Contrasting Seasonal and Fortnightly Variations in the Circulation of a Seasonally Inverse Estuary, Elkhorn Slough, California

Three and a half years of hydrographic, velocity, and meteorological observations are used to examine the dynamics of upper Elkhorn Slough, a seasonally inverse, shallow, mesotidal estuary in central California. The long-term observations revealed that residual circulation in Elkhorn Slough is seasonally variable, with classic estuarine circulation in the winter and inverse estuarine circulation in the summer. The strength of this exchange flow varied both within years and between years, driven by the annual cycle of dry summers and wet winters. Subtidal circulation is a combination of both tidal and density-driven mechanisms. The subtidal magnitude and reversal of the exchange flows is controlled primarily by the density gradient despite the significant tidal energy. As the density gradient weakens, the underlying tidal processes generate vertically sheared exchange flows with the same sign as that expected for an inverse density gradient. The inverse density gradient may then further strengthen this inverse circulation. These data were collected as part of the Land/Ocean Biogeochemical Observatory and demonstrate the utility of long-term in situ measurements in a coastal system, as consideration of such a wide range of forcing conditions would not have been possible with a less comprehensive data set.