The M2 tide on the Amazon Shelf

Abstract

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 semidiurnal tidal constituent, the M 2 . These results have been combined with coastal sea level data from within the Amazon and Para Rivers, the adjacent 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 progressive wave, with its amplitude decreasing and phase increasing upriver. Over the adjacent shelf north of Cabo Norte, the M 2 tide approaches a damped standing wave, with large amplitudes (greater than 1.5 m) near the coast due to near resonance within the coastal embayment formed by the Cabo Norte shoal to the south and Cabo Cassipore to the north. The observed M 2 tidal currents are nearly rectilinear and oriented primarily across the local isobaths. Comparisons between tidal observations in both the North Channel and the Cabo Norte‐Cabo Cassipore embayment and a simple variable‐width channel tidal model indicate that (1) most of the M 2 tidal energy dissipation occurs over the mid‐ and inner shelf (in water depths less than 20 m) and (2) fluid muds found there cause a significant reduction (of order 50%) in the effective bottom friction felt by the M 2 tide. The approximate resonant period of the Cabo Norte‐Cabo Cassipore embayment is 11.9 hours, and at resonance the average energy dissipation per forcing period is roughly 2.2 times the average mechanical energy in the embayment. This damping rate is large enough that the tidal amplification is rather insensitive to forcing frequency, so that the response of the embayment to forcing over the semidiurnal band should be essentially the same. The vertical structure of the M 2 tidal current is examined at one outer shelf site located in 65‐m water depth. The observed semimajor axis increases 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 bottom. The mean ellipticity is small (less than 0.1) and positive, indicating clockwise rotation of a nearly rectilinear current, and the semimajor axis is oriented within 10° of the local cross‐isobath direction. The M 2 phase increases with height above bottom, with flood at the bottom leading flood at the surface by about 1 hour. A simple, local homogeneous tidal model with time‐ and space‐dependent eddy viscosity simulates the observed near‐bottom velocity reasonably well, however, the model suggests that stratification above the lowest few meters may significantly affect the tidal boundary layer structure at this site. The M 2 energy flux onto the Amazon shelf and into the Amazon and Para Rivers has been estimated using current and surface elevation 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 0.47×10 10 W and 0.19×10 10 W, respectively. A net M 2 energy flux of about 3.3×10 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.