Interannual variability of South Equatorial Current bifurcation and western boundary currents along the Madagascar coast

Yamagami, Yoko; Tozuka, Tomoki

doi:10.1002/2015jc011069

Cited by 21 publications

(23 citation statements)

References 52 publications

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“…ENSO predominantly impacts the Indian Ocean south of 10°S, whereas IOD is more influential north of 10°S (Rao & Behera, ). ENSO induces wind stress curl anomalies northwest of Australia, generating interannual variations in the SEMC (Yamagami & Tozuka, ). Hence, during La Niña (El Niño) events, a stronger (weaker) SEMC is observed, at a 5‐ to 15‐months lag (Palastanga et al, ; Yamagami & Tozuka, ).…”

Section: Discussionmentioning

confidence: 99%

A New Definition of the South‐East Madagascar Bloom and Analysis of Its Variability

et al. 2019

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The South‐East Madagascar Bloom occurs in an oligotrophic region of the southwest Indian Ocean. Phase locked to austral summer, this sporadic feature exhibits substantial temporal and spatial variability. Several studies, with different hypotheses, have focused on the initiation mechanism triggering the bloom, but none has been as yet clearly substantiated. With 19 years of ocean color data set available as well as in situ measurements (Argo profiles), the time is ripe to review this feature. The bloom is characterized in a novel manner, and a new bloom index is suggested, yielding 11 bloom years, including 3 major bloom years (1999, 2006, and 2008). Spatially, the bloom varies from a mean structure (22–32°S; 50–70°E) both zonally and meridionally. A colocation analysis of Argo profiles and chlorophyll‐a data revealed a bloom occurrence in a shallow‐stratified layer, with low‐salinity water in the surface layers. Additionally, a quantitative assessment of the previous hypotheses is performed and bloom occurrence is found to coincide with La Niña events and reduced upwelling intensity south of Madagascar. A stronger South‐East Madagascar Current during La Niña may support a detachment of the current from the coasts, dampening the upwelling south of Madagascar, and feeding low‐salinity waters into the Madagascar Basin, hence increasing stratification. Along with abundance of light, these provide the right conditions for a nitrogen‐fixing cyanobacterial phytoplankton bloom onset.

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Section: Discussionmentioning

confidence: 99%

A New Definition of the South‐East Madagascar Bloom and Analysis of Its Variability

et al. 2019

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“…Its two branches close the interior circulations of the tropical gyre in the north and the subtropical gyre in the south, respectively. Previous studies have shown that the SEC bifurcation typically occurs at 17–18°S in the upper thermocline and extends to as far south as 22–27°S at depths near 1,000 m (e.g., Chen & Wu, ; Qu & Lindstrom, ; Rodrigues et al, ; Yamagami & Tozuka, ). We focus on the SEC south of its bifurcation, which lies roughly around 20°S taking the vertical average, and define its transport as a meridional integral of westward flow between 20°S and 40°S in the upper 2,000 m at 170°W for the South Pacific, 30°W for the South Atlantic, and 60°E for the South Indian Ocean.…”

Section: Inter‐ocean Connectionmentioning

confidence: 97%

Spin‐Up of the Southern Hemisphere Super Gyre

Qu¹,

Fukumori

Fine

2019

JGR Oceans

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This study investigates the variability of the Southern Hemisphere super gyre (SHSG), using remotely sensed altimeter measurements, in situ Argo observations, and results from an ocean state estimate of the Consortium for Estimating the Circulation and Climate of the Ocean. Analyses of altimeter data show large trends of sea surface height, and their positive-negative contrast suggests a strengthening of subtropical gyres in all the three Southern Hemisphere oceans since 1993. Analyses of Argo data and the Estimating the Circulation and Climate of the Ocean estimate indicate that these dynamic signals of southern subtropical gyres extend to at least 2,000 m. The three southern subtropical gyres are interconnected through the Tasman and Agulhas leakages and vary consistently during the period 1993-2016. The Tasman and Agulhas leakages also show an increasing trend of inter-ocean water exchange with a typical increase of~2 Sv (1 Sv = 10 6 m 3 /s) per decade, indicative of a two-decade-long spin-up of the SHSG. The strengthening and poleward shift of westerly winds are associated with an increasing southern annular mode, which affect the midlatitude and high-latitude Southern Hemisphere oceans and contribute to the spin-up of the SHSG.Plain Language Summary The Southern Hemisphere super gyre (SHSG) is an interconnected circulation system of the southern subtropical gyres. It sweeps out of the Tasman Sea, transports water westward, and feeds the upper limb of the Atlantic thermohaline circulation through the Tasman and Agulhas leakages. The SHSG absorbs substantial quantities of atmospheric gases, such as oxygen and CO 2 , thus directly contributing to climate change. In this study, we analyze remotely sensed altimeter measurements, in situ observations, and outputs of an ocean general circulation model. The results reveal a two-decade-long spin-up of the SHSG. The inter-ocean exchange through the Tasman and Agulhas leakages shows an increasing trend during the period 1993-2016. This long-term trend may be attributed to changes in westerly winds associated with an increasing southern annular mode.

