The Subsurface Mode Tropical Instability Waves in the Equatorial Pacific Ocean and Their Impacts on Shear and Mixing

The tropical instability waves (TIWs) in the eastern tropical Pacific have generally been considered as surface-intensified structures resembling the first baroclinic mode. Here, we report on the existence of subsurface-intensified TIWs on the equator. These TIWs are primarily manifested in zonal velocities, inducing maximum velocity oscillations at 70-90 m depth with amplitudes of 0.1-0.2 m/s and periods of 5-20 days. They account for similar to 20% of the variance at 5- to 30-day periods, with another similar to 50% being contributed by the surface-intensified TIWs. These waves are most significant during the TIW seasons; they are energized in part by barotropic instabilities and usually last for 3-7 months. Via interacting with the mean flow, they can induce strong out-of-phase shear changes between similar to 50-m depth and just above the Equatorial Undercurrent core and may lead to complex diapycnal mixing structures. Their horizontal structures, generation mechanism(s), and large-scale impacts remain to be disclosed.