The importance of monitoring the Greater Agulhas Current and its inter-ocean exchanges using large mooring arrays

The 2013 Intergovernmental Panel on Climate Change report, using CMIP5 and EMIC model outputs suggests that the Atlantic Meridional Overturning Circulation (MOC) is very likely to weaken by 11–34% over the next century, with consequences for global rainfall and temperature patterns. However, these coupled, global climate models cannot resolve important oceanic features such as the Agulhas Current and its leakage around South Africa, which a number of studies have suggested may act to balance MOC weakening in the future. To properly understand oceanic changes and feedbacks on anthropogenic climate change we need to substantially improve global ocean observations, particularly within boundary current regions such as the Agulhas Current, which represent the fastest warming regions across the world’s oceans. The South African science community, in collaboration with governing bodies and international partners, has recently established one of the world’s most comprehensive observational networks of a western boundary current system, measuring the Greater Agulhas Current System and its inter-ocean exchanges south of Africa. This observational network, through its design for long-term monitoring, collaborative coordination of resources and skills sharing, represents a model for the international community. We highlight progress of the new Agulhas System Climate Array, as well as the South African Meridional Overturning Circulation programme, which includes the Crossroads and GoodHope hydrographic transects, and the South Atlantic MOC Basin-wide Array. We also highlight some of the ongoing challenges that the programmes still face.

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The INALT family – a set of high-resolution nests for the Agulhas Current system within global NEMO ocean/sea-ice configurations

Geoscientific Model Development ◽

10.5194/gmd-12-3329-2019 ◽

2019 ◽

Vol 12 (7) ◽

pp. 3329-3355 ◽

Cited By ~ 6

Author(s):

Franziska U. Schwarzkopf ◽

Arne Biastoch ◽

Claus W. Böning ◽

Jérôme Chanut ◽

Jonathan V. Durgadoo ◽

...

Keyword(s):

High Resolution ◽

Boundary Conditions ◽

Current System ◽

Western Boundary Current ◽

Global Ocean ◽

Western Boundary ◽

Boundary Current ◽

The South ◽

Agulhas Current ◽

Mesoscale Dynamics

Abstract. The Agulhas Current, the western boundary current of the South Indian Ocean, has been shown to play an important role in the connectivity between the Indian and Atlantic oceans. The greater Agulhas Current system is highly dominated by mesoscale dynamics. To investigate their influence on the regional and global circulations, a family of high-resolution ocean general circulation model configurations based on the NEMO code has been developed. Horizontal resolution refinement is achieved by embedding “nests” covering the South Atlantic and the western Indian oceans at 1/10∘ (INALT10) and 1/20∘ (INALT20) within global hosts with coarser resolutions. Nests and hosts are connected through two-way interaction, allowing the nests not only to receive boundary conditions from their respective host but also to feed back the impact of regional dynamics onto the global ocean. A double-nested configuration at 1/60∘ resolution (INALT60) has been developed to gain insights into submesoscale processes within the Agulhas Current system. Large-scale measures such as the Drake Passage transport and the strength of the Atlantic meridional overturning circulation are rather robust among the different configurations, indicating the important role of the hosts in providing a consistent embedment of the regionally refined grids into the global circulation. The dynamics of the Agulhas Current system strongly depend on the representation of mesoscale processes. Both the southward-flowing Agulhas Current and the northward-flowing Agulhas Undercurrent increase in strength with increasing resolution towards more realistic values, which suggests the importance of improving mesoscale dynamics as well as bathymetric slopes along this narrow western boundary current regime. The exploration of numerical choices such as lateral boundary conditions and details of the implementation of surface wind stress forcing demonstrates the range of solutions within any given configuration.

