Destabilization of mixed Rossby gravity waves and the formation of equatorial zonal jets

Hua, Bach Lien; d’Orgeville, Marc; Fruman, Mark D.; Ménesguen, Claire; Schopp, Richard; Klein, Patrice; Sasaki, Hideharu

doi:10.1017/s0022112008002656

Cited by 48 publications

(90 citation statements)

References 34 publications

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“…15) on monthly timescales, but are almost always unidirectional (especially in the Northern Hemisphere); their longitudinal dependence along the Equator is currently uncertain. The EDJ are also thought to be generated by downward propagating Yanai waves, in this case by barotropic instability of these waves as discussed by Hua et al (2008) Ménesguen et al, 2009). The EDJ show downward phase propagation but upward energy propagation, consistent with the above theory, and lead to variability with a roughly 4.5 year period throughout the water column within 2 degrees latitude on either side of the Equator and above about 3000 m in depth (Brandt et al, 2011).…”

Section: Advectionmentioning

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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Section: Advectionmentioning

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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“…Furthermore, the vertical symmetry of their experiment (the experiment is continually forced by a second baroclinic mode, intraseasonal Yanai wave, uses vertically uniform stratification and free slip/no normal flux boundary conditions at the top and bottom boundaries) suggests that this would not change in a longer model run. We also note a distinction between a generation mechanism, that sets the scale of the deep jets, like the theories of D 'Orgeville et al (2007) or Hua et al (2008), and a sustaining mechanism, that can maintain the deep jets in a quasi-equilibrium, as in Ascani et al (2015) or Muench and Kunze (2000). The two mechanisms are thought to operate together and are not in any way contradictory to each other.…”

Section: Introductionmentioning

confidence: 79%

“…Regarding the second question, one possibility is that in addition to the enhancement mechanism for pre-existing basin modes, there is also a generation mechanism at work that re-establishes the energy in the higher modes. A candidate for the later generation mechanism is the mechanism of Hua et al (2008), in which the vertical scale of the EDJs is set either by the destabilization of intraseasonal Yanai waves or by resonant triad interactions involving intraseasonal Yanai waves. All these topics require further investigation and are topics for future research.…”

Section: Discussionmentioning

confidence: 99%

“…The two mechanisms are thought to operate together and are not in any way contradictory to each other. Indeed, the mechanism described by Ascani et al (2015) somewhat resembles, and may simply be another way to interpret, the resonant triad mechanism described in Appendix A of Hua et al (2008).…”

Section: Introductionmentioning

confidence: 88%

“…D 'Orgeville et al (2007) showed, in an idealised study, that destabilisation of Yanai waves (also called Mixed Rossby Gravity waves) generated near the western boundary and that have eastward group velocity into the interior of the ocean, can drive a high baroclinic mode structure following the argument of Hua et al (2008). Eden and Dengler (2008) also investigated the generation of vertically stacked jets in a high resolution ocean model where the energy source was thought to be fluctuations of the Deep Western Boundary Current in the model.…”

Section: Introductionmentioning

confidence: 99%

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The emergence of equatorial deep jets in an idealised primitive equation model: an interpretation in terms of basin modes

et al. 2017

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Ocean circulation models do not generally exhibit equatorial deep jets (EDJs), even though EDJs are a recognised feature of the observed ocean circulation along the equator and they are thought to be important for tracer transport along the equator and even equatorial climate. EDJs are nevertheless found in nonlinear primitive equation models with idealised box geometry. Here we analyse several such model runs. We note that the variability of the zonal velocity in the model is dominated by the gravest linear equatorial basin mode for a wide range of baroclinic vertical normal modes and that the EDJs in the model are dominated by energy contained in vertical modes between 10 and 20. The emergence of the EDJs is shown to involve the linear superposition of several such neighbouring basin modes. Furthermore, the phase of these basin modes is set at the start of the model run and, in the case of the reference experiment, the same basin modes can be found in a companion experiment in which the amplitude of the forcing has been reduced by a factor of 1000. We also argue that following the spin-up, energy must be transferred between different vertical modes. This is because the model simulations are dominated by downward phase propagation following the spin-up whereas our reconstructions imply episodes of upward and downward propagation. The transfer of energy between the vertical modes is associated with a decadal modulation of the EDJs.

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Mooring observations of equatorial currents in the upper 1000 m of the western Pacific Ocean during 2014

Wang

Guan

et al. 2016

JGR Oceans

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Destabilization of mixed Rossby gravity waves and the formation of equatorial zonal jets

Cited by 48 publications

References 34 publications

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

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

The emergence of equatorial deep jets in an idealised primitive equation model: an interpretation in terms of basin modes

Mooring observations of equatorial currents in the upper 1000 m of the western Pacific Ocean during 2014

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