Interior versus boundary mixing of a cold intermediate layer

The relative importance of interior versus boundary mixing is examined for the erosion of the cold intermediate layer (CIL) of the Gulf of St. Lawrence. Based on 18 years of historical temperature profiles, the seasonal erosion of the core temperature, thickness and heat content of the CIL are, respectively, (T) over dot(min) = 0.24 +/- 0.04 degrees C mo(-1), (d) over dot(min) = -11 +/- 2 m mo(-1) and (H) over dot = 0.59 +/- 0.09 MJ m(-3) mo(-1). These erosion rates are remarkably well reproduced with a one-dimensional vertical diffusion model fed with turbulent diffusivities inferred from 892 microstructure casts. This suggests that the CIL is principally eroded by vertical diffusion processes. The CIL erosion is best reproduced by mean turbulent kinetic energy dissipation rate and eddy diffusivity coefficient of epsilon similar or equal to 2 x 10(-8) W kg(-1) and K similar or equal to 4 x 10(-5) m(2) s(-1), respectively. It is also suggested that while boundary mixing may be significant it may not dominate CIL erosion. Interior mixing alone accounts for about 70% of this diffusivity with the remainder being attributed to boundary mixing. The latter result is in accordance with recent studies that suggest that boundary mixing is not the principal mixing agent in coastal seas.