Dynamics of the combined extra-equatorial and equatorial deep jets in the Atlantic

Abstract

The available meridional sections of zonal velocity with high vertical and meridional resolution reveal tall eastward jets at 2N and 2S, named the extra-equatorial jets (EEJ), straddling the stacked eastward and westward jets of smaller vertical scales right at the equator, the so-called equatorial deep jets (EDJ). In contrast to the semi-annual to interannual fluctuations in the zonal velocity component, the measured meridional velocity component is dominated by intraseasonal period. We argue here that the formation mechanism for both types of jets is linked to the intraseasonal variability in meridional velocity and the associated wave motions. A process study is complemented by high resolution primitive equation simulations based on a realistic background stratification and an oscillating forcing inside the western boundary layer. The forcing confined to the upper 2500 m excites a spectrum of waves, including a baroclinic short Mixed Rossby-Gravity (MRG), whose instability leads to the formation of the EDJ and short barotropic Rossby waves, whose instability gives rise to the EEJ. The modeled EEJ and EDJ response is confined to the same depth range as the forcing. Potential vorticity is homogenized within specific depth ranges of westward EDJ and is found to be latitudinally confined between 2N and 2S by the EEJ. The combined EDJ and EEJ increase lateral mixing at the equator but also act as barriers at ±2 degrees of latitude.

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

“…However, particularly the intermediate circulation (below 250 m) is still underestimated by these high-resolution simulations in realistic settings. To identify the physical mechanism responsible for the mean and variable zonal jets, idealised high-resolution models have been employed (Ménesguen et al, 2009;Ascani et al, 2010;Qiu et al, 2013). Such idealised models could furthermore be used, by including oxygen in the simulations, to study the roles of mean and variable advection in maintaining the tropical OMZs and to identify the mechanisms driving oxygen variability on interannual to multidecadal timescales.…”

“…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).…”

The Deep Equatorial Ocean Circulation in Wind-Forced Numerical Solutions

“…The scenario of d 'Orgeville et al (2007) and Ménesguen et al (2009) A second scenario for producing EICs over a long zonal extent has been proposed by Ascani et al (2010). Instead of DEIV being generated along the western boundary by instabilities of a DWBC, Ascani et al (2010) propose that the DEIV is generated by the instabilities of the upper-ocean equatorial current system, in particular those associated with TIWs (Fig.…”

“…To remedy this discrepancy, Ménesguen et al (2009) explore the case where the forcing is still along the western boundary but is now confined to the upper 2500 m -instead of appearing as a single baroclinic mode. In this case, the forcing excites not only the short low-baroclinic-mode MRG waves but also short barotropic equatorial Rossby waves.…”

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

“…Nevertheless, idealised model set-ups do support EDJs (e.g. D 'Orgeville et al 2007;Ménesguen et al 2009;Ascani et al 2015;Matthießen et al 2015) and hopefully analysis of these model runs can shed light on why realistic models are unsuccessful in simulating EDJs.…”