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Three decades of research on the greater Agulhas Current

Ocean Science ◽

10.5194/os-3-129-2007 ◽

2007 ◽

Vol 3 (1) ◽

pp. 129-147 ◽

Cited By ~ 41

Author(s):

J. R. E. Lutjeharms

Keyword(s):

Thermohaline Circulation ◽

Current Knowledge ◽

Current System ◽

Meridional Overturning Circulation ◽

Global Ocean ◽

Agulhas Current ◽

Controversial Subject ◽

The Past ◽

New Information ◽

Meridional Overturning

Abstract. The greater Agulhas Current has been shown to be a key link in the global thermohaline circulation and an increased understanding of this current system is therefore of more than just local interest. Knowledge on the Agulhas Current system has in fact increased enormously over the past 30 years. This review covers some aspects of what has been learnt on the northern and the southern parts of the Agulhas Current proper and their influence on the waters and circulation of the adjoining continental shelf. It also discusses the Natal Pulse and new information that has been gained on how it is triggered and what influence it has. It deals with the Agulhas retroflection, the shedding of Agulhas rings and the movement and characteristics of these rings that contributes to the meridional overturning circulation of the global ocean. The Agulhas Return Current forms part of the final outflow of the system and current knowledge on that current is appraised. The sources of the Agulhas Current have been a controversial subject for many years and this dispute continues. This is described and discussed, based on what information has been gained from research over the past three decades. Building on what is currently known, some suggestions are given on the most important remaining knowledge gaps and how these could most efficaciously be filled.

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Intrinsic Variability of the Atlantic Meridional Overturning Circulation at Interannual-to-Multidecadal Time Scales

Journal of Physical Oceanography ◽

10.1175/jpo-d-14-0163.1 ◽

2015 ◽

Vol 45 (7) ◽

pp. 1929-1946 ◽

Cited By ~ 37

Author(s):

Sandy Grégorio ◽

Thierry Penduff ◽

Guillaume Sérazin ◽

Jean-Marc Molines ◽

Bernard Barnier ◽

...

Keyword(s):

Time Scales ◽

Gulf Stream ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

Global Ocean ◽

Intrinsic Variability ◽

Meridional Heat Transport ◽

Overturning Circulation ◽

Spectral Peaks ◽

Meridional Overturning

AbstractThe low-frequency variability of the Atlantic meridional overturning circulation (AMOC) is investigated from 2, ¼°, and ° global ocean–sea ice simulations, with a specific focus on its internally generated (i.e., “intrinsic”) component. A 327-yr climatological ¼° simulation, driven by a repeated seasonal cycle (i.e., a forcing devoid of interannual time scales), is shown to spontaneously generate a significant fraction R of the interannual-to-decadal AMOC variance obtained in a 50-yr “fully forced” hindcast (with reanalyzed atmospheric forcing including interannual time scales). This intrinsic variance fraction R slightly depends on whether AMOCs are computed in geopotential or density coordinates, and on the period considered in the climatological simulation, but the following features are quite robust when mesoscale eddies are simulated (at both ¼° and ° resolutions); R barely exceeds 5%–10% in the subpolar gyre but reaches 30%–50% at 34°S, up to 20%–40% near 25°N, and 40%–60% near the Gulf Stream. About 25% of the meridional heat transport interannual variability is attributed to intrinsic processes at 34°S and near the Gulf Stream. Fourier and wavelet spectra, built from the 327-yr ¼° climatological simulation, further indicate that spectral peaks of intrinsic AMOC variability (i) are found at specific frequencies ranging from interannual to multidecadal, (ii) often extend over the whole meridional scale of gyres, (iii) stochastically change throughout these 327 yr, and (iv) sometimes match the spectral peaks found in the fully forced hindcast in the North Atlantic. Intrinsic AMOC variability is also detected at multidecadal time scales, with a marked meridional coherence between 35°S and 25°N (15–30 yr periods) and throughout the whole basin (50–90-yr periods).

