Agulhas leakage into the Atlantic estimated with subsurface floats and surface drifters

Richardson, Philip L.

doi:10.1016/j.dsr.2007.04.010

Cited by 160 publications

(198 citation statements)

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“…The mean Agulhas leakage transports derived from using +0.1 spice anomaly are 18 Sv and 17 Sv for pentad and monthly model output, respectively, similar to the mean Lagrangian Agulhas leakage of 18 Sv and at the upper range of observational estimates of 3-20 Sv [Gordon et al, 1992;de Ruijter and Biastoch, 1999;Doglioli et al, 2006;Richardson, 2007]. These estimates are half that found in coarse-resolution CCSM4 [Weijer and van Sebille, 2014].…”

Section: Discussionsupporting

confidence: 62%

“…In most rings, +0.8 spice anomaly does not capture the upper 100 m of Agulhas waters ( Figure S1). Overall, we choose the +0.1 spice anomaly threshold since it yields significant correlation between Eulerian and Lagrangian estimates, a mean Agulhas leakage of 18 Sv that is comparable to observations [Gordon et al, 1992;de Ruijter and Biastoch, 1999;Doglioli et al, 2006;Richardson, 2007], and captures surface Agulhas waters as well as penetration depth beyond 1000 m.…”

Section: Comparing Eulerian Leakage Transport To Lagrangian Estimatesmentioning

confidence: 99%

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A new Eulerian method to estimate “spicy” Agulhas leakage in climate models

Putrasahan

Beal

Kirtman

2015

Geophysical Research Letters

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The Agulhas leakage brings warm and saline water from the Indian Ocean into the South Atlantic, forming part of the returning branch of the global thermohaline circulation, an important component of climate. We develop a new method using the passive tracer spice, to estimate Agulhas leakage transport in an ocean eddy‐resolving climate model that properly captures Agulhas Retroflection and leakage. We identify Agulhas leakage waters as positive spice anomalies (≥ +0.1) and are able to trace leakage to depths greater than 1000 m. Spice‐based Eulerian Agulhas leakage captures spikes in Lagrangian leakage transport which coincide with the passage of Agulhas rings, yielding a statistically significant correlation (0.47). On interannual times scales, differences between Eulerian and Lagrangian transports are reduced and correlation increases to 0.77. We obtain a mean spice‐based Eulerian Agulhas leakage of 17 ± 4 sverdrup (Sv) and Lagrangian Agulhas leakage of 18 ± 2 Sv, which are within the range of observational estimates.

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Section: Discussionsupporting

confidence: 62%

Section: Comparing Eulerian Leakage Transport To Lagrangian Estimatesmentioning

confidence: 99%

A new Eulerian method to estimate “spicy” Agulhas leakage in climate models

Putrasahan

Beal

Kirtman

2015

Geophysical Research Letters

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“…2.4 AL transports are calculated at each time step by integrating the normal velocities over the area defined by the bias adjusted thresholds. ARC112i and AGUa0.10 have a mean AL of 11.2 and 9.5 Sv (Table 3), which are the most similar to the estimates from literature and Lagrangian estimates of 10-20 Sv (Speich et al 2001;Doglioli et al 2006;Richardson 2007). ORCA12 has half the mean NET transport of ARC112i and is lower than observed values.…”

Section: Preliminary Description Of Agulhas Leakagesupporting

confidence: 65%

“…They are not easily measured, as turbulence in the Cape Basin leads to the decay of rings (Boebel et al 2003). As one ring is estimated to carry a volume transport of 0.5-1.5 Sv (Schouten et al 2000), with only 5-6 rings per year, rings cannot be the only vehicle of exchange as alone they cannot produce the estimated 15 Sv of transport reported by Richardson (2007). The extra contribution is likely provided through sub-mesoscale processes including filaments and jets (Lutjeharms and Cooper 1996) and by a background flux component (Loveday et al 2015).…”

Section: Introductionmentioning

confidence: 99%

Spatio-temporal characteristics of Agulhas leakage: a model inter-comparison study

et al. 2016

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west of the southern tip of Africa , is used to identify features that are robust regardless of the model used and takes into account the thermohaline biases of each model. When determined by a passive tracer method, 60 % of the magnitude of Agulhas leakage is captured and more than 80 % of its temporal fluctuations, suggesting that the method is appropriate for investigating the variability of Agulhas leakage. In all simulations but one, the major driver of variability is associated with mesoscale features passing through the section. High resolution (<1/10 • ) hindcast models agree on the temporal (2-4 cycles per year) and spatial (300-500 km) scales of these features corresponding to observed Agulhas Rings. Coarser resolution models (<1/4 • ) reproduce similar time scale of variability of Agulhas leakage in spite of their difficulties in representing the Agulhas rings properties. A coarser resolution climate model (2 • ) does not resolve the spatio-temporal mechanism of variability of Agulhas leakage. Hence it is expected to underestimate the contribution of Agulhas Current System to climate variability.

