SEMINAR: OI Seminar Series - Barry Ruddick

Barry Ruddick from Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada.

Title: Eddy stirring, spiral arms, and thermohaline intrusions: seismic imaging of the ocean’s thermal variance cascade

Abstract: 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 Meddy core but anomalously high in the top, bottom, and surrounding frontal region. This is in accord with the observations and model of Ruddick et al. (JMR 2010) that linked the erosion rate and thermal structure of Meddy “Sharon” with direct microstructure measurements to quantitatively confirm the importance of intrusions in laterally mixing the Meddy. The immediate periphery of the Meddy exhibits bands of water containing numerous reflectors consistent with thermohaline intrusions and with anomalously high mixing rate. We suggest these may be “spiral arms” of water that was removed from the Meddy periphery by external stirring – the Meddy’s “salty trail”.

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 from mesoscales to microscale dissipation could involve a partnership between eddy stirring and thermohaline intrusions.