Bio-Optical, Physical, and Chemical Properties of a Loop Current Eddy in the Gulf of Mexico

Multi-sensor data collected with in situ and satellite instruments during August 2015 were used to understand how the three-dimensional bio-optical properties of a Loop Current Eddy (LCE) in the Gulf of Mexico (GoM) contrast those of the background waters, and how these properties are related to physical and chemical properties. With a surface radius of similar to 150 km and vertical extension to 1,400-1,500 m, the LCE was found to have highly stratified waters in two layers, with one lying just below the mixed layer (16 m) and the other coinciding with the pycnocline (similar to 120-200 m within the eddy). Strong contrasts were found in the bio-optical properties (chlorophyll -a concentration, absorption of particulate and dissolved matters, particulate backscattering, and beam-c attenuation) across the eddy core, eddy edge, and surrounding waters. Absorption coefficients (400 nm) of surface particulate and dissolved matters were similar to 4 times higher in the surrounding waters than in the eddy core, while surface reflectance (400 nm) in the eddy core was similar to 7 times higher than in the surrounding waters. The magnitude of deep chlorophyll maximum (DCM) was comparable (0.3-0.33 mg/m3) in all waters, but the depth of DCM in the eddy core (similar to 115 m) was much deeper than in the surrounding waters (60-75 m). These contrasts were found to correspond to different water masses with different physical (temperature, density, and buoyancy frequency) and chemical properties (salinity and dissolved oxygen concentration), where physical processes (river plume advection and eddy-induced downwelling) appeared to drive the changes in bio-optical properties.Loop Current Eddies (LCEs) are dominant circulation features in the Gulf of Mexico (GoM), which sporadically shed from the Loop Current (LC) with westward propagations. LCEs can transport heat, salt, dissolved oxygen, and other particulate and dissolved matters both horizontally and vertically. LCEs have been studied extensively, including their formations, shedding variability and prediction, moving paths, and interactions with topography. However, little is known about their bio-optical properties, especially in the context of their physical and chemical contrasts from the background waters. Here, using multiple observational data sets collected during August 2015, we characterize the three-dimensional bio-optical, physical, and chemical properties of a LCE. With a mean radius of similar to 150 km at the sea surface, the influence of the LCE extended to 1,400-1,500 m depth. The LCE was also found to have different bio-optical, physical, and chemical properties from the surrounding waters at both surface and depth, where physical processes appeared to drive the different water masses that led to the observed changes.