Dynamics of the Indian-Ocean oxygen minimum zones

McCreary, Julian P.; Yu, Zuojun; Hood, Raleigh R.; Vinaychandran, P. N.; Furue, Ryo; Ishida, Akio; Richards, Kelvin J

doi:10.1016/j.pocean.2013.03.002

Cited by 147 publications

(138 citation statements)

References 60 publications

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“…The major role of northward lateral advection and eddyinduced vertical advection in controlling the supply of oxygen to the OMZ is in agreement with the study of McCreary et al (2012). In a suite of sensitivity experiments, they identified the horizontal transport of oxygenated waters from the south and the additional vertical mixing induced by eddies as essential to properly resolve the OMZ.…”

Section: Discussionsupporting

confidence: 72%

“…Later, the transport of oxygenated waters by the western boundary current (Swallow, 1984;Sarma, 2002), mixing by mesoscale eddies (Kim et al, 2001) and strong detrital sinking rates enhancing export to the deep ocean (Wiggert and Murtugudde, 2007), were invoked to explain the presence of relatively high oxygen concentration below the region of highest productivity. The recent process study of McCreary et al (2012) confirmed the prominent role of oceanic circulation in controlling the spatial distribution of the OMZ in the Arabian Sea. Using sensitivity experiments, they identified the transport of oxygen by the eastern boundary current and eddy mixing as essential processes to reproduce the eastward shift of the OMZ.…”

Section: Introductionmentioning

confidence: 68%

“…This difference probably arises from the spatial resolution and the subsequent representation of eddies in the two models. Whereas mesoscale eddies are explicitly resolved in our model, they are parameterised by an enhanced mixing process and not an advective process in the model of McCreary et al (2012). Although their parameterisation of enhanced mixing takes into account the spatial patterns of the eddy kinetic energy observed by satellite, it is unable to represent the horizontal transport mediated by the 1000 km long mesoscale filaments emanating from the coast of Oman or by the eastward propagating mesoscale eddies formed along the coast of India.…”

Section: Discussionmentioning

confidence: 99%

See 2 more Smart Citations

Controlling factors of the oxygen balance in the Arabian Sea's OMZ

Resplandy

Lévy²,

Bopp³

et al. 2012

Biogeosciences

101

100

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Abstract. The expansion of OMZs (oxygen minimum zones) due to climate change and their possible evolution and impacts on the ecosystems and the atmosphere are still debated, mostly because of the unability of global climate models to adequatly reproduce the processes governing OMZs. In this study, we examine the factors controlling the oxygen budget, i.e. the equilibrium between oxygen sources and sinks in the northern Arabian Sea OMZ using an eddy-resolving biophysical model.Our model confirms that the biological consumption of oxygen is most intense below the region of highest productivity in the western Arabian Sea. The oxygen drawdown in this region is counterbalanced by the large supply of oxygenated waters originated from the south and advected horizontally by the western boundary current. Although the biological sink and the dynamical sources of oxygen compensate on annual average, we find that the seasonality of the dynamical transport of oxygen is 3 to 5 times larger than the seasonality of the biological sink. In agreement with previous findings, the resulting seasonality of oxygen concentration in the OMZ is relatively weak, with a variability of the order of 15 % of the annual mean oxygen concentration in the oxycline and 5 % elsewhere. This seasonality primarily arises from the vertical displacement of the OMZ forced by the monsoonal reversal of Ekman pumping across the basin. In coastal areas, the oxygen concentration is also modulated seasonally by lateral advection. Along the western coast of the Arabian Sea, the Somali Current transports oxygen-rich waters originated from the south during summer and oxygenpoor waters from the northeast during winter. Along the eastern coast of the Arabian Sea, we find that the main contributor to lateral advection in the OMZ is the Indian coastal undercurrent that advects southern oxygenated waters during summer and northern low-oxygen waters during winter. In this region, our model indicates that oxygen concentrations are modulated seasonally by coastal Kelvin waves and westward-propagating Rossby waves.Whereas on seasonal time scales the sources and sinks of oxygen are dominated by the mean vertical and lateral advection (Ekman pumping and monsoonal currents), on annual time scales we find that the biological sink is counterbalanced by the supply of oxygen sustained by mesoscale structures (eddies and filaments). Eddy-driven advection hence promotes the vertical supply of oxygen along the western coast of the Arabian Sea and the lateral transport of ventilated waters offshore the coast of Oman and southwest India.