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“…We set the lag as 12 months because the lag correlation between wind stress and the Natal Pulse is maximum (r=0.52) at this lag (Figure b). This time lag is reasonable for the propagation of Rossby waves and eddies from the interior region to the Natal Bight (Yamagami & Tozuka, ). Since the regression model shows a significant effect (Figure c) and reproduces the number of the Natal Pulse relatively well, the interannual variations of the Natal Pulse are linked with the ENSO via atmospheric teleconnections to the southern Indian Ocean.…”

Section: Interannual Variability Of the Natal Pulsementioning

confidence: 74%

“…Several studies have investigated the low-frequency ocean circulation and climate in the southern Indian Ocean associated with the El Niño/Southern Oscillation (ENSO) (e.g. Rao & Behera, 2005;Ridderinkhof et al, 2013;Yamagami & Tozuka, 2015;Zhuang et al, 2013), Indian Ocean Dipole (Palastanga et al, 2006), and Ningaloo Niño (Kataoka et al, 2014). Since wind stress curl variability to the south of 10°is explained by the ENSO (Rao & Behera, 2005), here we assume wind stress curl variability in the southern Indian Ocean represents ENSO variability.…”

Section: Connection Between the Wind Stress Field In The Southern Indmentioning

confidence: 99%

Interannual Variability of the Natal Pulse

2019

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Satellite altimetry data and high-resolution model output are used to investigate the mechanism of interannual variations of the solitary meander in the Agulhas Current, called the Natal Pulse. It is shown that most of the Natal Pulses are triggered by anticyclonic eddies originating from the Southeast Madagascar Current (SEMC) and anticyclonic eddies of the Mozambique Channel are the secondary source. In addition to the barotropic conversion suggested by previous studies, the advection of eddy kinetic energy (EKE) plays a statistically significant role in the growth of the Natal Pulse. The number of anticyclonic eddies arriving at the Natal Bight largely explains the interannual variations in the Natal Pulse. The barotropic conversion from the SEMC, whose interannual variations are likely related to the remote wind forcing, is the main EKE source and corresponds well with the interannual variations of anticyclonic eddies generated off the southern coast of Madagascar. It is also shown that the interannual variations of the Natal Pulse are linked to large-scale wind variability in the southern Indian Ocean.Plain Language Summary Natal Pulse is the large meander of the Agulhas Current originating in the Natal Bight and it affects both the local marine environment and global ocean circulation. Previous observational studies suggested that the occurrence of the Natal Pulse varies from year-to-year. Since oceanic eddies tend to trigger the Natal Pulse, we investigate the relationship between the Natal Pulse and incoming eddies in this study. We show that variations of the Natal Pulse are mainly explained by variations of eddies propagating from the south of Madagascar and the Mozambique Channel. We also show that variations in the Natal Pulse are linked to the large-scale wind variability over the southern Indian Ocean. This study is the first to show the mechanism of the year-to-year variations in the Natal Pulse.

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Interannual variability of South Equatorial Current bifurcation and western boundary currents along the Madagascar coast

Cited by 21 publications

References 52 publications

A New Definition of the South‐East Madagascar Bloom and Analysis of Its Variability

A New Definition of the South‐East Madagascar Bloom and Analysis of Its Variability

Spin‐Up of the Southern Hemisphere Super Gyre

Interannual Variability of the Natal Pulse

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