Observations of Intense Internal Mixing at the Mouth of the Laurentian Channel

Abstract:

The Laurentian Channel (LC) acts as a pathway for the Atlantic Slope Water propagating inland into the Gulf of St. Lawrence and playing important role in ventilation of the Gulf. The volumetric transport of this inland flow is in part controlled by mixing processes and the entrainment of Cold Intermediate Water. The present study assesses the conditions for the LC internal mixing in the upper 200 m. Two high resolution transects were conducted across the channel, one near the LC mouth (at the shelfbreak), and another further inshore at the Cabot Strait. These transects comprised two data types, CTD measurements from a Moving Vessel Profiler (MVP) that yielded a high resolution density sampling, and velocity profiles from a 75 kHz shipboard ADCP. The energetic internal mixing at the mouth was evident in frequent areas of inverse stratification. These inverses were much less numerous in the Cabot St. The gradient Richardson number (Ri) was estimated on the transects and it was found that at the mouth more than 50% of the sampled area was characterized by Ri < 0.25. Energetic interior overturnings at the mouth could be caused by the internal wave energy convergence into the channel. The ADCP measurements and the geostrophic velocity estimate revealed a rich submesoscale eddy field at the mouth in the upper layer of the water column with a vorticity shear at ~80 m depth. The observed vortical features could further enhance the internal wave breaking.