Dynamics of transport and variability in the Denmark Strait overflow

The overflow of dense water from the Nordic Seas through the Denmark Strait is one of the primary sources of the deep water in the world's oceans. In 1998, a rapid high-resolution survey on the F/S Poseidon with expendable profilers (XCP/XCTD) collected velocity, temperature and salinity data from the region of the Denmark Strait sill to study the initial descent of the overflow into the deep North Atlantic. The major results from this and an earlier, more modest, survey in 1997 on the R/V Aranda, along with additional analysis of satellite and current meter data, can be summarized as follows: (1) The flow near the sill is characterized by a strongly barotropic structure associated with a nearly-vertical temperature front. As the denser water descends the Greenland slope, it develops the bottom-intensified structure characteristic of a gravity current. (2) Initial transport of σ_&thetas; > 27.8 water at the sill is measured by the synoptic sections to be 2.7 ± 0.6 Sv, essentially identical both in mean and variability to that measured in 1973 by a 5-week current meter array deployment. (3) Despite large spatial and temporal variability in velocity, thickness and transport, the overflow's pathway and descent with distance from the sill are remark ably steady. (4) Measurements of near-bottom shear stress (from logarithmic velocity fits) confirm the importance of bottom friction in controlling the rate of overflow descent. (5) Satellite sea-surface temperature images confirm the birth and downstream propagation of cyclonic eddies starting at approximately 125 km southwest of the sill. This same point is also marked by a change in the rate of overflow entrainment and a maximum in overflow speed. (6) The presence of subsurface eddies upstream of the appearance of the surface features suggests a geographical separation between the region of flow instability and the site of eddy generation and vortex stretching. These two distinct processes occur in the approach to the sill and over the steepest descent, respectively.