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Decadal changes in the Atlantic Meridional Overturning Circulation in high-resolution simulations of the subpolar North Atlantic

10.5194/egusphere-egu21-7789 ◽

2021 ◽

Author(s):

Claus W. Böning ◽

Arne Biastoch ◽

Klaus Getzlaff ◽

Patrick Wagner ◽

Siren Rühs ◽

...

Keyword(s):

High Resolution ◽

North Atlantic ◽

Deep Convection ◽

Alternative Interpretation ◽

Meridional Overturning Circulation ◽

Global Ocean ◽

Labrador Sea ◽

Overturning Circulation ◽

First Results ◽

Meridional Overturning

A series of global ocean - sea ice model simulations is used to investigate the spatial structure and temporal variability of the sinking branch of the meridional overturning circulation (AMOC) in the subpolar North Atlantic. The experiments include hindcast simulations of the last six decades based on the high-resolution (1/20&#176;) VIKING20X-model forced by the CORE and JRA55-do reanalysis products, supplemented by sensitivity studies with a 1/4&#176;-configuration (ORCA025) aimed at elucidating the roles of variations in the wind stress and buoyancy fluxes. The experiments exhibit different multi-decadal trends in the AMOC, reflecting the well-known sensitivity of ocean-only models to subtle details in the configuration of the subarctic freshwater forcing. All experiments, however, concur in that the dense, southward branch of the overturning is mainly fed by &#8220;sinking&#8221; (in density space) in the Irminger and Iceland Basins, in accordance with the first results of the OSNAP observational program. Remarkably, the contribution of the Labrador Sea has remained small throughout the whole simulation period, even during the phase of extremely strong convection in the early 1990s: i.e., the rate of deep water exported from the subpolar North Atlantic by the DWBC off Newfoundland never differed by more than O(1 Sv) from the DWBC entering the Labrador Sea at Cape Farewell. The model solutions indicate a particular concentration of the sinking along the deep boundary currents south of the Denmark Straits and south of Iceland, pointing to a prime importance for the AMOC of the outflows from the Nordic Seas and their subsequent enhancement by the entrainment of intermediate waters. Since these include the water masses formed by deep convection in the Labrador and southern Irminger Seas, our study offers an alternative interpretation of the dynamical role of decadal changes in Labrador Sea convection intensity in terms of a remote effect on the deep transports established in the outflow regimes.

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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 ◽

Meridional Overturning Circulation ◽

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.

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An Adjoint Analysis of the Meridional Overturning Circulation in a Hybrid Coupled Model

Journal of Climate ◽

10.1175/jcli3821.1 ◽

2006 ◽

Vol 19 (15) ◽

pp. 3751-3767 ◽

Cited By ~ 19

Author(s):

Véronique Bugnion ◽

Chris Hill ◽

Peter H. Stone

Keyword(s):

Boundary Conditions ◽

Wind Stress ◽

Ocean Circulation ◽

General Circulation ◽

Meridional Overturning Circulation ◽

Global Ocean ◽

Model Parameters ◽

Overturning Circulation ◽

Near Surface ◽

Meridional Overturning

Abstract Multicentury sensitivities in a realistic geometry global ocean general circulation model are analyzed using an adjoint technique. This paper takes advantage of the adjoint model’s ability to generate maps of the sensitivity of a diagnostic (i.e., the meridional overturning’s strength) to all model parameters. This property of adjoints is used to review several theories, which have been elaborated to explain the strength of the North Atlantic’s meridional overturning. This paper demonstrates the profound impact of boundary conditions in permitting or suppressing mechanisms within a realistic model of the contemporary ocean circulation. For example, the so-called Drake Passage Effect in which wind stress in the Southern Ocean acts as the main driver of the overturning’s strength, is shown to be an artifact of boundary conditions that restore the ocean’s surface temperature and salinity toward prescribed climatologies. Advective transports from the Indian and Pacific basins play an important role in setting the strength of the overturning circulation under “mixed” boundary conditions, in which a flux of freshwater is specified at the ocean’s surface. The most “realistic” regime couples an atmospheric energy and moisture balance model to the ocean. In this configuration, inspection of the global maps of sensitivity to wind stress and diapycnal mixing suggests a significant role for near-surface Ekman processes in the Tropics. Buoyancy also plays an important role in setting the overturning’s strength, through direct thermal forcing near the sites of convection, or through the advection of salinity anomalies in the Atlantic basin.

