Quantifying Agulhas Leakage in a High-Resolution Climate Model

Cheng, Yu; Beal, Lisa M.; Kirtman, Ben P.

doi:10.1175/jcli-d-15-0568.1

Cited by 31 publications

(56 citation statements)

References 48 publications

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“…The subsequent water and salt zonal transports display substantial intermittency (Figure 8). This agrees fairly well with observations that show no prominent seasonal pattern in the AL, with the actual monthly transports changing as a function of the shedding of rings from the AC [36,37].…”

Section: Agulhas Leakagesupporting

confidence: 90%

Seasonal Variability of Retroflection Structures and Transports in the Atlantic Ocean as Inferred from Satellite-Derived Salinity Maps

et al. 2019

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Three of the world's most energetic regions are in the tropical and South Atlantic: the North Brazil Current Retroflection, the Brazil-Malvinas Confluence, and the Agulhas Current Retroflection. All three regions display offshore diversions of major boundary currents, which define the intensity of the returning limb of the Atlantic meridional overturning circulation. In this work, we use a sea-surface salinity (SSS) satellite product, combined with a high-resolution numerical model and in situ measurements, in order to explore the seasonal variation of the surface currents and transports in these three regions. The analysis of the model output shows that the SSS patterns reflect the surface velocity structure, with the largest horizontal SSS gradients coinciding with those areas of highest velocity and the most predominant velocity vector being 90 • anticlockwise (clockwise) from the horizontal SSS gradient in the northern (southern) hemisphere. This information is then applied to the SSS satellite product to obtain maps of water velocity and salt transports, leading to a quantitative tool to estimate both water and salt transports in key regions of the world ocean.

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Section: Agulhas Leakagesupporting

confidence: 90%

Seasonal Variability of Retroflection Structures and Transports in the Atlantic Ocean as Inferred from Satellite-Derived Salinity Maps

et al. 2019

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“…This improvement was also reported by von for ocean-only simulations. There are two reasons for this warm bias reduction in ER pp : (1) the Agulhas Return Current, Agulhas retroflection, and Agulhas leakage are now bet- ter resolved, producing a more realistic circulation and watermass transfer from the Indian Ocean into the South Atlantic, as seen in other similar studies (McClean et al, 2011;Cheng et al, 2018); and (2) the eddyinduced cooling and freshening of the intermediate ocean further reduces the warm bias.…”

Section: Ocean Interiormentioning

confidence: 83%

“…The latter bias could not be removed by just increasing the atmospheric resolution. In the ocean, warm and saline biases in the Southern Atlantic were removed, because of the better representation of the Agulhas Current system (Putrasahan et al, 2015;Cheng et al, 2016) and because of eddy-induced upward transport of fresh and cold water masses, as described in von . In general, swifter and narrower boundary currents are simulated in all basins with a high resolution.…”

Section: Effects Of High-resolution Oceanmentioning

confidence: 99%

Max Planck Institute Earth System Model (MPI-ESM1.2) for the High-Resolution Model Intercomparison Project (HighResMIP)

et al. 2019

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As a contribution towards improving the climate mean state of the atmosphere and the ocean in Earth system models (ESMs), we compare several coupled simulations conducted with the Max Planck Institute for Meteorology Earth System Model (MPI-ESM1.2) following the High-ResMIP protocol. Our simulations allow to analyse the separate effects of increasing the horizontal resolution of the ocean (0.4 to 0.1 • ) and atmosphere (T127 to T255) submodels, and the effects of substituting the Pacanowski and Philander (PP) vertical ocean mixing scheme with the K-profile parameterization (KPP).The results show clearly distinguishable effects from all three factors. The high resolution in the ocean removes biases in the ocean interior and in the atmosphere. This leads to the important conclusion that a high-resolution ocean has a major impact on the mean state of the ocean and the atmosphere. The T255 atmosphere reduces the surface wind stress and improves ocean mixed layer depths in both hemispheres. The reduced wind forcing, in turn, slows the Antarctic Circumpolar Current (ACC), reducing it to observed values. In the North Atlantic, however, the reduced surface wind causes a weakening of the subpolar gyre and thus a slowing down of the Atlantic meridional overturning circulation (AMOC), when the PP scheme is used. The KPP scheme, on the other hand, causes stronger open-ocean convection which spins up the subpolar gyres, ultimately leading to a stronger and stable AMOC, even when coupled to the T255 atmosphere, thus retaining all the positive effects of a higher-resolved atmosphere.

