Structure, variability, and dynamics of the West Greenland Boundary Current System

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2022.The ventilation of intermediate waters in the Labrador Sea has important implications for the strength of the Atlantic Meridional Overturning Circulation. Boundary current-interior interactions regulate the exchange of properties between the slope and the basin, which in turn regulates the magnitude of interior convection and the export of ventilated waters from the subpolar gyre. This thesis characterizes theWest Greenland Boundary Current System near Cape Farewell across a range of spatio-temporal scales. The boundary current system is composed of three velocity cores: (1) the West Greenland Coastal Current (WGCC), transporting Greenland and Arctic meltwaters on the shelf; (2) the West Greenland Current (WGC), which advects warm, saline Atlantic-origin water at depth, meltwaters at the surface, and newly-ventilated Labrador Sea Water (LSW); and (3) the Deep Western Boundary Current, which carries dense overflow waters ventilated in the Nordic Seas. The seasonal presence of the LSW and Atlantic-origin water are dictated by air-sea buoyancy forcing, while the seasonality of the WGCC is governed by remote wind forcing and the propagation of coastally trapped waves from East Greenland. Using mooring data and hydrographic surveys, we demonstrate mid-depth intensified cyclones generated at Denmark Strait are found offshore of the WGC and enhance the overflow water transport at synoptic timescales. Using mooring, hydrographic, and satellite data, we demonstrate that the WGC undergoes extensive meandering due to baroclinic instability that is enhanced in winter due to LSW formation adjacent to the current. This leads to the production of small-scale, anticyclonic eddies that can account for the entirety of wintertime heat loss within the Labrador Sea. The meanders are shown to trigger the formation of Irminger Rings downstream. Using mooring, hydrographic, atmospheric, and Lagrangian data, and a mixing model, we find that strong atmospheric storms known as forward tip jets cause upwelling at the shelfbreak that triggers offshore export of freshwater. This freshwater flux can explain the observed lack of ventilation in the eastern Labrador Sea. Together, this thesis documents previously unobserved interannual, seasonal, and synoptic-scale variability and dynamics within the West Greenland boundary current system that must be accounted for in future modeling.The work in this dissertation was funded by the National Science Foundation grants OCE-1259618 and OCE-1756361