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Regional Patterns of Sea Level Change Related to Interannual Variability and Multidecadal Trends in the Atlantic Meridional Overturning Circulation*

Journal of Climate ◽

10.1175/2010jcli3341.1 ◽

2010 ◽

Vol 23 (15) ◽

pp. 4243-4254 ◽

Cited By ~ 36

Author(s):

K. Lorbacher ◽

J. Dengg ◽

C. W. Böning ◽

A. Biastoch

Keyword(s):

Sea Level ◽

Time Scales ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

Global Ocean ◽

Overturning Circulation ◽

Regional Patterns ◽

Model Experiments ◽

Basin Scale ◽

Meridional Overturning

Abstract Some studies of ocean climate model experiments suggest that regional changes in dynamic sea level could provide a valuable indicator of trends in the strength of the Atlantic meridional overturning circulation (MOC). This paper describes the use of a sequence of global ocean–ice model experiments to show that the diagnosed patterns of sea surface height (SSH) anomalies associated with changes in the MOC in the North Atlantic (NA) depend critically on the time scales of interest. Model hindcast simulations for 1958–2004 reproduce the observed pattern of SSH variability with extrema occurring along the Gulf Stream (GS) and in the subpolar gyre (SPG), but they also show that the pattern is primarily related to the wind-driven variability of MOC and gyre circulation on interannual time scales; it is reflected also in the leading EOF of SSH variability over the NA Ocean, as described in previous studies. The pattern, however, is not useful as a “fingerprint” of longer-term changes in the MOC: as shown with a companion experiment, a multidecadal, gradual decline in the MOC [of 5 Sv (1 Sv ≡ 106 m3 s−1) over 5 decades] induces a much broader, basin-scale SSH rise over the mid-to-high-latitude NA, with amplitudes of 20 cm. The detectability of such a trend is low along the GS since low-frequency SSH changes are effectively masked here by strong variability on shorter time scales. More favorable signal-to-noise ratios are found in the SPG and the eastern NA, where a MOC trend of 0.1 Sv yr−1 would leave a significant imprint in SSH already after about 20 years.

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14C Ventilation ages suggest a brief reversal of ocean Meridional Overturning Circulation during deglacial ‘Heinrich Stadial 1’

10.5194/egusphere-egu2020-5354 ◽

2020 ◽

Author(s):

Michael Sarnthein ◽

Pieter M. Grootes

Keyword(s):

