The Annual Modulation of Subtidal Estuarine Dynamics by the Gulf Stream

Abstract Boundary currents transport salt and heat along shelf breaks and exhibit distinct subtidal variability. The proximity of boundary currents to the coast suggests they could influence estuarine boundary conditions. However, the influence of boundary currents on estuarine dynamics is underexplored, and a mechanistic framework for the interaction remains underdeveloped. This study explores the subtidal interaction between the Gulf Stream, a western boundary current, and the St. Lucie Estuary, Florida, USA. The direct and comparative effects of the Gulf Stream on estuarine water levels, salinity, and temperature were quantified through statistical analyses and Variance Partitioning. The influence of the Gulf Stream on subtidal estuarine mass transport was isolated using scenario analyses through process‐based numerical modeling. Results show that variations in Gulf Stream transport are transmitted into the estuary through a quasi‐geostrophic balance, establishing a physical pathway linking boundary current variability to subtidal estuarine dynamics. Gulf Stream transport was inversely correlated with estuarine water levels and explained more variance than river discharge and local winds. Additionally, the variations in Gulf Stream transport impacted density distribution and subtidal mass transport. Decreases in Gulf Stream transport increased average estuarine density, strengthened longitudinal density gradients, and engendered net landward mass transport. Increases in Gulf Stream transport decreased average estuarine density, weakened longitudinal density gradients, and engendered net seaward mass transport. These responses should have implications for contaminant dispersion, residence times, and sediment transport.