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 ~20% of the variance at 5‐ to 30‐day periods, with another ~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 ~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. Plain Language Summary: The eastern equatorial Pacific is an area of intense ocean heat uptake due to the strong equatorial Sun. The fate of this heat depends on turbulent mixing processes in the upper ocean: heat may be returned quickly to the atmosphere or be carried into the deeper ocean and stored for many years, with obvious consequences for climate. The same is true of other important water properties such as CO2 content. Equatorial turbulence originates largely with vertically sheared currents. One important mechanism for this is tropical instability waves (TIWs), 1,000‐km waves that generate and amplify vertical shear (i.e., vertical differences in the horizontal current), which in turn generates the turbulence that mixes heat downward. Previous studies have shown that TIWs concentrated at the ocean surface carry the most energy. Here, we show that TIWs focused below the surface, while less energetic, can carry a disproportionate fraction of the shear, and it is of course that shear that drives turbulence and the resulting fluxes of heat, CO2 and other properties. We describe the times of the year when these subsurface TIWs are strongest, the periods at which they oscillate, and the depths at which they generate enhanced shear. Key Points: Subsurface tropical instability waves, with zonal velocity oscillation peaking at 70–90 m, are identified in the eastern equatorial PacificThe waves have periods of 5–20 days and amplitudes of 0.1–0.2 m/s and can persist for 3–7 months from July to the following FebruaryThe waves can induce periodically enhanced and reduced shear and hence mixing at ~50 m and above the core of the Equatorial Undercurrent [ABSTRACT FROM AUTHOR]