Observed Deep Intraseasonal Variability in the Western Equatorial Pacific Ocean: Yanai Waves Originated from the Western Boundary

Abstract

Substantial deep intraseasonal kinetic energy (IKE) over 1000–3000-m depth with a 20–90-day period was detected from 5-yr mooring data in the western equatorial Pacific Ocean. Deep intraseasonal variability (ISV) is mainly concentrated in the meridional velocity V with a peak period of 50 days. The frequency–vertical mode number spectrum of V suggests that 81% of IKE is related to Yanai waves. The IKE is mainly concentrated on vertical modes of 2–6. Furthermore, the core frequency of deep ISV at 2464 m has a seasonal variation and negative correlation with the zonal velocity, which belongs to the upper deep branch of Pacific meridional overturning circulation (U-PMOC), and has a strong seasonal variability. Such a relationship suggests the Doppler shift effect and westward phase propagation of Yanai waves. The outputs from a reanalysis product identify observed characteristics of deep ISV and are used to study its energy source. The IKE over 1500–3000 m at the mooring site of 1428E originates from the intermediate layer over 1000–1750 m between 1348 and 1378E, which is generated by barotropic instability associated with the horizontal shear of the intermediate currents. Yanai waves are responsible for energy transmission from the intermediate layer of the western side to the deep layer of the eastern side. This research highlights the critical role of Yanai waves in the energy transfer between the intermediate and deep layers of the equatorial ocean.