Biophysical Drivers of Surface Chlorophyll‐a in the Loop Current and Frontal Eddies in the Gulf of Mexico

Abstract

Loop Current Frontal Eddies (LCFEs) are eddies that generate through instabilities of the Loop Current (LC). LCFEs have long been considered oases for fish communities due to their elevated chlorophyll‐a (Chl‐a) concentrations, typically attributed to their cyclonic nature. However, the specific drivers of the increased Chl‐a within LCFEs remain unexplored. Using multiple satellite data sets (absolute dynamic topography and Chl‐a), a high‐resolution (∼4 km) Hybrid Coordinate Ocean Model (HYCOM) reanalysis and riverine data, we identify several key physical and biogeochemical factors influencing Chl‐a concentrations within LCFEs and in the eastern Gulf. These include the LC phase, LCFE position, eddy‐driven horizontal and vertical advection, seasonal mixed layer variability, and Mississippi‐Atchafalaya River discharge and associated nutrient concentrations. We find that mixed layer seasonality and riverine nutrient input often compete to shape intra‐annual Chl‐a variability. In winter, when riverine nutrients are minimal, deeper mixed layers and reduced stratification facilitate Chl‐a increase in offshore regions. In contrast, during summer, increased riverine nutrient concentration and stronger wind‐induced cross‐shelf transport compensate for reduced vertical fluxes caused by strong stratification and shallow mixed layers. Additionally, eddy‐driven horizontal and vertical advection transports nutrient‐rich waters from the continental shelf and the deep ocean into the eddy, playing a key role in modulating their Chl‐a concentrations. Our findings reveal that the relationship between LCFEs and Chl‐a is more complex than previously thought and underscore the importance of physical and biogeochemical processes in shaping Chl‐a patterns. Understanding these dynamics can improve fishery management by increasing Chl‐a predictability across temporal and spatial scales.