Coupling between the California Current System and a coastal plain estuary in low riverflow conditions

Willapa Bay, a partially mixed coastal plain estuary, is located on the shoreward side of a narrow, deep continental shelf whose water properties fluctuate on several day scales in response to alternating periods of upwelling and downwelling. Hydrographic surveys as well as water property and velocity time series at a number of sites both within the estuary and on the adjacent coast are used to examine water property and circulation patterns in the estuary during a low runoff period. The data demonstrate that variability is significant (up to 3 psu, ∼2°C and 10 cm s −1 at 5 km from the estuary mouth) and that this variability is determined primarily by the variability in the coastal ocean rather than by estuarine processes such as changes in riverflow or neap‐spring variation in mixing. Density changes near the mouth of the estuary that result from upwelling or downwelling of coastal water are consistent with transmission to the estuary primarily through a gravity current mechanism, which modifies the along‐estuary density gradient and hence the gravitational circulation within the estuary. Tidal stirring is likely also important to the modification of estuary water properties. New water moves up the estuary at a rate on the order of 10 cm s −1 . Associated Eulerian residual velocity fluctuations propagate up estuary about 50% faster than water properties, indicating that up‐estuary transmission of the ocean water perturbation may also have internal wave‐like characteristics. The modulations in estuarine circulation and water properties lag local wind stress fluctuations (hence upwelling or downwelling) by more than a day near the estuary mouth and several days farther up the estuary.