THE M(2) TIDE ON THE AMAZON SHELF

As part of A Multidisciplinary Amazon Shelf Sediment Study (AMASSEDS), moored and shipboard current measurements made over the Amazon shelf during 1990-1991 have been analyzed to determine the dominant semidiur nal tidal constituent, the M(2). These results have been combined with coastal sea level data from within the Amazon and Para Rivers, the ad jacent shelf, and with satellite-derived tidal elevation data from off the shelf to provide a more complete description of the M(2) tide in this complex river/shelf system. Near the Amazon River mouth the M(2) tide propagates across the shelf and through the mouth as a damped pro gressive wave, with its amplitude decreasing and phase increasing upri ver. Over the adjacent shelf north of Cabo Norte, the M(2) tide approa ches a damped standing wave, with large amplitudes (greater than 1.5 m ) near the coast due to near resonance within the coastal embayment fo rmed by the Cabo Norte shoal to the south and Cabo Cassipore to the no rth. The observed M(2) tidal currents are nearly rectilinear and orien ted primarily across the local isobaths. Comparisons between tidal obs ervations in both the North Channel and the Cabo Norte-Cabo Cassipore embayment and a simple variable-width channel tidal model indicate tha t (1) most of the M(2) tidal energy dissipation occurs over the mid- a nd inner shelf (in water depths less than 20 m) and (2) fluid muds fou nd there cause a significant reduction (of order 50%) in the effective bottom friction felt by the M(2) tide. The approximate resonant perio d of the Cabo Norte-Cabo Cassipore embayment is 11.9 hours, and at res onance the average energy dissipation per forcing period is roughly 2. 2 times the average mechanical energy in the embayment. This damping r ate is large enough that the tidal amplification is rather insensitive to forcing frequency, so that the response of the embayment to forcin g over the semidiurnal band should be essentially the same. The vertic al structure of the M(2) tidal current is examined at one outer shelf site located in 65-m water depth. The observed semimajor axis increase s logarithmically with height above bottom within the lowest 1-2 m and reaches a maximum in excess of 0.5 m/s at approximately 11 m above bo ttom. The mean ellipticity is small (less than 0.1) and positive, indi cating clockwise rotation of a nearly rectilinear current, and the sem imajor axis is oriented within 10 degrees of the local cross-isobath d irection. The M(2) phase increases with height above bottom, with floo d at the bottom leading flood at the surface by about 1 hour. A simple , local homogeneous tidal model with time- and space-dependent eddy vi scosity simulates the observed near-bottom velocity reasonably well, h owever, the model suggests that stratification above the lowest few me ters may significantly affect the tidal boundary layer structure at th is site. The M(2) energy flux onto the Amazon shelf and into the Amazo n and Para Rivers has been estimated using current and surface elevati on data and the best fit variable-width channel model results. The net M(2) energy flux into the mouths of the Amazon and Para Rivers is 0.4 7 x 10(10)W and 0.19 x 10(10)W, respectively. A net Mt energy flux of about 3.3 x 10(10)W occurs onto the shelf between the North Channel of the Amazon River and Cabo Cassipore. This stretch of the Amazon shelf accounts for about 1.3% of the global dissipation of the M(2) tide.