New Direct Estimates of Iceland-Scotland Overflow Water Transport Through the Charlie-Gibbs Fracture Zone

Abstract:

Detailed observations of the pathways, transports and water properties of dense overflows associated with the AMOC provide critical benchmarks for climate models and mixing parameterizations. A recent two-year time series from eight moorings provides the first long-term simultaneous observations of the hydrographic properties and transport of Iceland-Scotland Overflow Water (ISOW) flowing westward through the Charlie-Gibbs Fracture Zone. Using the isohaline 34.94 to define the ISOW layer, 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 using the same isohaline. Differences in the two estimates could reflect the difficulty of defining the long-term mean in the presence of strong transport variability on multiple time scales: ten 13-month mean transport estimates from the new two-year record range from -2.0 to -1.4 Sv. Furthermore, the temperature/salinity time series indicate large fluctuations in ISOW layer thickness: for example, thickness over the northern rift valley had a mean value of about 1400 m and ranged from 400 to 2100 m (compared to Saunders’ fixed value of 2200 m at this site). A time series of ISOW transport through CGFZ estimated from a multi-decadal run of the Hybrid Coordinate Ocean Model (HYCOM) shows a similar level of interannual variability as the observations.