analyzed. These moorings formed the western boundary array of a transbasin observing system designed to continuously monitor the meridional overturning circulation and meridional heat flux in the subtropical North Atlantic, under the framework of the joint U.K.–U.S. Rapid Climate Change (RAPID)–Meridional Overturning Circulation (MOC) Program. Important features of the western boundary circulation include the southward-flowing deep western boundary current (DWBC) below 1000 m and the northward-flowing “Antilles” Current in the upper 1000 m. Transports in the western boundary layer are estimated from direct current meter observations and from dynamic height moorings that measure the spatially integrated geostrophic flow between moorings. The results of these methods are combined to estimate the time-varying transports in the upper and deep ocean over the width of the western boundary layer to a distance of 500 km offshore of the Bahamas escarpment. The net southward transport of the DWBC across this region, inclusive of northward deep recirculation, is −26.5 Sv (Sv ≡ 106 m3 s−1), which is divided nearly equally between upper (−13.9 Sv) and lower (−12.6 Sv) North Atlantic Deep Water (NADW). In the top 1000 m, 6.0 Sv flows northward in a thermocline-intensified jet near the western boundary. These transports are found to agree well with historical current meter data in the region collected between 1986 and 1997. Variability in both shallow and deep components of the circulation is large, with transports above 1000 m varying between −15 and +25 Sv and deep transports varying between −60 and +3 Sv. Much of this transport variability, associated with barotropic fluctuations, occurs on relatively short time scales of several days to a few weeks. Upon removal of the barotropic fluctuations, slower baroclinic transport variations are revealed, including a temporary stoppage of the lower NADW transport in the DWBC during November 2004.

Mean Circulation and Variability of the Tropical Atlantic during 1952–2001 in the GECCO Assimilation Fields

Journal of Physical Oceanography ◽

10.1175/2007jpo3541.1 ◽

2008 ◽

Vol 38 (1) ◽

pp. 177-192 ◽

Cited By ~ 16

Author(s):

Benjamin Rabe ◽

Friedrich A. Schott ◽

Armin Köhl

Keyword(s):

Model Comparison ◽

Heat Content ◽

Western Boundary ◽

Boundary Current ◽

Equatorial Undercurrent ◽

Equatorial Upwelling ◽

Meridional Overturning ◽

Mean Circulation ◽

The Tropics

Abstract The shallow subtropical–tropical cells (STC) of the Atlantic Ocean have been studied from the output fields of a 50-yr run of the German partner of the Estimating the Circulation and Climate of the Ocean (GECCO) consortium assimilation model. Comparison of GECCO with time-mean observational estimates of density and meridional currents at 10°S and 10°N, which represent the boundaries between the tropics and subtropics in GECCO, shows good agreement in transports of major currents. The variability of the GECCO wind stress in the interior at 10°S and 10°N remains consistent with the NCEP forcing, although temporary changes can be large. On pentadal and longer time scales, an STC loop response is found between the poleward Ekman divergence and STC-layer convergence at 10°S and 10°N via the Equatorial Undercurrent (EUC) at 23°W, where the divergence leads the EUC and the convergence, suggesting a “pulling” mechanism via equatorial upwelling. The divergence is also associated with changes in the eastern equatorial upper-ocean heat content. Within the STC layer, partial compensation of the western boundary current (WBC) and the interior occurs at 10°S and 10°N. For the meridional overturning circulation (MOC) at 10°S it is found that more than one-half of the variability in the upper limb can be explained by the WBC. The explained MOC variance can be increased to 85% by including the geostrophic (Sverdrup) part of the wind-driven transports.

Inferring the zonal distribution of measured changes in the meridional overturning circulation

Ocean Science ◽

10.5194/os-3-55-2007 ◽

2007 ◽

Vol 3 (1) ◽

pp. 55-57 ◽

Cited By ~ 7

Author(s):

A. M. de Boer ◽

H. L. Johnson

Keyword(s):

Dynamic Method ◽

Western Boundary ◽

Boundary Current ◽

Overturning Circulation ◽

Zonal Distribution ◽

Deep Western Boundary Current ◽

Meridional Overturning ◽

Florida Straits ◽

Southward Flow

Abstract. Recently, hydrographic measurements have been used to argue that the meridional overturning circulation at 25° N has decreased by 30% over the last 50 years. Here we show that the most likely interpretation consistent with this approach (i.e., with the dynamic method together with a level-of-no-motion assumption and Ekman dynamics) is that any decrease in strength of the deep western boundary current must have been compensated, not by a basin-wide increase in upper layer southward flow, but by changes in the nonlinear region immediately outside of the Florida Straits.

