TIDE-INDUCED MIXING IN THE AMAZON FRONTAL ZONE

The freshwater discharge from the Amazon River produces a bottom salinity front on the Amazon shelf that extends many hundreds of kilometers along the shelf, between the 10- and 20-m isobaths. Strong cross-shelf tidal currents cause advective excursions of the front of more than 20 km and supply the major source of kinetic energy for mixing on the shelf. This mixing is manifested in spring-neap variations in the frontal structure and position, with weaker stratification and seaward migration of the front during periods of strong tidal currents, and stronger stratification and landward migration during neap tides. Mixing appears to be driven by shear instability, based on local estimates of gradient Richardson number and bulk estimates of the water column stability. Maximum mixing occurs during ebb, when vertical shear is greatest. Suspended sediment reaches such high concentrations in the frontal zone that it makes a significant contribution to the stratification, thus increasing the stability of the flow and reducing the effectiveness of tide-induced mixing.