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“…Good reviews of the subject are given by Schmitz (1995, 1996a,b), de Ruijter et al (1999, Gordon (2001), andLutjeharms (2006). The amount of Indian Ocean water leaking into the Atlantic and contributing to the overturning circulation, its distribution in depth, and its variability remain controversial, although some recent studies support a large leakage transport (Gordon, 2001;Sloyan and Rintoul, 2001;Mercier et al, 2003;Richardson, 2007). Measuring the transport is difficult because of the complexity of the highly energetic Agulhas retroflection region including Agulhas rings and cyclones, which translate into the Atlantic, contributing a major part of the Indian Ocean leakage.…”

Section: Merz 1925mentioning

confidence: 99%

On the history of meridional overturning circulation schematic diagrams

Richardson

2008

Progress in Oceanography

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Recent global warming caused by humans and the prediction of a reduced Atlantic Ocean meridional overturning circulation in the future has increased interest in the role of the overturning circulation in climate change. A schematic diagram of the overturning circulation called the "Great Ocean Conveyor Belt," published by Wallace Broecker in 1987, has become a popular image that emphasizes the inter-connected ocean circulation and the northward flux of heat in the Atlantic. This would appear to be a good time to review the development of the conveyor belt concept and summarize the history of overturning circulation schematics.In the nineteenth century it was thought that symmetric overturning circulation cells were located on either side of the equator in the Atlantic. As new hydrographic measurements were obtained in the late nineteenth century and early twentieth century, circulation schematics in the early twentieth century began to show the inter-hemispheric overturning circulation in the Atlantic. In the second half of the twentieth century schematics showed the global ocean overturning circulation including connections between the Atlantic and the Pacific and Indian Oceans. Some recent schematics of the overturning circulation show its complexities, but as more information is included these schematics have also become complex and not as easy to understand as the simple Lenz 1845 4The earliest recorded deep temperature measurements in the subtropical Atlantic were made in 1750 by Captain Henry Ellis of the British slave ship Earl of Halifax. They revealed that warm subtropical water was confined to a relatively thin surface layer overlying much colder water (Ellis, 1851; Wüst, 1968; Deacon, 1971; Warren, 1981).Count Rumford (1798) who was born American as Benjamin Thompson and who served with the British, was knighted, eventually became Count of the Holy Roman Empire and ran the government of Bavaria, first described a circulation system to explain these temperature observations. He suggested that cold water from north and south polar regions spreads on the bottom of the sea toward the equator, and this produces currents at the surface in the opposite directions. A further development of this idea was made by Emil von Lenz (1845), a German-Russian physicist, who noted that the thermocline was shallower near the equator than in the subtropics and proposed an upwelling of deep water into the surface layer near the equator to compensate for poleward flow of warm near surface water. He derived a conceptual model of the meridional overturning circulation in the Atlantic, which consisted of two circulation cells located symmetrically on either side of the equator. A schematic summarizing this circulation (Fig. 2 The overturning circulation is a complicated system of currents driven by winds, by buoyancy forcing through evaporation, precipitation, heating, and cooling, and by mixing due to winds and tides. Knowledge of it developed slowly as subsurface temperature and salinity began to be accurately measu...

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Agulhas leakage into the Atlantic estimated with subsurface floats and surface drifters

Abstract: Surface drifters and subsurface floats drifting at depths near 800 m were used to study the pathways of warm salty Indian Ocean water leaking into the South Atlantic that is a

Cited by 160 publications

References 74 publications

A new Eulerian method to estimate “spicy” Agulhas leakage in climate models

A new Eulerian method to estimate “spicy” Agulhas leakage in climate models

Spatio-temporal characteristics of Agulhas leakage: a model inter-comparison study

On the history of meridional overturning circulation schematic diagrams

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