Abstract
Estuarine exchange flow plays an important role in the biogeochemical properties exchange between the coastal ocean and the estuaries. Circulation patterns inside the Rias Baixas (NW Iberian Peninsula) have been widely studied, while the Ría de Pontevedra has received less attention. A high-resolution ROMS-AGRIF system configuration with three online two-way nesting domains was used and validated to enhance the knowledge about the circulation patterns of this Ría. Two representative estuarine circulation patterns were studied: a coastal upwelling event and a downwelling event associated with a maximum river runoff of 84 m3/s. An analysis using Empirical Orthogonal Functions (EOF) was applied to the horizontal subtidal velocity fields at the surface layer and at 20 m depth and to the subtidal normal velocity across vertical cross-sections. Upwelling events are characterized by a reinforcement of the positive estuarine circulation, in which the surface waters outflow and deep waters inflow at the main sill of the Ría, whilst full water column inflow (outflow) is observed at the northern (southern) openings. Conversely, downwelling events induce an opposite behavior, consisting of a negative estuarine circulation with the surface (deep) inflow (outflow) at the main sill of the Ría and outflow (inflow) at the northern (southern) openings. Evidences of a front splitting two vertical clockwise circulation cells were found. These cells are associated with the ocean water subduction that enters through the surface layer under downwelling favorable winds and this surface inflow opposes to the direction of the inner estuarine circulation. The EOF study confirms that shelf wind forcing has an important contribution to the variability of the velocity field, inducing positive or negative estuarine like circulation. The temporal principal components (PC) of EOF analysis present correlations of 0.92 between the meridional wind component and the first mode, and 0.77 between the zonal wind component and the second mode. Third EOF mode represents the circulation during transitional winds when some vorticity structures are visible. The realistic high-resolution simulation of the physical conditions at the Ría de Pontevedra provides new insights into the ocean-estuarine water exchanges and allow the visualization of recirculation processes and characteristic structures that have consequences in the marine ecosystem and in their productivity.
Highlights
Estuarine systems are among the most active areas of the ocean
Model validation is presented through comparison of sea surface height, temperature and salinity data
Results of horizontal and vertical Empirical Orthogonal Functions (EOF) analysis investigating the relationship between wind and surface circulation are presented
Summary
Estuarine systems are among the most active areas of the ocean. They provide critical habitat for species that are valued culturally, recreationally, and commercially, but they are under increased pressures, threatening the water and land integrities (Giddings and MacCready, 2017). The Ría de Pontevedra is an estuary located on the West Galician coast (NW Spain; Figure 1A), between latitudes 42.24◦ N and 42.44◦ N and longitudes 8.65◦ W and 9◦ W and is the second southernmost of the Rias Baixas (Figure 1B) It has a surface area of ∼147 km and a volume of ∼3.45 km (Aguiar Fernandez, 2016), and as the other Rias Baixas, it is characterized by a V-shaped coastline and bathymetry, which gets broader in the NE–SW direction. The Cíes islands are separated from the continent by another sill (southern opening) with a width of 2.92 km and a maximum depth of ∼30 m This latter opening is the natural connection that facilitates the exchange between the Ría de Vigo and the Ría de Pontevedra (Figure 1C). Even though fishing and aquaculture (e.g., mussel raft culture) resources are socially and economically important in the Ría, there are industrial activities including a paper production factory and several small shipyards that release chemicals and effluents to the estuary
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