Flow Regimes and Adjustment to Wind‐Driven Motions in Lake Pontchartrain Estuary: A Modeling Experiment Using FVCOM. Issue 11 (25th November 2018)

• Record Type: Journal Article
• Authors: Li, Chunyan; Huang, Wei; Chen, Changsheng; Lin, Huichan
• Is Part Of: Journal of geophysical research. Volume 123:Issue 11(2018)
• Page Start: 8460
• Page End: 8488
• Publication Date: 2018-11-25
• Subjects: wind‐driven seiche -- short adjustment -- quasi steady state -- FVCOM -- wind‐driven circulation -- Lake Pontchartrain; Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2018JC013985
• Languages: English
• ISSNs: 2169-9275

Abstract

Abstract: Lake Pontchartrain is a brackish estuary and restricted lagoon in the microtidal northern Gulf of Mexico. Using a validated Finite Volume Community Ocean Model (FVCOM), we study the flow regimes and wind‐driven adjustment of circulation in this system. It is found that tidal currents are only significant in the eastern end near the open boundary. Local wind inside the estuary is important in driving the circulation including coastal currents. Numerical experiments confirm that the subtidal wind‐driven flow is in a quasi‐steady balance between wind stress and surface slope. Our results show that the adjustment of surface level inside the system to a sudden change in wind is a transient seiche, which dissipates within two to three cycles, each lasting ~3 hr. After the oscillations, the system reaches a new equilibrium. We show in theory that this is a damped oscillation and as wind changes, an adjustment to a new equilibrium goes much faster than a complete tidal period. This leads to the quasi steady state balance of subtidal wind‐driven flows with essentially no phase lag with wind even though wind changes with time continuously. Wind‐induced circulation behaves differently from the Csanady model with Earth rotation but similar to the prismatic lake model of Engelund without Coriolis: The coastal and shallow regions tend to have flows in the direction of wind, while the interior and deeper waters have return flows against the wind especially in bottom layer. The surface flow may not be in the direction of wind, while the bottom usually has broad and uniform counter wind flows. Plain Language Summary: In a choked or restricted shallow lagoon, wind‐driven flows are confirmed to be a surprising quasi steady state. Numerical experiments show that any change in wind would produce a hydrodynamic perturbation in the form of seiche that dissipates quickly in two to three cycles, leading to a fast return to a new equilibrium or quasi steady state. This warrants examination of wind‐driven flows at constant speed for different scenarios (wind from different directions) and not to worry too much about the temporal variation of wind or history of wind. Earth rotation in such system is found to be negligible for wind‐driven circulations: The experiment shows that the relative error is only 0.3% if Coriolis force is turned off for Lake Pontchartrain. Key Points: Seiche generated by wind is dissipated in two to three cycles, adjusting the system to a new balance, leading to a quasi steady state Subtidal cross bay slopes are determined by local wind, generating strong coastal currents; neglecting rotation only produces 0.3% error Bottom flows are counterwind, broad, and uniform; Engelund model is applicable for circulation