Origin of the Intraseasonal Current Variability and Its Interannual Modulation at 1,000 m Depth in the Eastern Equatorial Pacific Ocean

It has been shown that the intraseasonal variability in the eastern Pacific Ocean at the intermediate depth (∼1,000 m) has a large amplitude along the equator. However, our understanding of the origin and detailed characteristics of the Equatorial Deep Intra-seasonal Variability (DEIV) is still limited. In this study, we use observed data near the surface and at 1,000 m depth as well as numerical simulations to evaluate the interannual modulation of the DEIV and its relation to the upper-ocean variability. The observed meridional component of eddy kinetic energy (EKE) at 1,000 m depth depicts large intraseasonal variability in the equatorial eastern Pacific, showing a significant seasonality with its maximum in boreal winter. The meridional EKE in winter also indicates a clear interannual variation of the DEIV. The upper-layer equatorially antisymmetric intraseasonal variability, associated with the tropical instability waves, also shows significant interannual modulation with a high correlation to the meridional EKE at 1,000 m with a 3-month lag. Though temperature and velocity observations above 500 m indicate two types of oscillation with 17- and 30-day periods in the upper layer, numerical experiments reveal that during non-La Niña years, only the 30-day component can generate vertically propagating Yanai waves, which effectively form the DEIV.