Observations of Stratified Turbulent Mixing in an Estuary: Neap-to-spring Variations During High River Flow

Abstract

Observations of turbulent mixing, stratification and currents were made with a free-falling microstructure profiler and a 600-kHz ADCP in the Hudson River off Manhattan in May 1994. The ship was anchored in a relatively straight and uniform channel about 15m deep. Over 4 days of intense observations from 19 to 24 May 1994, the progression from neap tides to spring tides caused a severe reduction in stratification from an initial top-to-bottom salinity difference of 18 to 3–6. While low gradient Richardson numbers,Ri, occurred only in the weakly stratified bottom layer on flood during neap tides, lowRispanned the water column on the second half of ebb during spring tides. Turbulent dissipation rates varied roughly inversely toRi. During neap conditions, turbulent mixing was intense only in the bottom layer on flood, where eddy diffusivities,Kρ, reached more than 10−2m2s−1, while mixing was weak in the centre of the halocline near 5m. During spring tides, strong mixing occurred throughout the water column during the latter part of ebb with typicalKρ≈5×10−3m2s−1and typical vertical salt flux of 0·0015kg m−2s−1. If it existed in isolation, this flux would eliminate the haline stratification on a time scale of 4h. Ebb-time bands of lowRiand intense mixing showed increasing phase lag with increasing distance from the bottom. The spring-tide, ebb-time internal turbulent stress was relatively uniform with depth with a typical magnitude of 0·4Pa (4dyne cm−2). Turbulent overturning (Thorpe) scales ranged from below 1cm in the neap-tide halocline to a few metres in the weakly stratified bottom layer on flood. The strong spring-tide, ebb-time mixing had Thorpe scales of typically 0·5m throughout the water column. Ozmidov scales tracked Thorpe scales except in the weakly stratified layer above the bottom.