Interior versus boundary mixing of a cold intermediate layer

Abstract

[1] 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 min = 0.24 ± 0.04°C mo −1 , _ d min = −11 ± 2 m mo −1 and _ H = 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 ' 2 × 10 −8 W kg −1 and K ' 4 × 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.

Turbulent nitrate fluxes in the Lower St. Lawrence Estuary, Canada

“…This interval has also been chosen to represent the flux at the base of the euphotic zone, typically found at a depth of 10-20 in the LSLE [Therriault and Levasseur, 1985;Vezina, 1994 Note that for fluxes calculations at Station 23, the diffusivity profiles used are from the station itself, i.e., far from the channel sloping boundaries. As suggested by Cyr et al [2011], the fluxes reported for this station could be increased by 60% to account for boundary mixing processes. Figure 10 (bottom inset) suggests that the observations at the HLC are biased within a semidiurnal cycle, with more observations in the reversal phase near the high tide (t HT 5 0 h).…”

Behavior and mixing of a cold intermediate layer near a sloping boundary

“…For the rest of the year, the system is stratified into three water masses after a surface layer is formed as a result of the spring freshet and increasing air temperature. The previous winter surface layer then becomes trapped as a CIL between two warmer layers (e.g., El-Sabh, 1979;Koutitonsky and Bugden, 1991;Galbraith, 2006;Smith et al, 2006;Cyr et al, 2011). Once regenerated during the winter, the CIL properties are slowly eroded during summer months as a result of mixing (in the estuary, its core temperature warms at a rate of ∼ 0.24 • C per month while its thickness decrease at a rate of ∼ 11 m per month; Gilbert and Pettigrew, 1997;Cyr et al, 2011).…”

“…The previous winter surface layer then becomes trapped as a CIL between two warmer layers (e.g., El-Sabh, 1979;Koutitonsky and Bugden, 1991;Galbraith, 2006;Smith et al, 2006;Cyr et al, 2011). Once regenerated during the winter, the CIL properties are slowly eroded during summer months as a result of mixing (in the estuary, its core temperature warms at a rate of ∼ 0.24 • C per month while its thickness decrease at a rate of ∼ 11 m per month; Gilbert and Pettigrew, 1997;Cyr et al, 2011). Because of winter low surface layer salinities in the LSLE that inhibit mixing and convection, the CIL is not formed there but is rather advected from the Gulf during the summer months as the result of the estuarine circulation (e.g., Galbraith, 2006;Smith et al, 2006).…”

“…Along with other sensors, these profilers are equipped with two airfoil shear probes that allow measurements of microscale (∼ 1 cm) vertical shear u ′ z (see Cyr et al, 2011, for other sensors). They are also equipped with a fine-scale (∼ 1 dm) temperature-conductivity-depth (CTD) sensors manufactured by Sea-Bird Electronics.…”

“…In a recent study, Cyr et al (2011) concluded that although not dominant, boundary mixing can contribute significantly to the mixing budget of the Gulf of St. Lawrence, a semi-enclosed subarctic sea located in eastern Canada. That study was carried out at the Rimouski section, a transect extending across the Lower St. Lawrence Estuary (LSLE), off Rimouski ( Fig.…”