The generation of transverse flows by internal friction in the St. Lawrence Estuary

Internal friction in combination with strong vertical variations of the vertical shear of a current can set up a transverse circulation. The induced lateral flow field may be very important to the maintenance of the steady density field. To date there has been little work aimed at actually distinguishing this weak component of the flow in field data. We have compiled a set of velocity data for the St. Lawrence Estuary and we test for a relation between the second vertical derivative of the along-channel velocity and the lateral flow. We find that the frictionally driven circulation can only be identified when the second vertical derivative of the along-channel current is very large, and that in general the along-channel pressure gradient is at least as important as internal friction in generating cross-channel flow, in the St. Lawrence Estuary. These results suggest that the simplest possible picture of the lateral circulation of the St. Lawrence Estuary, a single cell with upwelling near the north shore and downwelling near the south shore, is not tenable.