The Northeast-Southwest Oscillating Equatorial Mode of the Tropical Instability Wave and Its Impact on Equatorial Mixing

The tropical instability waves (TIWs) in the eastern Pacific consist of waves with central periods of about 33 and 17days. While the former manifest as vortices north of the equator, and are known to modulate diapycnal mixing at their southern edge, the latter remain largely unexplored. Here the structure of the 17-day TIWs and the mechanism through which they may influence equatorial mixing are investigated based on long term in situ measurements and reanalysis data. Different from the well-recognized meridional velocity oscillation, the 17-day TIWs are found to induce northeast-southwest (NE-SW) velocity oscillations. They are confined within but asymmetric about the equator, differing from free Yanai waves, which they were commonly assumed to resemble. The vertical shear associated with the westward anomalous velocity superimposes on the shear of the mean flow, resulting in the strongest shear in the upper thermocline that is expected to facilitate diapycnal mixing therein. Plain Language Summary Observed from satellites, the tropical instability wave (TIW) in the eastern tropical Pacific Ocean is a 1,000-km-long gigantic combination of waves and vortexes. It emerges between the energetic zonally interleaving equatorial currents, impacts the atmosphere, and transfers enormous energy to the western Pacific Ocean and to the deep ocean as wellone of the research foci for both physical oceanographers and meteorologists. However, the structures of its huge body, and the associated complicated smaller-scale processes, remain unrevealed, primarily because the measurements that have been conducted, usually by research vessels, are too limited to derive a full picture. Here based on long-term observations at a hotspot of TIW and 4-D numerical model outputs, we identify the structure of TIW at the equator, which manifests as a pair of slanted clockwise and anticlockwise vortexes, and induces strong east-west oscillations. We also find the most efficient mechanism for the TIW to strengthen vertical velocity shear, shear instability, and ocean turbulent mixing. The findings therefore are important for understanding the TIW dynamics, TIWs' signatures in atmosphere-ocean interactions, and their impact on vertical heat transport from warmer sea surface to colder subsurface layers of the ocean.