An unusual two layered tidal circulation induced by stratification and wind in the Ría of Pontevedra (NW Spain)

A two-layered tidal circulation pattern has been observed in the Ria of Pontevedra at an anchored station (24 m deep) located at 42.3917 degree N and 8.7380 degree W. During a tidal cycle water leaves the estuary continuously through surface layers (till 8-12 m depth) and enters the estuary through bottom layers. This unusual pattern in a partially mixed estuary is observed to depend on the particular summer stratification conditions and on the presence of easterly winds inside the estuary. The vertical stratification is generated by solar heating and favourable upwelling conditions on the adjacent shelf, which reinforces the estuarine positive circulation and provokes the inflow of typical upwelling water (ENAW) through bottom layers. On the other hand, the easterly winds inside the estuary are strong enough (around 8 m s super(-1) ) to force the surface water to move, even against tide. This situation is compared to two typical patterns observed in the estuary, a winter situation where wind drives the surface water against tidal forcing and a spring situation where the water column follows the tidal pattern. In both cases, bottom layers are hardly influenced by wind forcing.