Three-Dimensional Simulations of Overflows on Continental Slopes

Abstract

Three-dimensional features and instabilities of dense overflows from marginal seas onto continental slopes are investigated using a three-dimensional, primitive equation numerical ocean model. The numerical simulations reveal important instability and three-dimensional features of the overflow plumes that have not been included in previous simulations with a one-dimensional streamtube model and a two-dimensional plume model. It is shown that the large primary plume breaks into a number of smaller subplumes on the offshore side of the plume due to instabilities manifested as growing topographic Rossby waves over the slope. The observed high temporal and spatial variabilities in the Denmark Strait overflow could be caused by the inherent dynamic instabilities as revealed by the numerical simulations. The simulations indicate that the initial overflow velocity and width, the properties of the source water, the planetary rotation, and the slope steepness play major roles in determining the scales of the breaking-away subplumes and the across-slope penetration of the large plume. The model simulations also show that a chain of surface cyclonic eddies form and travel almost parallel to the isobaths toward the right and downstream of the plume source. The eddies provide a surface signature of the sinking, breaking-away subplumes, as a result of vortex stretching in the upper pan of the water column above the subplumes. Such surface features may have been observed in satellite IR imagery along the East Greenland continental shelfbreak, and it may be possible to use satellite imagery and further modeling studies to monitor the Denmark Strait overflow, which produces most of the North Atlantic Deep Water.