Microstructure Observations of Turbulent Mixing in a Partially Mixed Estuary. Part II: Salt Flux and Stress

Abstract

Turbulent mixing in the water column was observed with a microstructure profiler in the Hudson River estuary during two cruises in summer and fall 1995. The focus is on the estimation of turbulent salt flux and turbulent stress from measured viscous dissipation rates (ε), and on the tidal and fortnightly variability of these fluxes. In estimating eddy viscosity (Km) and eddy diffusivity (K?), the authors follow measurement/modeling techniques of Busch and Osborn, while prescribing a variable flux Richardson number (Rf) dependent upon the gradient Richardson number (Ri). It is argued that a steady-state production?dissipation balance holds in the turbulent kinetic energy budget. All turbulence characteristics varied strongly over semidiurnal tidal cycles and over the fortnightly cycle. Subject to complications arising from nontidal flows, the ?strongest? mixing occurred during flood on neap tides, and during ebb on spring tides. In the lower part of the water column during floods and spring ebb Km and K? reached maxima of 1?5 (?10?2 m2 s?1) and decreased roughly exponentially with increasing height by 1?3 decades. The smallest eddy coefficients occurred in the halocline during neap tide with Km ≈ 10?4 m2 s?1 and K? ≈ 10?5 m2 s?1. Mostly, the internal turbulent stress (τy) was close to 0 in the upper third of the water column and approached the bottom shear stress with decreasing height. Neap ebb had small |τy| even close to the bottom in response to stable stratification. During spring ebb, in contrast, τy decayed approximately linearly from the bottom shear stress to 0 at the surface. The largest turbulent salt flux (JS) of 8?10 (?10?4 kg m?2 s?1) occurred through much of the water column during spring ebbs. Most floods also had significant JS, while neap ebbs showed small JS. Among the estimated turbulence characteristics, JS is subject to the most pronounced systematic uncertainty owing to lack of knowledge of the variation of Rf as a function of Ri. The stress profiles and the turbulent salt flux estimated from the microstructure profiling are compatible with independent estimates based on moored observations of currents, density, and pressure analyzed by Geyer et al. in terms of the integral momentum and salt balances of the estuary. The role of turbulent mixing within the observed flow is qualitatively that envisioned in the early concepts of Pritchard from the 1950s.