Eddies and the Transition Layer in Hawaiʻi

About this project

The ocean doesn’t hand off cleanly from the turbulent mixed layer above to the quiet interior below. Between them sits the transition layer — a dynamically charged zone where mesoscale stirring, submesoscale instabilities, and internal waves compete to move heat, salt, and energy across density surfaces. It is poorly constrained, rarely resolved, and consequential for everything from tracer transport to acoustic propagation.

The Hawaiian archipelago gives us a front-row seat. The islands interrupt the North Equatorial Current and disrupt the trade winds, spawning a persistent field of lee eddies through barotropic instability. At the same time, remotely generated eddies drift in from the eastern Pacific, carrying anomalous water masses and reshaping stratification on both sides of the island chain. The result is a two-source eddy environment that structures the transition layer between 100 and 400 m depth in ways that models don’t resolve and observations have only begun to capture.

We want to know where thermohaline variance transitions from stirring to dissipation, how lee eddies build submesoscale filaments along isopycnals, and whether the energy cascade runs forward to dissipation or feeds back to sustain the eddies themselves. These questions matter for tracers — and they matter for sound.

Approach

We combine existing observations from HOT, PacIOOS, and NOAA glider programs with targeted fieldwork using Seagliders, Wirewalkers, and microstructure profilers aboard the R/V Kaunānā. Adaptive deployments track eddy formation events in real time. Model comparison with UH’s high-resolution regional output links observed mixing scales to the spectral diagnostics that models can — and currently cannot — reproduce.