Generation of the deep zonal jets in the eastern equatorial Pacific Ocean

It has been shown that the Equatorial Intermediate Current (EIC) in the Pacific Ocean, which is a time-mean westward current along the equator at intermediated depth (from 500m to at least 2000 m), has a nearly basin-wide structure, which is a unique feature of the Pacific EIC. In addition to the EIC, slowly varying vertically alternating eastward and westward currents called Equatorial Deep Jets (EDJs) are observed along the equator. Moreover, intra-seasonal waves have also been observed near the equator at 1000 m depth with significant amplitude in the eastern Pacific Ocean. Although these deep intra-seasonal waves in the eastern basin are considered an energy source for the EIC and EDJs, the relationship between them in the Pacific Ocean remains an open question. In this study, we conduct an idealized numerical simulation, which reproduces basic features of the nearly basin-wide Pacific EIC, EDJs, and deep intra-seasonal Yanai waves in the eastern Pacific Ocean. The momentum budget indicates that the Yanai waves provide westward momentum to the background field (sum of the EIC and EDJs) in the eastern part of the basin. Specifically, the vertical profile of the momentum convergence associated with the Yanai waves indicates that Yanai waves strengthen the westward background flow while the Yanai waves have little impact on the eastward background flow. This is attributed to the fact that the shear of the background zonal flows, the sum of the EICs and EDJs, is stronger (weaker) in the case of the westward (eastward) EDJs. Consequently, the acceleration by the Yanai waves does not cancel out in the time-mean field, indicating the westward momentum supply to the time-mean EIC. This westward acceleration is stronger than the previously reported eastward acceleration of the EIC by the EDJs in the eastern part of the basin, leading to the unique nearly basin-wide EIC in the Pacific Ocean.