Circulation Patterns in the Ria of Pontevedra under Extreme Forcing

Two unusual circulation patterns were measured in the Ria of Pontevedra (NW Spain). The first one showed a positive two layered tidal pattern with water leaving the estuary through surface layers and entering it through the bottom ones. This unusual pattern is observed to depend on the particular summer stratification conditions, caused by upwelling events and solar radiation, and on the presence of easterly winds inside the estuary strong enough (~8 m s) to move surface water seawards. The second event showed the opposite behavior with a negative two layered tidal pattern with water leaving the estuary through bottom layers and entering it through the surface ones. This pattern is maintained by the existence of two water masses with different densities at intermediate layers. INTRODUCTION During the last few years, the hydrographical and hydrodynamical studies of the Ria of Pontevedra have been intensified. From a numerical point of view, the hydrodynamic pattern of this ria was simulated (Prego et al., 2000; Ruiz-Villareal et al., 2002) by means of a 3D hydrodynamic model (MOHID 2000) (Martins et al., 1998; Martins, 1999) adapted to the Galician estuaries. From these simulations it can be observed the tidal circulation along the estuary and the existence of a two layered residual circulation pattern. In GómezGesteira et al. 2003, the residence time at different points of the Ria of Pontevedra was calculated by means of a box model showing to be highly variable depending on fresh water input and sea water inflow. In addition to numeric models, the hydrodynamic behavior has also been studied by means of fieldwork measurements. In deCastro et al. (2000) the hydrodynamical characteristics of the ria was analyzed taking into account wind and tidal effect on water circulation from February to July 1998. They observed that wind direction is strongly dependent on topography in such a way that easterly winds force water to leave the estuary at surface layers and westerly winds force water to enter it. In addition, wind speeds higher than 4 m s are able to dominate the current at surface layers, even against tidal effect. In deCastro et al. (2003) a quasi-permanent * Grupo de Física Oceanográfica y de Costas, Facultad de Ciencias, Campus de Ourense, Universidad de Vigo, 32004 Ourense, Spain (1) [email protected] • Telephone: 00988387244 (2) [email protected] • Telephone: 00988387232 (3) [email protected] • Telephone: 00988387206 (4) [email protected] • Telephone: 00986214456 Grupo de Biogeoquímica Marina, Instituto de Investigaciones Marinas, CSIC, Eduardo Cabello 6, 36208 Vigo, Spain. Thalassas, 2003, 19 (2):55-63 An International Journal of Marine Sciences