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 (K _m ) and eddy diffusivity (K _ρ ), the authors follow measurement/modeling techniques of Busch and Osborn, while prescribing a variable flux Richardson number (R _f ) 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 K _m and K _ρ 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 K _m ≈ 10⁻⁴ m² s⁻¹ and K _ρ ≈ 10⁻⁵ m² s⁻¹. Mostly, the internal turbulent stress (τ _y ) 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 |τ _y | even close to the bottom in response to stable stratification. During spring ebb, in contrast, τ _y decayed approximately linearly from the bottom shear stress to 0 at the surface. The largest turbulent salt flux (J _S ) 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 J _S , while neap ebbs showed small J _S . Among the estimated turbulence characteristics, J _S is subject to the most pronounced systematic uncertainty owing to lack of knowledge of the variation of R _f 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.

Abstract

Variations of turbulent mixing in the water column and in the benthic boundary layer were observed with a microstructure profiler in the Hudson River estuary during two cruises in summer and fall of 1995. Variability patterns of stratification, shear, Richardson number (Ri), and turbulent dissipation rates (ϵ) were similar to those of earlier observations, with strong turbulence in the weakly stratified bottom layer, weak turbulence in the halocline during neap tides, and low Ri and high ϵ spanning the water column during spring ebbs. Depth-integrated turbulent dissipation rates ∫ ρϵ dz approximately equaled the work done by the tidal pressure gradient force adjusted for the change in tidal kinetic energy. Alternatively, a suitable scaling for ∫ ρϵ dz is also provided by the product of bottom shear stress and depth-average velocity τ _b υ , a relationship that fails at slack tide, however. At heights above bottom of z ≲ 0.3 m, and again excepting slack tides, observed ϵ were highly correlated with a law-of-the-wall dissipation rate ϵ _b = u ³ _∗ /(κz). Ratios ϵ/ϵ _b were close to 1. Here, the friction velocity is u∗, and von Kármán’s constant is κ. Most profiles of the normalized dissipation rate ϵ/ϵ _b showed a weak increase with z in the lowest 1.5 m, a departure from law of the wall scaling attributed to stable stratification. Such deviations from the law of the wall were smallest during spring floods when the near-bottom stratification was weak or unstable. Turbulence in the stratified water column well above the bottom appeared to be locally generated by shear instability even though ϵ _b and ϵ were correlated throughout the water column. During spring ebbs, ϵ exceeded ϵ _b by almost an order of magnitude at z ≳ 2 m.