Meddy, spiral arms, and mixing mechanisms viewed by seismic imaging in the Tagus Abyssal Plain (SW Iberia)

Seismic imaging of the water column is used to investigate the relative roles of thermohaline intrusive mixing and stirring (straining and shearing) by mesoscale eddies in the NE Atlantic, in and around a Meddy, and in waters between the Meddy and the coast of Portugal. The images show that mixing is virtually absent within the core but anomalously high in the top, bottom, and surrounding frontal region, as previously found by conventional means. In the immediate periphery of the Meddy, bands of water with numerous reflectors consistent with thermohaline intrusions, and with anomalously high mixing rate can be seen. We suggest these may be “spiral arms” of water that was removed from the Meddy periphery by instability or external stirring. The region between the Meddy and the Portuguese continental slope was also imaged, and shows numerous reflectors with relatively small slope and strength similar to the reflectors found in the Meddy periphery. This is in contrast to the predictions of individual thin filaments of slope O(f/N) by Smith and Ferrari (2009), and we suggest that eddy stirring in this region produces thermohaline fronts that are acted upon by thermohaline intrusions. Therefore, the cascade of thermal variance to microscale dissipation could involve a partnership between eddy stirring and thermohaline intrusions