NORTH ATLANTIC ON THE EDGE: OCEAN CIRCULATION THROUGH GATEWAYS AT THE SUBPOLAR-SUBTROPICAL BOUNDARY

Labrador Sea Water (LSW) is a major component of the deep limb of the Atlantic Meridional Overturning Circulation, yet LSW transport pathways and their variability lack a complete description. A portion of the LSW exported from the subpolar gyre is advected eastward along the North Atlantic Current and must contend with the Mid-Atlantic Ridge before reaching the eastern basins of the North Atlantic. In Chapter 1, we analyze observations from a mooring array and satellite altimetry, together with outputs from a hindcast ocean model simulation, to estimate the mean transport of LSW across the Charlie-Gibbs Fracture Zone (CGFZ), a primary gateway for the eastward transport of the water mass. The LSW transport estimated from the 25-year altimetry record is 5.3 ± 2.9 Sv, where the error represents the combination of observational variability and the uncertainty in the projection of the surface velocities to the LSW layer. Current velocities modulate the interannual to higher-frequency variability of the LSW transport at the CGFZ, while the LSW thickness becomes important on longer time scales. The modeled mean LSW transport for 1993{2012 is higher than the estimate from altimetry, at 8.2 ± 4.1 Sv. The modeled LSW thickness decreases substantially at the CGFZ between 1996 and 2009, consistent with an observed decline in LSW volume in the Labrador Sea after 1994. We suggest that satellite altimetry and continuous hydrographic measurements in the central Labrador Sea, supplemented by profiles from Argo floats, could be sufficient to quantify the LSW transport at the CGFZ.\nA decade ago, abrupt warming of the Northwest Atlantic Shelf prompted an ecosystem shift with significant consequences for fisheries, and the cause of this event has remained unclear. In Chapter 2, we show that a heightened presence of the Gulf Stream at the Tail of the Grand Banks (TGB) reduced the westward connectivity of the Labrador Current that otherwise supplies cold, fresh, oxygen-rich waters to the shelf. The appearance of anomalous properties at the TGB was followed by a southwestward progression of subsurface warming one year later. Thus, monitoring conditions at the TGB may offer predictability for shelf properties and ecosystem perturbations with substantial lead time. Historical observations suggest this recent shift was preceded by a similar phenomenon in the 1970s, indicating that just two events are responsible for much of the dramatic warming of the Northwest Atlantic Shelf over the past century. In Chapter 3, we performed a Lagragian analysis in the high-resolution ocean circulation model HYCOM to investigate the control of the Gulf Stream on the continuity of the Labrador Current past the Grand Banks of Newfoundland. The Labrador Current can be completely cut off at or upstream of the Tail of the Grand Banks by Gulf Stream eddies and meanders that impinge on the shelf-break. In the simulation, this process became more common after 2007, leading to warm/salty anomalous conditions in the Northwest Atlantic.

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