Joint IPRC-Oceanography Seminar

Andrei Natarov
International Pacific Research Center
University of Hawai’i at Manoa

"Persistent Presence of Small Vertical Scale Velocity (SVS) Features During Three-Dimensional Equilibration of Equatorial Inertial Instability"

Abstract: Small vertical scale velocity features (SVSs) are ubiquitous in the upper equatorial ocean but their life cycle and role in large scale dynamics are only beginning to be understood. At the seminar I will discuss the development of SVSs generated through inertial instability of a prototypical equatorial zonal flow with a uniform meridional shear. While previous studies employ a zonally symmetric setting, in which the flow is constrained to remain invariant in the stream-wise direction throughout its evolution, here we use a fully three-dimensional framework.

We choose a setting, in which the fastest growing linear modes are zonally symmetric and the initial perturbation is almost zonally symmetric, so that the flow remains nearly two-dimensional until the symmetric instability is fully neutralized. The secondary instabilities of the modified zonal mean flow favor non-symmetric modes, which leads to three-dimensionalization of the flow. It has been previously conjectured in the literature that the dominant secondary instability is an inflection point-type barotropic instability that selects disturbances with a large vertical scale and the necessary condition for which is the reversal of the meridional gradient of the background potential vorticity. We show that although the conditions necessary for the barotropic instability indeed arise after the neutralization of the symmetric instability, the dominant secondary modes form a sequence of high vertical mode disturbances excited through non-symmetric inertial instability of the evolving zonal mean flow. Barotropic eddies, on the other hand, remain undeveloped and do not play a significant role in the dynamics. The vertical mode spectrum is dominated by high baroclinic modes throughout the 800-day long simulation and is maintained everywhere in the computational domain, with the radiation of high vertical mode zonally non-symmetric inertia-gravity waves eventually filling the off-equatorial regions. While at the end of the simulation the SVSs are present in one form or another at every latitude, their intensity and kinetic energy dissipation is much higher closer to the equator. Long life span of these features suggests that linking high kinetic energy dissipation signals to particular generating events, such as periods of the mean flow instability, or wind bursts, is problematic close to the equator, where the background levels are high, but may be possible away from the equator where the background is less "noisy".

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Oceanography, Manoa Campus

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