Secondary circulation in a low-inflow, seasonally inverse estuary

Low inflow estuaries are ubiquitous among Mediterranean climate coastlines, but have been less well studied than systems with intense or regular freshwater input. An observational study was conducted to examine the impact of seasonal variation in longitudinal density gradients on secondary circulation in a low inflow estuary in Southern California. Between the summer to fall seasons, hypersalinity develops towards the estuary head and longitudinal temperature gradients are weakened, facilitating inverse longitudinal density gradients throughout the inner half of the bay. Full tidal cycle hydrographic surveys during spring tides confirm the inversion of longitudinal baroclinic pressure gradients. The second mode empirical orthogonal function of moored current profilers reveals that there is also an inversion in the sign of the transverse circulation patterns, suggesting that the alteration of lateral baroclinic pressure gradients by differential advection may contribute to the lateral momentum balance. However, full tidal velocity does not support a complete inversion of the flow field, which is likely a result of alternative mechanisms dominating the balance. Potential alternative mechanisms are assessed through analysis of momentum terms. A scaling is developed to determine when differential advection would be a dominant driver of secondary circulation in a low inflow estuary.