Tidal and atmospheric influences on near‐surface turbulence in an estuary

Abstract

Estuarine near‐surface turbulence is important for transport, mixing, and air‐water exchanges of many important constituents but has rarely been studied in detail. Here, we analyze a unique set of estuarine observations of in situ atmospheric and full water column measurements, estimated air‐sea exchanges, and acoustic measurements of several terms in the turbulent kinetic energy (TKE) budget. Observations from a 5.1 m deep site in the Hudson River estuary include dissipation at 50 cm depth (ɛ50), as well as profiles of TKE, shear production of TKE (P), and net turbulent vertical TKE transport (TD). Regressions suggest that the principal controlling factor for ɛ50 was wind (through the surface shear velocity, U*) and that the surface heat flux and tidal currents played a secondary role. For ebb spring tides, the TKE budget at 50 cm depth was closed within noise levels. Ebbs had high ɛ50 due to local shear production, which nearly balanced ɛ50. Floods had TD approaching P in the upper water column but generally weak near‐surface shear and turbulence. Examining buoyancy fluxes that impact near‐surface stratification and can indirectly control turbulence, solar heat input and tidal straining caused similar buoyancy fluxes on a sunny, calm weather day, promoting ebb tide restratification. Wind‐driven mixing was found to dominate during a fall season storm event, and strong overnight heat loss after the storm helped delay restratification afterward. These results demonstrate the utility of combining detailed air‐sea interaction and physical oceanographic measurements in future estuary studies.