North Pacific ◽

Model Simulation ◽

Ice Cores ◽

Meridional Overturning Circulation ◽

Global Ocean ◽

Age Difference ◽

Overturning Circulation ◽

Deep Waters ◽

Global Correlation ◽

Meridional Overturning

Changes in the geometry of ocean Meridional Overturning Circulation (MOC) are crucial in controlling changes of climate and the carbon inventory of the atmosphere. However, the accurate timing and global correlation of short-term glacial-to-deglacial changes in the MOC of different ocean basins still present a major challenge. The suite of jumps and plateaus in the record of past atmospheric radiocarbon (14C) concentrations offers a unique opportunity of age control and global correlation. The upper and lower boundaries of atmospheric 14C plateaus in the 14C records of both tree rings and Lake Suigetsu (age calibrated on the basis of Hulu U/Th model ages)&#173; provide a detailed stratigraphic &#8217;rung ladder&#8217; of ~30 age tie points from 29 to 10 ka that can be used for dating of planktic 14C records and an age correlation, by now employed to ~20 sediment cores obtained from key locations of MOC all over the global ocean. The age difference between paired planktic and benthic 14C ages provides an estimate of the ventilation age of deep waters since their last contact with the atmosphere. 14C ventilation ages of Last Glacial Maximum (LGM) deep waters reveal coeval opposed geometries of Atlantic and Pacific MOC. Similar to today, LGM Atlantic deep-water formation went along with an estuarine inflow of old abyssal waters from the Southern Ocean up to the northern North Pacific and an outflow of upper deep waters. Vice versa, low 14C ventilation ages of N.E. Pacific deep waters suggest a reversed, anti-estuarine MOC during early Heinrich Stadial 1 with a ~1500 year-long flushing of the deep North Pacific up to the South China Sea, when the North Atlantic was marked by an estuarine circulation geometry, gradually starting near 19 ka. Elevated 14C ventilation ages of LGM deep waters reflect a major drawdown of atmospheric carbon. Subsequent massive age drops accompanying changes in MOC reflect major events of carbon release to the atmosphere as recorded in Antarctic ice cores. These contemporaneous features of the MOC and the carbon cycle offer a great test case for comparison with model simulation.

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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 ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

Western Boundary ◽

Boundary Current ◽

Gaussian Stochastic Process ◽

Overturning Circulation ◽

Meridional Overturning

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.

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Optimal Surface Salinity Perturbations of the Meridional Overturning and Heat Transport in a Global Ocean General Circulation Model

Journal of Physical Oceanography ◽

10.1175/2008jpo3875.1 ◽

2008 ◽

Vol 38 (12) ◽

pp. 2739-2754 ◽

Cited By ~ 23

Author(s):

Florian Sévellec ◽

Thierry Huck ◽

Mahdi Ben Jelloul ◽

Nicolas Grima ◽

Jérôme Vialard ◽

...

Keyword(s):

Heat Transport ◽

General Circulation ◽

Meridional Overturning Circulation ◽

Global Ocean ◽

Transient Growth ◽

Surface Salinity ◽

Meridional Heat Transport ◽

Overturning Circulation ◽

Optimal Surface ◽

Meridional Overturning

Abstract Recent observations and modeling studies have stressed the influence of surface salinity perturbations on the North Atlantic circulation over the past few decades. As a step toward the estimation of the sensitivity of the thermohaline circulation to salinity anomalies, optimal initial surface salinity perturbations are computed and described for a realistic mean state of a global ocean general circulation model [Océan Parallélisé (OPA)]; optimality is defined successively with respect to the meridional overturning circulation intensity and the meridional heat transport maximum. Although the system is asymptotically stable, the nonnormality of the dynamics is able to produce a transient growth through an initial stimulation. Optimal perturbations are calculated subject to three constraints: the perturbation applies to surface salinity; the perturbation conserves the global salt content; and the perturbation is normalized, to remove the degeneracy in the linear maximization problem. Maximization using Lagrangian multipliers leads to explicit solutions (rather than eigenvalue problems), involving the integration of the model adjoint for each value to maximize. The most efficient transient growth for the intensity of the meridional overturning circulation appears for a delay of 10.5 yr after the perturbation by the optimal surface salinity anomaly. This optimal growth is induced by an initial anomaly located north of 50°N. In the same way, the most efficient transient growth for the intensity of the meridional heat transport appears for a shorter delay of 2.2 yr after the perturbation by the optimal surface salinity anomaly. This initial optimal perturbation corresponds to a zonal salinity gradient around 24°N. The optimal surface salinity perturbations studied herein yield upper bounds on the intensity of the response in meridional overturning circulation and meridional heat transport. Using typical amplitudes of the Great Salinity Anomalies, the upper bounds for the associated variability are 0.8 Sv (1 Sv ≡ 106 m3 s−1) (11% of the mean circulation) and 0.03 PW (5% of the mean circulation), respectively.

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