Under downwelling conditions, buoyant coastal currents can transport river-derived material hundreds of kilometers down the coast. The fate of this material is ultimately determined by the extent of mixing with the ambient coastal waters. Despite the importance that mixing plays in controlling the along-shelf dispersal of river-derived materials, few studies have directly measured mixing in the far-field of river plume. This study will quantify mixing and circulation in the Chesapeake Bay by combining a comprehensive field program with numerical models. The Chesapeake is ideal for this study because of its predictable coastal current and its proximity to extensive research infrastructure at the USACE facility in Duck, NC. The four primary objectives of the study are: to quantify mixing within the nose region, the interior, and the offshore edge of the plume during downwelling conditions, to obtain detailed measurements of plume propagation speeds and the structure of the circulation behind the nose in a buoyant coastal current under downwelling favorable winds, to identify the dominant mechanisms responsible for mixing and entrainment of salt into the coastal current, and to document the dominant dynamic balances that govern plume propagation during downwelling conditions under a range of wind forcing. This comprehensive field program will consist of moored, shipboard, and microstructure-AUV observations of circulation and mixing within the plume. High resolution simulations using ROMS will complement the field program by providing more comprehensive estimates of mixing, diagnose the dynamics of the plume, and assess the role of wind, waves and internal shear in driving plume mixing.
Coastally-trapped river plumes represent a class of gravity currents that have been extensively studied using numerical models and laboratory experiments. Detailed field observations resulting from this study will provide new insights into mixing and circulation processes in a buoyant gravity current. This project will provide unprecedented measurement documenting the relative roles of mixing and advective straining in modifying the plume?s stratification. The relative importance of mixing in the nose region versus broadly distributed mixing behind the plume due to winds and waves will also be examined. Results will also elucidate the role of ageostrophic dynamics in the nose region and along the off-shore edge of the plume, once isopycnals become vertical. The field program will use adaptive sampling that combines both ship-based surveys and AUV-microstructure observations of a propagating coastal current. Utilizing the AUV to adaptively sample an ephemeral coastal feature is expected to lead to technological advances in observational oceanography. Lagrangian data will be complemented by a mooring array that will provide unprecedented near-surface resolution of stratification and shear in the far field of a river plume. Detailed near surface observations of turbulence will help to identify the dominant mixing mechanisms responsible for entraining ambient sea water into the plume.
This project will serve as the basis for one graduate student's thesis at Rutgers. Several undergraduate students will also be included in the field effort through the NSF-funded Research Experience for Undergraduates at Rutgers and Woods Hole. The objectives for Broader Impacts are to translate the science themes and data sets from this study into innovative undergraduate teaching materials and to encourage STEM learning for future scientists (grades K-12) through hands-on activities related to the research. The expertise of the NSF-funded Centers for Ocean Science Education Excellence Networked Ocean World and the Ocean Observing Initiative's Education and Public Engagement (OOI EPE) Implementing Organization (IO) will be leveraged to create an online learning module for undergraduates. These lessons will be disseminated broadly through an OOI EPE online portal and workshops at ocean science meetings (e.g., AGU and ASLO Oceans). The modules will highlight the benefits of teaching with authentic data with the focus of engaging undergraduates in the scientific process.
