On the Spreading of Glacial Meltwater in the Western North Atlantic. Part I: Role of Dynamical Instabilities

Abstract

Glacial meltwater discharged in excessive amount into the ocean may disrupt ocean circulation on regional to larger scales. Here, the spreading of meltwater discharged from the Laurentide Ice Sheet is explored through idealized experiments with a numerical circulation model representing the western North Atlantic during the last ice age. The roles of dynamical instabilities and eddy generation in meltwater dispersal are isolated without the intricacies of a background circulation and atmospheric forcing (both considered in Part II). The meltwater released from the Laurentian Channel pro-duces a buoyant plume detached from the bottom and flowing to the southwest along the continental slope. The coastal current becomes unstable, meanders, and develops dipolar vortices all along the slope down to the South Atlantic Bight. These vortices move in the same direction as the parent current but slower by one order of magnitude. Their central jet tends to align perpendicularly to isobaths, and their cap is an anticyclonic flow produced by vortex compression of the sea-ward-flowing jet. They grow, interact, and ultimately lead to a complex and broad eddy field. Sensitivity tests show that meltwater spreads away from the slope at a consistent rate of O(104) m2 s21. A two-layer theory suggests that the meltwa-ter front moves offshore as a result of the ageostrophic velocity produced by the acceleration of the geostrophic alongfront flow. The instability of meltwater-induced currents alone can cool and freshen a region as large as the Slope Sea in,1 year.