Internal Waves and Turbulence in the Upper Central Equatorial Pacific: Lagrangian and Eulerian Observations

• Published in: Journal of physical oceanography Vol. 32; no. 9; pp. 2619 - 2639
• Main Authors: Lien, R-C., D'Asaro, E. A., McPhaden, M. J.
• Format: Journal Article
• Language: English
• Published: 01.09.2002
• ISSN: 0022-3670; 1520-0485
• DOI: 10.1175/1520-0485-32.9.2619

Abstract In the shear stratified flow below the surface mixed layer in the central equatorial Pacific, energetic near-N (buoyancy frequency) internal waves and turbulence mixing were observed by the combination of a Lagrangian neutrally buoyant float and Eulerian mooring sensors. The turbulence kinetic energy dissipation rate ε and the thermal variance diffusion rate χ were inferred from Lagrangian frequency spectral levels of vertical acceleration and thermal change rate, respectively, in the turbulence inertial subrange. Variables exhibiting a nighttime enhancement include the vertical velocity variance (dominated by near-N waves), ε, and χ. Observed high levels of turbulence mixing in this low-Ri (Richardson number) layer, the so-called deep-cycle layer, are consistent with previous microstructure measurements. The Lagrangian float encountered a shear instability event. Near-N waves grew exponentially with a 1-h timescale followed by enhanced turbulence kinetic energy and strong dissipation rate. The event supports the scenario that in the deep-cycle layer shear instability may induce growing internal waves that break into turbulence. Superimposed on few large shear-instability events were background westward-propagating near-N waves. The floats' ability to monitor turbulence mixing and internal waves was demonstrated by comparison with previous microstructure measurements and with Eulerian measurements.