Turbulence in an estuarine embayment: Observations from Beatrix Bay, New Zealand

Abstract

This study characterizes turbulence in a stratified estuarine embayment using measurements of temperature gradient microstructure. The water column was generally strongly stratified with only moderate tidal forcing. Dissipation levels and turbulence length scales in the surface mixing layer, the pycnocline, the fluid interior, and the benthic boundary layer were measured and compared with expectations based on previous work in different systems. Parameters important for modeling, including the measured dissipation ratio (Td) and the gradient Richardson number (Rig), were calculated. The observations highlight a number of points. (1) The lack of strong tidal flows means that buoyancy effects in estuarine embayments are more important than in the estuary-proper. (2) The combination of reduced tidal mixing and interacting thermohaline stratification suggests the importance, in certain situations, of diffusive convection. (3) Despite the shallow depths (<40 m), a number of hydrodynamic regimes, vertically adjacent to one another, exist, resulting in a highly variable distribution of turbulence properties.