Two Decades of the Atlantic Meridional Overturning Circulation: Anatomy, Variations, Extremes, Prediction, and Overcoming Its Limitations

Journal of Climate ◽

10.1175/jcli-d-12-00478.1 ◽

2013 ◽

Vol 26 (18) ◽

pp. 7167-7186 ◽

Cited By ~ 45

Author(s):

Carl Wunsch ◽

Patrick Heimbach

Keyword(s):

Ocean Circulation ◽

Extreme Values ◽

Statistical Significance ◽

Western Boundary ◽

Boundary Current ◽

Gaussian Stochastic Process ◽

Overturning Circulation ◽

Abstract The zonally integrated meridional volume transport in the North Atlantic [Atlantic meridional overturning circulation (AMOC)] is described in a 19-yr-long ocean-state estimate, one consistent with a diverse global dataset. Apart from a weak increasing trend at high northern latitudes, the AMOC appears statistically stable over the last 19 yr with fluctuations indistinguishable from those of a stationary Gaussian stochastic process. This characterization makes it possible to study (using highly developed tools) extreme values, predictability, and the statistical significance of apparent trends. Gaussian behavior is consistent with the central limit theorem for a process arising from numerous independent disturbances. In this case, generators include internal instabilities, changes in wind and buoyancy forcing fields, boundary waves, the Gulf Stream and deep western boundary current transports, the interior fraction in Sverdrup balance, and all similar phenomena arriving as summation effects from long distances and times. As a zonal integral through the sum of the large variety of physical processes in the three-dimensional ocean circulation, understanding of the AMOC, if it is of central climate importance, requires breaking it down into its unintegrated components over the entire basin.

Seasonal variability of the Atlantic Meridional Overturning Circulation at 11° S inferred from bottom pressure measurements

Ocean Science ◽

10.5194/os-17-265-2021 ◽

2021 ◽

Vol 17 (1) ◽

pp. 265-284

Author(s):

Josefine Herrford ◽

Peter Brandt ◽

Torsten Kanzow ◽

Rebecca Hummels ◽

Moacyr Araujo ◽

...

Keyword(s):

Seasonal Variability ◽

Western Boundary Current ◽

Western Boundary ◽

Bottom Pressure ◽

Boundary Current ◽

Overturning Circulation ◽

Western Basin ◽

Abstract. Bottom pressure observations on both sides of the Atlantic basin, combined with satellite measurements of sea level anomalies and wind stress data, are utilized to estimate variations of the Atlantic Meridional Overturning Circulation (AMOC) at 11∘ S. Over the period 2013–2018, the AMOC and its components are dominated by seasonal variability, with peak-to-peak amplitudes of 12 Sv for the upper-ocean geostrophic transport, 7 Sv for the Ekman and 14 Sv for the AMOC transport. The characteristics of the observed seasonal cycles of the AMOC and its components are compared to results from an ocean general circulation model, which is known to reproduce the variability of the Western Boundary Current on longer timescales. The observed seasonal variability of zonally integrated geostrophic velocity in the upper 300 m is controlled by pressure variations at the eastern boundary, while at 500 m depth contributions from the western and eastern boundaries are similar. The model tends to underestimate the seasonal pressure variability at 300 and 500 m depth, especially at the western boundary, which translates into the estimate of the upper-ocean geostrophic transport. In the model, seasonal AMOC variability at 11∘ S is governed, besides the Ekman transport, by the geostrophic transport variability in the eastern basin. The geostrophic contribution of the western basin to the seasonal cycle of the AMOC is instead comparably weak, as transport variability in the western basin interior related to local wind curl forcing is mainly compensated by the Western Boundary Current. Our analyses indicate that while some of the uncertainties of our estimates result from the technical aspects of the observational strategy or processes not being properly represented in the model, uncertainties in the wind forcing are particularly relevant for the resulting uncertainties of AMOC estimates at 11∘ S.

Inferring the zonal distribution of measured changes in the meridional overturning circulation

Ocean Science Discussions ◽

10.5194/osd-3-1653-2006 ◽

2006 ◽

Vol 3 (5) ◽

pp. 1653-1660 ◽

Cited By ~ 1

Author(s):

A. de Boer ◽

H. L. Johnson

Keyword(s):

Dynamic Method ◽

Western Boundary ◽

Boundary Current ◽

Overturning Circulation ◽

Zonal Distribution ◽

Deep Western Boundary Current ◽

Meridional Overturning ◽

Florida Straits ◽

Southward Flow

Abstract. Recently, hydrographic measurements have been used to argue that the meridional overturning circulation at 25° N has decreased by 30% over the last 50 years. Here, we show that the most likely interpretation consistent with this approach (i.e., with the dynamic method together with a level-of-no-motion assumption and Ekman dynamics), is that any decrease in strength of the deep western boundary current must have been compensated, not by a basin-wide increase in upper layer southward flow, but by changes in the nonlinear region, immediately outside of the Florida Straits.

