Cross-Front Mixing and Frontal Upwelling in a Controlled Quasi-Permanent Density Front in the Gulf of St. Lawrence

Abstract

CTD data obtained from three transects across a controlled quasi-permanent density front in the Gulf of St. Lawrence were analyzed for the purpose of investigating cross-front mixing, mechanisms for frontal convergence, secondary circulation induced by the front and relationship between surface mixed-layer properties and frontal structure. Water mass analysis indicates that mixing takes place mainly in the ambient water, from the lower boundary of the frontal layer down to ?100 m. On the side of heavier water, there is a region of low surface temperature. The water masses have a distribution suggestive of upwelling in the low surface temperature region. The thickness of the surface mixed layer varies across he front. Outside the frontal zone there is a well-developed mixed layer of a thickness of about 25 m. It disappears completely in the low surface temperature zone and is re-established in the frontal layer with a reduced thickness. Horizontal intrusions below the frontal layer and interleaving of thin layers in the intermediate cold layer (40?100 m) were observed. A cross-front circulation is proposed to explain the observations. Two mechanisms to generate the cross-front flow and upwelling, i.e. the centripetal acceleration of water parcels flowing along a curved density surface and suction of subsurface water by an internal Ekman flow beneath the frontal layer, are discussed.