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

Abstract

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.

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

“…6, the "flanking jets" are much stronger than the off-equatorial zonal jets noted earlier. Ascani et al (2010) have suggested that these jets are maintained by Yanai waves, generated at the surface (possibly by instability of the energetic near-surface flow field forming tropical instability waves: von Schuckmann et al, 2008;Jochum et al, 2004), which break at depth. The jets show considerable variability (see Fig.…”

“…Their strengths have been quantified using subsurface drift trajectories from floats (Maximenko et al, 2005;Ollitrault et al, 2006) and repeated shipboard sections . Such currents have been reproduced by idealised process modelling (Mé-nesguen et al, 2009;Ascani et al, 2010;Qiu et al, 2013),…”

Effects of increased isopycnal diffusivity mimicking the unresolved equatorial intermediate current system in an earth system climate model

“…These currents are divided into two groups: a group of currents trapped on the equator that change sign with depth with a wavelength of about 500 m (the Equatorial Deep Jets) and a group of currents of large vertical scale that alternate with latitude every 1-2 ı between 10 ı S and 10 ı N (the Equatorial Intermediate Currents). Here we define this set of deep zonal currents as EICS (note, this definition is different from the one used in Ascani et al [2010]). …”

“…Because these important currents are not well resolved neither in coarse resolution Earth System Models ( Figure 1b) nor in eddy-resolving ocean circulation models [Ascani et al, 2010], questions remain about the impact of this missing circulation on simulated nutrients and oxygen in the EEP.…”

Influence of the Barotropic Mean Flow on the Width and the Structure of the Atlantic Equatorial Deep Jets

“…Assuming a barotropic zonal mean flow, which depends only on latitude, a necessary condition for an increase in width is a positive curvature of the zonal flow at the equator. In the equatorial Atlantic Ocean, the Equatorial Intermediate Current (EIC), a large verticalscale westward flow, is flanked by eastward jets at 28N/S called the North and South Intermediate Counter Currents (NICC and SICC, respectively; Richardson and Fratantoni 1999;Gouriou et al 2001;Ascani et al 2010;Greatbatch et al 2012). These currents can create the necessary environment for a reduced meridional gradient of ambient vorticity and hence a widening of the EDJs.…”