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“…To estimate Agulhas leakage we consider all floats and drifters in the Agulhas Current flowing southwestward across the ACT (Agulhas Current Time‐series) section at 34°S (Beal et al, ) that either cross the GoodHope line into the southeast Atlantic (Ansorge et al, ) or enter the Agulhas Return Current and pass back east of 29°E, similar to previous Lagrangian estimates (Biastoch et al, ; Cheng et al, ; Richardson, ). If a float or drifter passes through the GoodHope line an odd number of times after passing through 34°S, it is considered a leaker.…”

Section: Methodsmentioning

confidence: 99%

“…Assuming six similar rings per year, the ring transport of Schmid et al () equates to 7.4 Sv of leakage on average while that of Wang et al () equates to 3 Sv, since they find that Indian Ocean waters make up only 25% of the waters trapped in rings. These estimates are low because they cannot account for the large amount of leakage that enters the Atlantic outside Agulhas Rings, which simulations suggest to be at least 50% of the total transport (Cheng et al, ; Loveday et al, ). Le Bars et al () and Biastoch et al () use satellite sea surface height and sea surface temperature, respectively, to infer changes in Agulhas leakage, but these techniques do not provide an estimate of absolute leakage.…”

Section: Introductionmentioning

confidence: 99%

A New Improved Estimation of Agulhas Leakage Using Observations and Simulations of Lagrangian Floats and Drifters

2020

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A new estimate of Agulhas leakage transport is calculated using profiling floats and drifters.Since Richardson's seminal estimate of 15 Sv in 2007, the number of floats and drifters passing through the Agulhas Current has quadrupled. Within uncertainties we find the same leakage percentages as Richardson, with 34% of drifters leaking at the surface and 21% of floats leaking at 1,000 m depth. We find that the drifters tend to follow a northward leakage pathway via the Benguela Current compared to the northwestward leakage pathway of the floats along the Agulhas Ring corridor. We simulate the isobaric and profiling behavior of the floats and drifters using two high resolution models and two offline Lagrangian tracking tools, quantifying for the first time the sampling biases associated with the observations. We find that the isobaric bias cannot be robustly simulated but likely causes an underestimate of observed leakage by one or two Sverdrups. The profiling behavior of the floats causes no significant bias in the leakage. Fitting a simulated vertical leakage profile to the observed leakage percentages from the floats and drifters and using the mean Agulhas transport observed by a moored array at 34 • S we find an improved Agulhas leakage transport of 21.3 Sv, with an estimated error of 4.7 Sv. Our new leakage transport is higher primarily because we account for leakage at depths down to 2,000 m, while Richardson considered only the top 1,000 m of the water column. Plain Language SummaryWe calculate a new estimation of Agulhas leakage using floats and drifters. Since most recent estimate of 15 Sv was calculated in 2007, we have had four times as many observations pass through the Agulhas Current. We find more drifters leak at the surface than floats at intermediate depth. The drifters follow a northward leakage passage compared to the northwestward passage of the floats. Using two models and two particle tracking tools, we mimic the behavior of these floats and drifters to better understand and quantify the sampling biases associated with these observations. Combining the results from the observations and these simulations, we calculate a new improved leakage transport of 21.3 Sv with an error of 4.7 Sv.Quantifying Agulhas leakage is difficult, with previous estimates ranging from 2 to 20 Sv (1 Sv = 1 × 10 6 m 3 s −1 ). The Cape Basin features the highest eddy kinetic energy outside a western boundary current system in the world (Chelton et al., 2011). Due to this highly turbulent nature, the mixing and stirring

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Quantifying Agulhas Leakage in a High-Resolution Climate Model

Cited by 31 publications

References 48 publications

Seasonal Variability of Retroflection Structures and Transports in the Atlantic Ocean as Inferred from Satellite-Derived Salinity Maps

Seasonal Variability of Retroflection Structures and Transports in the Atlantic Ocean as Inferred from Satellite-Derived Salinity Maps

Max Planck Institute Earth System Model (MPI-ESM1.2) for the High-Resolution Model Intercomparison Project (HighResMIP)

A New Improved Estimation of Agulhas Leakage Using Observations and Simulations of Lagrangian Floats and Drifters

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