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

Abstract 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.