Spatial variation of cold front wind-driven circulation and quasi-steady state balance in Lake Pontchartrain Estuary

Abstract

Circulation in a low-salinity estuary with restricted openings to the ocean is mainly driven by wind and to a lesser extent by water level fluctuations at the open boundaries. Numerical model experiments are conducted using a baroclinic Finite Volume Community Ocean Model (FVCOM) for wind-driven circulations during a period encompassing a total of 16 cold fronts in the Lake Pontchartrain Estuary. The work is focused on the examination of the spatial structure of circulation in response to local and remote winds. A quasi-steady state balance between the surface slope and wind stress is further analyzed. The accuracy of the balance is evaluated by comparing the FVCOM calculated surface slope with that from the steady state balance equation. Results show that this balance is more accurate in the cross-estuary direction than that in the along-estuary direction (R2 ∼ 0.94 vs. 0.60). This difference in the accuracy of the quasi-steady state balance between the cross- and along-estuary directions is caused by the open boundary - a tidally-induced mean slope exists. In addition, even if the tidal effect is removed, the accuracy still decreases toward the open end for slopes in both directions. Remote wind effect and residual flow through the eastern open boundary tend to introduce a departure from the quasi-steady state balance in both along- and cross-estuary directions. Remote wind effect decreases into the interior due to bottom friction. Local wind effect tends to produce downwind flows in coastal, shallow water regions and on the surface, but upwind flows near the bottom, a result consistent with barotropic wind-driven circulations; while the remote wind effect is important mostly near the open boundary. Furthermore, quasi-steady state balance is more accurate in the along-estuary direction before cold front passages then after, because of the relatively higher occurrence of the wind in that direction before the cold fronts than after. In contrast, this quasi-steady state balance is less accurate in the cross estuary direction before cold front passages than after, because of the relatively lower occurrence of the wind in that direction before the cold fronts than after.