Assessing the Impact of Tides and Atmospheric Fronts on Submesoscale Physical and Bio-Optical Distributions near a Coastal Convergence Zone

Abstract

Optically-active constituents vary over short time and space scales in coastal waters, and they are impacted by a variety of complex, inter-related forcing processes. As part of the Integrated Coastal Bio-Optical Dynamics (ICoBOD) project, we conducted a field campaign in Mississippi Sound in the northern Gulf of Mexico during spring 2018 to examine the impact of the passage of atmospheric and tidal fronts on fine-scale physical and bio-optical property distributions in a shallow, dynamic, coastal environment. During a 25-day experiment, we deployed eight moorings over a roughly 7 × 7 km box encompassing a frontal zone, to collect a time series of physical and bio-optical measurements. We describe changes in diver visibility related to the passage of a short-duration, high-turbidity surface plume and nepheloid layer development/decay during a tidal cycle. Maximum nepheloid layer development was observed during low tide and lasted about 9–12 h. The strongest turbidity signal extended about 4–5 m above the bottom (approximately half of the water column), although anomalously elevated values were observed all the way to the surface. In addition, high-resolution (50 m) hydrodynamic model simulations provide insight into the frontal dynamics and aid interpretation of the observed patterns. Mooring observations confirmed model-predicted heat flux changes associated with the passage of an atmospheric cold front.

Bio-Optical Properties near a Coastal Convergence Zone Derived from Aircraft Remote Sensing Imagery and Modeling

“…During this data collection activity on 04/05/18, personnel in the aircraft that housed thermal and reflectance sensors observed a front created during flood tide from the south and northerly winds, as shown in Figure 2b. The vessel was then directed to the front, where subsequently uranine and rhodamine dye were deployed to aid in the understanding of mixing and subduction processes [1]. Radiometer readings were taken in and around the front.…”

“…The hydrodynamic model in this study is the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) [1]. This is the three-dimensional hydrodynamic model with a 50 m resolution and 50 vertical levels.…”

“…The coastal zone is the interface between land and the open ocean, and it is impacted by many processes, such as blooms, river discharge, and sediment resuspension. The previous publication examined the impact of tides and atmospheric fronts on bio-optical variability, including the development and decay of the nepheloid layers, using mooring datasets and compared them with models [1]. Here, we expand that work to use remote sensing and modeling to further examine fine-scale coastal variability.…”

“…In March and April of 2018, a variety of data were collected in the Mississippi Sound in and around an ocean front. A previous publication discussed the breadth of instrumentation used to collect data during these acquisitions and provided context and rationale for the study [1]. Hyperspectral remote sensing reflectance data, collected from airborne and Unmanned Aerial Vehicle (UAV) platforms, were included in this dataset.…”