The Charlie-Gibbs Fracture Zone: A Crossroads of the Atlantic Meridional Overturning Circulation

The Charlie-Gibbs Fracture Zone (CGFZ), a deep gap in the Mid-Atlantic Ridge at ~52N, is the primary conduit for westward-flowing Iceland-Scotland Overflow Water (ISOW), which merges with Denmark Strait Overflow Water to form the Deep Western Boundary Current. The CGFZ has also been shown to "funnel" the path of the northern branch of the eastward-flowing North Atlantic Current (NAC), thereby bringing these two branches of the AMOC into close proximity. A recent two-year time series of hydrographic properties and currents from eight tall moorings across the CGFZ offers the first opportunity to investigate the NAC as a source of variability for ISOW transport. The two-year mean and standard deviation of ISOW transport was -1.7 ± 1.5 Sv, compared to -2.4 ± 3.0 Sv reported by Saunders for a 13-month period in 1988-1989. Differences in the two estimates are partly explained by limitations of the Saunders array, but more importantly reflect the strong low-frequency variability in ISOW transport through CGFZ (which includes complete reversals). Both the observations and output from a multi-decadal simulation of the North Atlantic using the Hybrid Coordinate Ocean Model (HYCOM) forced with interannually varying wind and buoyancy fields indicate a strong positive correlation between ISOW transport and the strength of the NAC through the CGFZ (stronger eastward NAC related to weaker westward ISOW transport). Vertical structure of the low-frequency current variability and water mass structure in the CGFZ will also be discussed. The results have implications regarding the interaction of the upper and lower limbs of the AMOC, and downstream propagation of ISOW transport variability in the Deep Western Boundary Current.