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 ◽

Western Boundary ◽

Boundary Current ◽

Overturning Circulation ◽

Deep Western Boundary Current ◽

Wave Basin ◽

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.

Seasonal variability of salt in the western tropical Atlantic

10.5194/egusphere-egu2020-22633 ◽

2020 ◽

Author(s):

Yaci Alvarez ◽

Andre Luiz Belem

Keyword(s):

North Atlantic ◽

Deep Ocean ◽

Salt Transport ◽

Western Boundary ◽

Intermediate Water ◽

The North ◽

Meridional Overturning ◽

The North Atlantic ◽

The Antarctic

<p>The western boundary regime of the tropical South Atlantic Ocean is the main pathway of an important meridional transfer of warm and cold water masses that balances the global temperature on Earth, known as Atlantic Meridional Overturning Circulation (AMOC). The AMOC is a system that depends on a delicate balance of temperature and salinity effects on density, and is considered one of the main elements of the terrestrial system. The objective of this work was to study the variability of the salinity in the Western Tropical Atlantic Ocean, in order to identify salt transport anomalies in the circulation of the Atlantic Meridional Overturning Circulation as a result of climate change. Based on 3 decades of hydrographic observations of the Northern Brazilian Current and of the Deep Western Boundary Current, neutral density surfaces, salinity anomalies, geostrophic transport and salt transport were calculated. In general, the results reveal a coherent decadal change in salinity in 5&#176;S and 11&#176;S. In the upper ocean, both water masses, the South Atlantic Central Water and the Antarctic Intermediate Water, presented an increase of the salinity. The Antarctic Intermediate Water shows small trends with a decrease in salinity values in the upper part of the layer and an increase at the border to the North Atlantic Deep Water. In the deep ocean, the North Atlantic Deep Water layers the salinity generally decreases and, as expected for a warmer ocean in the Southern Hemisphere, the Antarctic Bottom Water layer shows an increase in salinity. The geostrophic and salt transports suggest a multidecadal variability and the changes in upper layer salinity are consistent with an increased Agulhas leakage, as described in literature. In the deep ocean, water mass changes seem to be likely related to changes in weather patterns in the North Atlantic as well as in tropical circulation changes.</p>

Seasonal variability of the Atlantic Meridional Overturning Circulation at 11° S inferred from bottom pressure measurements

10.5194/os-2020-55 ◽

2020 ◽

Author(s):

Josefine Herrford ◽

Peter Brandt ◽

Torsten Kanzow ◽

Rebecca Hummels ◽

Moacyr Araujo ◽

...

Keyword(s):

Seasonal Variability ◽

Rossby Waves ◽

Western Boundary Current ◽

Western Boundary ◽

Bottom Pressure ◽

Boundary Current ◽

Overturning Circulation ◽

Abstract. Bottom pressure observations on both sides of the Atlantic basin, combined with satellite measurements of sea level anomalies and wind stress data, are utilized to estimate variations of the Atlantic Meridional Overturning Circulation (AMOC) at 11° S. Over the period 2013–2018, the AMOC and its components are dominated by seasonal variability, with peak-to-peak amplitudes of 12 Sv for the upper-ocean geostrophic transport, 7 Sv for the Ekman and 14 Sv for the AMOC transport. The observed seasonal cycles of the AMOC, its components as well as the Western Boundary Current as observed with current meter moorings are in general good agreement with results of an ocean general circulation model. The seasonal variability of zonally integrated geostrophic velocity in the upper 300 m is controlled by pressure variations at the eastern boundary, while at 500 m depth contributions from the western and eastern boundaries are similar. The model tends to underestimate the seasonal pressure variability at 300 and 500 m depth, slightly stronger at the western boundary. In the model, seasonal AMOC variability at 11° S is governed by the variability in the eastern basin. Here, long Rossby waves originating from equatorial forcing are known to be radiated from the Angolan continental slope and propagate westward into the basin interior. The contribution of the western basin to AMOC seasonal variability is instead comparably weak as transport variability due to locally forced Rossby waves is mainly compensated by the Western Boundary Current. Our analyses indicate, that while some of the uncertainties of our estimates result from the technical aspects of the observational strategy or processes being not properly represented in the model, uncertainties in the wind forcing are particularly relevant for AMOC estimates at 11° S.