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?1 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 2°N and 2°S. In the present study, the authors use idealized numerical simulations and analytical solutions to demonstrate that the EICS currents within 2.5° 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 wave-induced 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.