On the Overflow through the Western Valley of the Iceland-Faroe Ridge

On the Overflow through the Western Valley of the Iceland-Faroe Ridge

Bogi Hansen, Karin Margretha Húsgarð Larsen, Steffen M. Olsen, Detlef Quadfasel, Kerstin Jochumsen, Svein Østerhus

Abstract

The Iceland-Faroe Ridge (IFR) is one of the gaps in the Greenland-Scotland Ridge that allows an exchange of water masses between the Nordic Seas and the North Atlantic. The overflow of dense water across the IFR was discovered already in the late 19th century, and more recent estimates of its volume transport are typically around 1 Sv, but still fairly uncertain. Additionally, the flow across the IFR has hitherto not been adequately simulated in climate models, emphasizing the need for better knowledge of the flow. The Western Valley (WV) is the northernmost passage crossing the ridge and it has been considered to carry about half of the total overflow across the ridge. Here, we present results from a field experiment, lasting more than nine months, with one bottom-mounted Acoustic Doppler Current Profiler (ADCP) and two bottom temperature loggers deployed along a section crossing the WV close to the sill. From our observations, the bottom waters at the ADCP site are consistently cold indicating presence of overflow water, but typical velocities are lower than previously anticipated and what would be expected based on simple hydraulic models. We find that the volume transport of WV-overflow is inversely related to the intensity of the inflow of Atlantic waters in the upper layers and the average WV-overflow is estimated to be less than 0.1 Sv. Based on the observations and using a simple two-layer model, we discuss the main reasons for the apparently weak overflow across the WV. Our results indicate that total IFR-overflow is probably at the low end of previous estimates.