The Patos Lagoon hydrodynamics during an El Niño event (1998)

Abstract

TELEMAC-2D, a two-dimensional depth-averaged finite element flow model, has been chosen for modelling the hydrodynamics of the Patos Lagoon during the 1998 El Niño event. The model is initially calibrated against measurements carried out between 20 and 29/10/98, and good agreement is achieved between measured and predicted longitudinal velocities. Model validation is carried out by comparing measurements and predictions for one reference station in the estuarine area for the period between 23/05 and 06/06/98. Results indicate an excellent agreement between measurements and predictions, and the model is considered calibrated and validated for the Patos Lagoon hydrodynamics.A study of the Patos Lagoon hydrodynamics under the El Niño extreme conditions shows that the local and non-local wind effects associated with the freshwater discharge at the top of the lagoon promote longitudinal and lateral set-up/set-down mechanisms that influence the circulation. Results indicate that velocities in the lagoon and estuary during the extreme conditions observed in the El Niño period are much stronger than the normal periods.The response of the Patos Lagoon entrance to the local and non-local forcing is studied for the first time in terms of the inflow and outflow of water in the system. Results indicate that during the 1998 El Niño event the non-local forcing dominated, promoting exchanges that represent around 7% of the lagoon initial volume in the simulation. Strong freshwater discharges at the top of the lagoon combined with pressure gradients due to changes in elevation generated by the local and non-local wind favours the predominant seawards flow at the mouth, with outflows in the order of 5000m3s−1. The strong outflow observed during the El Niño extreme flow conditions generates huge plumes of freshwater into the adjacent coastal area.