Internal Wave Interactions with Equatorial Deep Jets. Part II: Acceleration of the Jets

Muench, Joanna E.; Kunze, Eric

doi:10.1175/1520-0485(2000)030<2099:iwiwed>2.0.co;2

Cited by 14 publications

(19 citation statements)

References 31 publications

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“…Resolution alone is not the key; simulations with up to 400 levels and 1 /88 resolution (not shown here) did not substantially strengthen the EICS. Other factors could be the numerical code itself, especially the advective scheme that may not conserve PV, or small-scale phenomena such as inertia-gravity waves that are neglected in OGCM simulations but may contribute to the PV dissipation (Muench and Kunze 2000).…”

Section: Discussionmentioning

confidence: 99%

Deep Equatorial Ocean Circulation Induced by a Forced–Dissipated Yanai Beam*

Ascani

Firing

Dutrieux

et al. 2010

Journal of Physical Oceanography

100

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A complex pattern of zonal currents below the thermocline has been observed in the equatorial Pacific and Atlantic Oceans. The currents have typical speeds from 10 to 15 cm s 21 and extend as deep as 2500 m. Their structure can be divided into two overlapping parts: the equatorial deep jets (EDJs), centered on the equator and alternating in the vertical with a wavelength of several hundred meters, and the Equatorial Intermediate Current system (EICS), composed of currents with large vertical scale and alternating with latitude over several degrees on either side of the equator. The strongest EICS current is a westward flow on the equator flanked by eastward currents at 28N and 28S.In the present study, the authors use idealized numerical simulations and analytical solutions to demonstrate that the EICS currents within 2.58 from the equator could result from the self-advection with dissipation of a downward-propagating beam of monthly periodic Yanai (Rossby gravity) waves. The zonally restricted beam is generated in the eastern part of the basin by instabilities of the swift near-surface equatorial currents. For a weak Yanai wave amplitude and no dissipation, mean Eulerian currents resembling the three strongest EICS currents are obtained but only within the beam; in this case, the Eulerian flow is balanced by the waveinduced Stokes drift, yielding a zero-mean Lagrangian flow, and the water parcels conserve their potential vorticity (PV) and are stationary over a wave cycle. For larger amplitudes, the Yanai waves break, losing their energy to small vertical scales where it is dissipated. This dissipation changes the mean (wave averaged) PV of a water parcel within the beam, allowing the parcel to have a persistent equatorward drift across PV contours. This can be viewed as a wave-induced Sverdrup transport; by continuity and by virtue of the westward group velocity of long Rossby waves, this Lagrangian-mean meridional flow requires a Lagrangian-mean zonal flow within and to the west of the beam, with a meridional structure consistent with the three strongest EICS currents. This mechanism of EICS formation is active in some ocean general circulation models; its importance in the ocean remains to be evaluated.

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Section: Discussionmentioning

confidence: 99%

Deep Equatorial Ocean Circulation Induced by a Forced–Dissipated Yanai Beam*

Ascani

Firing

Dutrieux

et al. 2010

Journal of Physical Oceanography

100

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“…In the model simulations, momentum flux divergence of the background internal wave field at critical layers as proposed by Muench and Kunze [1999] and Muench and Kunze [2000] can be ruled out as a forcing mechanism for the stacked jets, because the internal wave field is almost unresolved by the hydrostatic models. We can thus conclude that instability of the large‐scale flow is sufficient to generate stacked jets with characteristics that are in agreement with observations.…”

Section: Discussionmentioning

confidence: 99%

“…In recent years nonlinear mechanisms have also been proposed to explain the stacked jets. Muench and Kunze [1999] and Muench and Kunze [2000] showed that energy can be transferred from the internal wave field to the equatorial stacked jets by momentum flux divergence in vertical critical layers (given by the large‐scale flow). This process could thus sustain the stacked jets despite energy loss due to dissipation within the jets and energy reflection at meridional boundaries.…”

Section: Introductionmentioning

confidence: 99%

Stacked jets in the deep equatorial Atlantic Ocean

Eden¹,

Dengler

2008

J. Geophys. Res.

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[1] Middepth current measurements in the equatorial Atlantic are characterized by elevated levels of energy contained in zonal flows of high baroclinic mode number. These alternating zonal flows, often called equatorial stacked jets, have amplitudes up to 20 cm s À1 and vertical wavelengths of 600 m. The jets are most pronounced in the depth range between 500 and 2500 m. Repeated direct velocity observations at 35°W indicate that the jets are coherent within ±1°of the equator. Individual jets can persist for 1-2 years, but they appear and decay rather irregularly. The equatorial stacked jets are also found in realistic general circulation model simulations. The features grow in amplitude with increasing horizontal and vertical model resolution. However, even at very high model resolutions, their amplitudes are still underestimated. In all model simulations, high levels of energy related to the stacked jets are found in the vicinity of the western boundary currents (WBCs). Depth range and strength of the WBCs in different experiments are related to depth range and strength of the jets. In the interior, stacked jets are characterized by eastward wave propagation suggesting that high baroclinic mode Kelvin waves radiate energy generated in the WBC into the interior and form the stacked jets.

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“…In the model to be described below, we apply an oscillatory forcing to balance the dissi- (2010) show that downward propagating Yanai waves, generated by tropical instability waves and that break at depth, are able to generate the quasi-steady flanking jets with large vertical scale terestingly, Muench and Kunze (1999) and Muench and Kunze (2000) an analysis of both ARGO float data (Lebedev et al 2007) and cruise data (the cruises are 87 listed in Table 1). Section 4 provides a summary and discussion.…”

mentioning

confidence: 99%