Formation of an Azores Current Due to Mediterranean Overflow in a Modeling Study of the North Atlantic

Abstract

A mechanism for the formation of the Azores Current is proposed. On the basis of observations and model results, it is argued that the primary cause of the Azores Current is the water mass transformation associated with the Mediterranean overflow in the Gulf of Cadiz. Observations show that the transport of the Mediterranean outflow water through the Strait of Gibraltar increases significantly as it descends the continental slope by entraining the overlying North Atlantic Central Water. This entrainment process introduces a sink at the eastern boundary to the ocean upper layer in addition to the inflow into the Mediterranean. Such a sink is capable of inducing strong zonal flows such as the Azores Current. This mechanism is confirmed by numerical experiments with and without the representation of the Mediterranean overflow process. The numerical model is based on the Miami Isopycnic Coordinate Ocean Model. The model does not include the Mediterranean overflow explicitly, but restores the model density fields in the Gulf of Cadiz toward the observations. This restoring condition produces a reasonable representation of the water mass transformation deduced from observations. The formation of the Azores Current in response to the water mass transformation in the Gulf of Cadiz suggests that the Mediterranean overflow is not only a source of warm and saline water at depth, but also has a strong dynamic impact on the ocean upper layer. This study emphasizes the need to improve the representation of the Mediterranean overflow process in general circulation models in order to capture the correct characteristics of the flow fields and water masses in the subtropical eastern North Atlantic.