The Atlantic/meridional overturning circulation (A/MOC) has been the focus of growing attention in recent years. Much of this arises from the widespread recognition that the flow of warm, salty water to high latitudes - the Nordic Seas - helps shape the mild climate experienced across central and northern Europe and no doubt influences climate across the northern hemisphere. Thus the strength and stability of the MOC is a key parameter in the climate puzzle and therefore of great societal relevance. For this reason a project was funded in 2005 to monitor the inflow of warm water into the Nordic Seas across the Iceland-Faroe- Scotland ridge, a natural choke point through which virtually all north-flowing water must pass. What made the undertaking so attractive was the existence of an ocean-going ferry, the Norröna, from which current measurements using a vessel-mounted acoustic Doppler current meter (ADCP) could be made. The Norröna operates weekly across the Faroe-Shetland Channel and along the Iceland-Faroe ridge, the two major inflows. Some unexpected, but very serious start-up problems (bubble drawdown that blocks the acoustics) delayed the project by two years. The company that owns the ferry has gone out of its way to be helpful and substantial amounts of data are now being collected. The original four-year grant received a one-year add-on last year.
This award is to continue and strengthen the project for an additional 4 years. First, although the bubble problem has been ameliorated thanks to a fairing and two vortex generators that bring clear water up from below, more can and should be done. Specifically, with the help of numerical calculations in collaboration with the ship designer in September an area free of bubbles will be sought, perhaps closer to the bow. Second, using an underwater camera will be used to verify the performance and wake structure of the vortex generators. Norröna is scheduled for dry-dock this coming winter-spring at which time the final adjustments will be implemented and should lead to substantially improved performance. The unanticipated problem of bubbles under the Norröna has been an extremely valuable learning process regarding the use and placement of ADCPs on commercial vessels. Second, thanks to the recent development of a new automated XBT launcher, heat fluxes can also be measured directly by combining the ADCP and XBT data. Third, the team will begin to report new findings about tidal signals, EKE activity, and inflows. Scientifically, the biggest surprise by far has been the substantial flow (to the south) on the western side of the Faroe-Shetland Channel such that the net flow through the channel appears to be to the south.
Intellectual Merit: The Iceland-Faroe-Scotland ridge separates two large and distinctly different water mass systems (the North Atlantic and the Arctic Mediterranean). The exchange between the two is driven by both winds and buoyancy forcing over a wide range of scales. By monitoring the inflows and outflows (thanks to sustained moored ADCP programs) one can examine in considerable detail the space-time response of these fluxes to time-varying atmospheric forcing including the well-known North-Atlantic Oscillation.
Broader Impacts: This team has developed extensive expertise with vessel-operated ADCPs. Repeat ADCP enables one to measure currents and transports in the ocean at high horizontal resolution and great confidence. Measuring currents directly adds to dynamical methods that use hydrography or altimetry. High horizontal resolution means resolving the most energetic scales in the ocean - often defined by the radius of deformation. We hope that wider knowledge and use of this technology will, in time, lead to an improved knowledge of ocean currents over a wide range of space and time scales.
