Axial Wind Effects on Stratification and Longitudinal Salt Transport in an Idealized, Partially Mixed Estuary

• Published in: Journal of physical oceanography Vol. 39; no. 8; pp. 1905 - 1920
• Main Authors: CHEN, Shih-Nan, SANFORD, Lawrence P
• Format: Journal Article
• Language: English
• Published: Boston, MA American Meteorological Society 01.08.2009
• Subjects: Brackish; Earth, ocean, space; Estuaries; Exact sciences and technology; External geophysics; Meteorology; Physics of the oceans; Salts; Sea level; Sediment transport; Stratification; Water column; Water depth; Wind; United States; Chesapeake Bay; ANW, USA, Chesapeake Bay; Axial stress; Salinity gradient; Entrainment (atmosphere, ocean); stratification; Regional scope; Wind effect; Pressure gradient; North America; Ocean model; winds; three-dimensional models; Hydrodynamic model; estuaries; Forcing; transient phenomena; Richardson number; Wind shear
• ISSN: 0022-3670; 1520-0485
• DOI: 10.1175/2009JPO4016.1

Abstract A 3D hydrodynamic model [Regional Ocean Model System (ROMS)] is used to investigate how axial wind influences stratification and to explore the associated longitudinal salt transport in partially mixed estuaries. The model is configured to represent a straight estuarine channel connecting to a shelf sea. The results confirm that wind straining of the along-channel salinity gradient exerts an important control on stratification. Two governing parameters are identified: the Wedderburn number (W) defined as the ratio of wind stress to axial baroclinic pressure gradient force, and the ratio of an entrainment depth to water depth (hs/H). Here W controls the effectiveness of wind straining, which promotes increases (decreases) in stratification during down-estuary (up-estuary) wind. The ratio hs/H determines the portion of the water column affected by direct wind mixing. While stratification is always reduced by up-estuary wind, stratification shows an increase-then-decrease transition when down-estuary wind stress increases. Such transition is a result of the competition between wind straining and direct wind mixing. A horizontal Richardson number modified to include wind straining/mixing is shown to reasonably represent the transition, and a regime diagram is proposed to classify the wind’s role on stratification. Mechanisms driving salt flux during axial wind events are also explored. At the onset and end of the wind events, barotropic adjustment drives strong transient salt fluxes. Net salt flux is controlled by the responses of subtidal shear dispersion to wind forcing. Moderate down-estuary winds enhance subtidal shear dispersion, whereas up-estuary winds always reduce it. Supporting observations from upper Chesapeake Bay are presented.