On the Spreading of Meltwater Plumes in the Ocean during the Last Deglaciation

The dispersal of meltwater discharged from the St. Lawrence Valley is investigated from numerical experiments with a regional configuration of the MIT general circulation model representing the western North Atlantic during the last ice age. These experiments assume a horizontal resolution of 1/20^o and a vertical grid with 21 levels in the upper 100 m, so that both the mesoscale eddy field and the vertical structure of the meltwater plume could be simulated in detail. Our goals are to identify possible mechanisms responsible for the offshore spreading of meltwater in the ocean during the deglaciation and to help the interpretation of paleoceanographic observations from the seafloor and sediment cores.

We find that meltwater discharged from the St. Lawrence Valley forms a buoyant plume which turns to the southwest along the continental slope under the action of the Coriolis force. Part of the meltwater is entrained away from the slope by meander crests and warm-core rings of the Gulf Stream (GS) between the St. Lawrence Valley and Cape Hatteras. The other part is diverted offshore by the opposing GS near Cape Hatteras, where the GS leaves the continental margin. In one experiment, meltwater is incorporated into a meander trough that pinches off and produces a cold-core ring, leading to meltwater transport into the subtropical gyre, or it flows southward along the slope inshore of the GS to the South Atlantic Bight. Sensitivity tests show that the buoyant plume spreads at a consistent rate of O(10⁵ m² s^-1). A reduced-gravity two-layer model suggests that the spreading of the plume is governed by (i) the net ageostrophic motion produced by the total acceleration and the upwelling-favorable winds along the front of the plume and (ii) the advection of the front of the plume by the ambient geostrophic flow. In our experiments, meltwater in turn alters the upper part of the GS through meltwater-induced changes in cross-stream density gradients. Our results put constraints on the interpretation of ice-rafted debris found in (de)glacial sediments from the Sargasso Sea and of iceberg scours observed on the slope south of Cape Hatteras.