Internal Solitary Waves off the Amazon River Mouth: coherence crestlengths and generation mechanism

A comprehensive dataset of Synthetic Aperture Radar (SAR) images (RADARSAT, ERS, ENVISAT, TerraSAR-X) is used, for the first time, to investigate the full 2-D structure of internal solitary waves (ISWs) over the continental shelf off the Amazon River mouth (ARM) influenced by the North Brazil Current (NBC) along the northern coast of South America. Although three different groups of ISWs are identified according to their direction of propagation and possible generation mechanisms, our focus in this paper will be on those waves which propagate southeastward, i.e. over the continental shelf in the opposite direction to the NBC flow. Unambiguously, the primary region of ISW activity is the mid- and outer-shelf regions approximately between 1^o-6^oN and 51^o-47^oW. ISW packets emerging from this area are regularly observed to reach crestlengths from a few kilometers up to 35 km, although the majority of observations is characterized by crestlengths < 13 km with a mean value of ~ 8 km for all the waves observed. The inter-packet separation is ~ 5 km with the distance between consecutive crests within a given packet of ~ 0.5 km. The generation mechanism of the ISWs does not seem to be solely related with the semidiurnal tides, but appears instead to result from a combination between the NBC, tidal currents and the localized bottom topographic features around to the 80-m isobath. Solutions of a boundary-value problem at different moments in time, and at the positions where the SAR-derived ISW signatures are identified, show that these waves are nearly in the transcritical regime for mode-1 non-hydrostatic linear waves propagating upstream the NBC. The longevity of these waves is unknown at present, and they may play a relevant role for mixing between fresh riverine and ocean water.