Density stratification in an estuary with complex geometry: Driving processes and relationship to hypoxia on monthly to inter‐annual timescales

The density field in Narragansett Bay (NB), a northeast U.S. estuary with complex geometry that suffers hypoxia, is described and related to driving factors using monthly means from time series observations at 9 sites during late spring to early fall 2001–2009. Stratification (deep‐shallow density difference) is dominated by salinity and strongest (4–7 kg m−3 in late spring) near rivers in the north and east. Shallow horizontal density gradients are about 0.2 kg m−3 km−1; deep densities have minor spatial and seasonal variations. Geographic structure in density, and its inter‐annual anomalies, is weaker than expected based on the complex geometry and large size relative to the internal deformation radius. Inter‐annual variability is primarily driven by river flow and weakly influenced by winds, contrasting nearby systems (Chesapeake Bay, Long Island Sound), likely due to reduced fetch and/or unfavorable alignment with prevailing winds. Stratification response to river flow follows 2/3 power scaling despite that the theory omits important NB attributes (complex geometry, depth‐varying horizontal gradients). Contrasting other systems (Delaware Bay, San Francisco Bay), horizontal gradients are at least as responsive to river forcing as theoretical 1/3 power scaling; depth‐dependent horizontal gradients or finite basin constraint of intrusion length may be responsible. Bay‐wide inter‐annual variations in seasonal hypoxia correlate with late spring stratification, though stratification peaks in the north and east with hypoxia most severe in the north and west. Long‐term response of stratification, and thus its role in hypoxia, to climate‐driven increases in river flow and temperatures will be dominated by the former.