Shear dispersion in the thermocline and the saline intrusion

Over the mid-Atlantic shelf of the North America, there is a pronounced shoreward intrusion of the saltier slope water along the seasonal thermocline, whose genesis remains unexplained. Taking note of the observed broad-band baroclinic motion, we postulate that it may propel the saline intrusion via the shear dispersion. Through an analytical model, we first examine the shear-induced isopycnal diffusivity (“shear diffusivity” for short) associated with the monochromatic forcing, which underscores its varied even anti-diffusive short-term behavior and the ineffectiveness of the internal tides in driving the shear dispersion. We then derive the spectral representation of the long-term “canonical” shear diffusivity, which is found to be the baroclinic power band-passed by a diffusivity window in the log-frequency space. Since the baroclinic power spectrum typically plateaus in the low-frequency band spanned by the diffusivity window, canonical shear diffusivity is simply 1/8 of this low-frequency plateau — independent of the uncertain diapycnal diffusivity. Applied to the mid-Atlantic shelf, this canonical shear diffusivity is about 20 m2 s−1, which is sufficient to account for the observed tracer dispersion or saline intrusion in the thermocline.

► Isopycnal shear diffusivity is enhanced in the seasonal thermocline. ► Internal tides are ineffective in driving the shear dispersion. ► Shear diffusivity is proportional to the low-frequency spectral density. ► Shear diffusivity is insensitive to the diapycnal diffusivity. ► Shear diffusivity is several tens  m2 s−1 over typical shelf environment.