Warming in the Agulhas Current system since the 1980's

Rouault, Mathieu; Penven, Pierrick; Pohl, Benjamin

doi:10.1029/2009gl037987

Cited by 179 publications

(192 citation statements)

References 27 publications

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“…[22] In addition, the Mozambique Channel is the source of the Agulhas Current, the most intense western boundary current in the Southern Hemisphere, which plays an important role in the climate of Southern Africa and acts as a source of heat and salt to the Atlantic Ocean [Backeberg et al, 2012;Rouault et al, 2009]. There is evidence of the formation of an anticyclonic eddy in the northern section of the channel during the boreal winter that subsequently moves southward advecting subtropical waters into the Agulhas Current [Schouten et al, 2005].…”

Section: Discussionmentioning

confidence: 99%

“…The characteristics of the background environment are however such as to place the flow close to the point where this bifurcation vanishes. There is evidence of a trend toward an intensification of the easterly trade winds over the southern Indian Ocean [Han et al, 2010;Rouault et al, 2009] and such changes would result in a decrease in the inverse Rossby number that may result in changes in the flow splitting by Madagascar.…”

Section: Discussionmentioning

confidence: 99%

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Impact of the high topography of Madagascar on the structure of the Findlater Jet

Moore

2013

Geophysical Research Letters

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The cross‐equatorial flow over the western Indian Ocean, known as the Findlater Jet, plays an important role in the monsoonal circulation of the region. During the boreal summer, there is southerly flow across the equator that is concentrated along the East African highlands. During the boreal winter, there is a reversal in wind direction across the equator. Madagascar, the world's fourth largest island, with heights in excess of 1 km represents a significant obstacle to the flow whose impact on this jet has not been fully characterized. Here we use diagnostic tools developed to investigate atmospheric flow distortion by Greenland's high topography to study this interaction. We show that there is a bifurcation of the Findlater Jet by Madagascar during the boreal summer and localized tip jets at the island's northern and southern ends. During the boreal winter, the northern tip jet reverses direction and weakens, while the southern tip jet maintains its direction and magnitude. We show that rotational effects are important for these interactions but not dominant and result in an enhancement of the northern tip jet; while allowing for existence of the southern tip jet. As will also be shown, this flow distortion has impacts on the meteorology and oceanography of the region including the forcing of oceanic eddies in the Mozambique Channel, a modulation of the southward displacement of the Inter‐Tropical Convergence Zone (ITCZ) and a splitting of the boreal summer cross‐equatorial mass transport associated with the Findlater Jet into two branches.

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Impact of the high topography of Madagascar on the structure of the Findlater Jet

Moore

2013

Geophysical Research Letters

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“…In this study, we explore the possibility that remote forcing from outside the tropical Atlantic Ocean, namely, because of increased Agulhas leakage transport south of Africa (Rouault et al 2009;Biastoch et al 2009), has led to the warming of the subsurface tropical Atlantic. These tropical Atlantic subsurface waters, which are warmer than in previous decades, are subsequently upwelled to the surface by the increased trade winds, resulting in the observed warming trend at the surface.…”

Section: Introductionmentioning

confidence: 99%

Contribution of Increased Agulhas Leakage to Tropical Atlantic Warming

2015

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The upper tropical Atlantic Ocean has markedly warmed since the 1960s. It has been shown that this warming was not due to local heat fluxes and that the trade winds that drive the coastal and equatorial upwelling have intensified rather than weakened. Remote forcing might thus have played an important role. Here, model experiments are used to investigate the contribution from an increased inflow of warm Indian Ocean water through Agulhas leakage. A high-resolution hindcast experiment with interannually varying forcing for the time period 1948-2007, in which Agulhas leakage increases by about 45% from the 1960s to the early 2000s, reproduces the observed warming trend. To tease out the role of Agulhas leakage, a sensitivity experiment designed to only increase Agulhas leakage is used. Compared to a control simulation, it shows a pronounced warming in the upper tropical Atlantic Ocean. A Lagrangian trajectory analysis confirms that a significant portion of Agulhas leakage water reaches the upper 300 m of the tropical Atlantic Ocean within two decades and that the tropical Atlantic warming in the sensitivity experiment is mainly due to water of Agulhas origin. Therefore, it is suggested that the increased trade winds since the 1960s favor upwelling of warmer subsurface waters, which in part originate from the Agulhas, leading to higher SSTs in the tropics.

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“…The simplest Eulerian approach assumes that the waters of Indian Ocean origin represent the warmest and most saline waters across the Goodhope section. These thermohaline properties can then be utilised in order to establish an integration domain over which volume transport is calculated (Rouault et al 2009). The limitation of such an approach is that by imposing water mass criteria, the leakage in eddy-rich regions may be underestimated, although similar methods have been shown to reproduce the variability well (van Sebille et al 2010b;Loveday et al 2014;Putrasahan et al 2015).…”

Section: Introductionmentioning

confidence: 99%

“…This should be considered a first step to verify whether coarser resolution ocean models hence climate models are able to reproduce the processes of variability of AL. Rouault et al (2009) and Biastoch et al (2009) identified evidence that AL is increasing with climate change, linked to a warming trend in the Agulhas system since the 1960s and southward shift of the Subtropical front. Over 2-4 decades the warm salty water that enters the South Atlantic is advected northwards (Ruhs et al 2013;Weijer et al 2001), its influence on the buoyancy balance affects deep water formation, the AMOC and thus global climate.…”

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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Warming in the Agulhas Current system since the 1980's

Cited by 179 publications

References 27 publications

Impact of the high topography of Madagascar on the structure of the Findlater Jet

Impact of the high topography of Madagascar on the structure of the Findlater Jet

Contribution of Increased Agulhas Leakage to Tropical Atlantic Warming

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

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