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Section: Discussionsupporting

confidence: 72%

Section: Introductionmentioning

confidence: 68%

Section: Discussionmentioning

confidence: 99%

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Controlling factors of the oxygen balance in the Arabian Sea's OMZ

Resplandy

Lévy²,

Bopp³

et al. 2012

Biogeosciences

101

100

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“…It effectively mixes oxygen in regions with strong isopycnal oxygen gradients, thus substantially contributing to the ventilation of an OMZ across its lateral boundaries (Luyten et al, 1983a;McCreary et al, 2013;Gnanadesikan et al, 2013;Hahn et al, 2014). For the ETNA OMZ, the eddy-driven meridional oxygen flux could be quantified along 23 • W, both by a diffusive flux parameterisation and by eddy correlation (Hahn et al, 2014).…”

Section: Lateral Mixingmentioning

confidence: 99%

On the role of circulation and mixing in the ventilation of oxygen minimum zones with a focus on the eastern tropical North Atlantic

et al. 2015

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Abstract. Ocean observations are analysed in the framework of Collaborative Research Center 754 (SFB 754) "ClimateBiogeochemistry Interactions in the Tropical Ocean" to study (1) the structure of tropical oxygen minimum zones (OMZs), (2) the processes that contribute to the oxygen budget, and (3) long-term changes in the oxygen distribution. The OMZ of the eastern tropical North Atlantic (ETNA), located between the well-ventilated subtropical gyre and the equatorial oxygen maximum, is composed of a deep OMZ at about 400 m in depth with its core region centred at about 20 • W, 10 • N and a shallow OMZ at about 100 m in depth, with the lowest oxygen concentrations in proximity to the coastal upwelling region off Mauritania and Senegal. The oxygen budget of the deep OMZ is given by oxygen consumption mainly balanced by the oxygen supply due to meridional eddy fluxes (about 60 %) and vertical mixing (about 20 %, locally up to 30 %). Advection by zonal jets is crucial for the establishment of the equatorial oxygen maximum. In the latitude range of the deep OMZ, it dominates the oxygen supply in the upper 300 to 400 m and generates the intermediate oxygen maximum between deep and shallow OMZs. Water mass ages from transient tracers indicate substantially older water masses in the core of the deep OMZ (about 120-180 years) compared to regions north and south of it. The deoxygenation of the ETNA OMZ during recent decades suggests a substantial imbalance in the oxygen budget: about 10 % of the oxygen consumption during that period was not balanced by ventilation. Long-term oxygen observations show variability on interannual, decadal and multidecadal timescales that can partly be attributed to circulation changes. In comparison to the ETNA OMZ, the eastern tropical South Pacific OMZ shows a similar structure, including an equatorial oxygen maximum driven by zonal advection but overall much lower oxygen concentrations approaching zero in extended regions. As the shape of the OMZs is set by ocean circulation, the widespread misrepresentation of the intermediate circulation in ocean circulation models substantially contributes to their oxygen bias, which might have significant impacts on predictions of future oxygen levels.

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“…On the other hand, three-dimensional coupled physical-biogeochemical models that include the oxygen cycle have contributed to improve the description and understanding of dynamics of hypoxic events in various coastal regions (Peña et al, 2010), such as the Gulf of Mexico (e.g., Hetland and DiMarco, 2008), Black Sea (e.g., Gregoire and Friedrich, 2004), and Baltic Sea (e.g., Eilola et al, 2009). Such models have also been used to investigate the mechanisms driving the spatial distribution (McCreary et al, 2013) and seasonal evolution of the OMZ in the interior Arabian Sea (Resplandy et al, 2012). There is, however, no dedicated modeling study to date, addressing the mechanisms that drive the interannual oxygen variability along the WCI.…”

Section: Introductionmentioning

confidence: 99%

Positive Indian Ocean Dipole events prevent anoxia off the west coast of India

et al. 2017

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Abstract. The seasonal upwelling along the west coast of India (WCI) brings nutrient-rich, oxygen-poor subsurface waters to the continental shelf, favoring very low oxygen concentrations in the surface waters during late boreal summer and fall. This yearly-recurring coastal hypoxia is more severe during some years, leading to coastal anoxia that has strong impacts on the living resources. In the present study, we analyze a 1/4 • resolution coupled physical-biogeochemical regional oceanic simulation over the 1960-2012 period to investigate the physical processes influencing the oxycline interannual variability off the WCI, that being a proxy for the variability on the shelf in our model. Our analysis indicates a tight relationship between the oxycline and thermocline variations in this region on both seasonal and interannual timescales, thereby revealing a strong physical control of the oxycline variability. As in observations, our model exhibits a shallow oxycline and thermocline during fall that combines with interannual variations to create a window of opportunity for coastal anoxic events. We further demonstrate that the boreal fall oxycline fluctuations off the WCI are strongly related to the Indian Ocean Dipole (IOD), with an asymmetric influence of its positive and negative phases. Positive IODs are associated with easterly wind anomalies near the southern tip of India. These winds force downwelling coastal Kelvin waves that propagate along the WCI and deepen the thermocline and oxycline there, thus preventing the occurrence of coastal anoxia. On the other hand, negative IODs are associated with WCI thermocline and oxycline anomalies of opposite sign but of smaller amplitude, so that the negative or neutral IOD phases are necessary but not the sufficient condition for coastal anoxia. As the IODs generally start developing in summer, these findings suggest some predictability to the occurrence of coastal anoxia off the WCI a couple of months ahead.

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Dynamics of the Indian-Ocean oxygen minimum zones

Cited by 147 publications

References 60 publications

Controlling factors of the oxygen balance in the Arabian Sea's OMZ

Controlling factors of the oxygen balance in the Arabian Sea's OMZ

On the role of circulation and mixing in the ventilation of oxygen minimum zones with a focus on the eastern tropical North Atlantic

Positive Indian Ocean Dipole events prevent anoxia off the west coast of India

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