A High-Resolution Modeling Study of the Circulation Patterns at a Coastal Embayment: Ria de Pontevedra (NW Spain) Under Upwelling and Downwelling Conditions

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 Ria 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 Ria. Two representative estuarine circulation patterns were studied: a coastal upwelling event and a downwelling event associated with a maximum river runoff of 84 m(3)/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 Ria, 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 Ria 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 Ria 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.

Automated summary

Estuarine exchange flow plays a significant role in exchanging biogeochemical properties between coastal ocean and estuaries. This study focused on the circulation patterns inside Rias Baixas and Ria de Pontevedra using a high-resolution ROMS-AGRIF system configuration. The results revealed two representative estuarine circulation patterns, the impact of different wind forcing on velocity field variability, and the presence of vertical clockwise circulation cells associated with ocean water subduction. These findings provide new insights into ocean-estuarine water exchanges and their implications for marine ecosystem productivity.