Faroe Bank Channel Overflow: Dynamics and Mixing

A large fraction of the deep cold waters from the Nordic Seas flows through the Faroe Bank Channel (FBC), and enters the North Atlantic like an underwater river that we call the dense, Faroe Bank Channel overflow plume. The FBC overflow is very energetic and turbulent, and carries approximately 2 million cubic meters of cold water every second. As the voluminous plume flows from the Nordic Seas to the North Atlantic, its properties such as salinity content, oxygen concentration and temperature are mixed with the overlying warm Atlantic water, and becomes a crucial part of the large scale ocean circulation and climate. Due to a lack of observations and the shortcomings in understanding the mechanisms by which they mix, overflows are very poorly represented in climate models. The mixing of water properties occurs on small (metre to centimetre) scales, but is forced by various processes at larger scales (10 m to 100 km). The understanding of the link between the forcing and the turbulent mixing is crucial, and requires dedicated and complex measurement systems and process-based observations. In the OVERFLOW project, we assembled the expertise and instrumental capability to address this. The aim of OVERFLOW is to improve the state-of-the-art on dynamics and mixing of dense overflows in the ocean with particular attention to the Faroe Bank Channel, primarily through observations. A challenge in ocean observations, and particularly for the deep (greater than 800 m) FBC overflow, is to achieve sufficient sampling of the turbulence and mixing in the water column. In our project we show that underwater gliders offer a noise-free platform suitable for ocean turbulence measurements and return data of sufficient quality suitable for studies of both ocean mixing processes and long-term monitoring and mapping of mixing. Gliders are robots that are extensively utilized for hydrographic and biogeochemical observations, and now vertical mixing, in the interior ocean. This is a complementary sampling scheme to that of measurements from a ship, and improves the spatial and time coverage during a research cruise. In OVERFLOW we collected detailed measurements covering from large scales at which forcing occurs to small scales at which energy dies out. These measurements made from a ship and gliders were supplemented by one-year long data from bottom-anchored moored instruments. The unique aspect of the moored instruments is that, for the first time, the entire lateral and vertical extent of the plume was covered at two sections separated by 25 km. The coverage by the mooring arrays allowed us to construct a heat budget for the overflow, yielding, for the first time, long time series of vertical mixing and volume flow and heat content evolution. The analysis of the OVERFLOW data set improved our understanding of the transport and mixing variability in the region on daily to seasonal time scales.

The aim of this project is to investigate the mixing and entrainment of the dense oceanic overflow from the Faroe Bank Channel. The project will utilize the legacy of International Polar Year (IPY) project Bipolar Atlantic Thermohaline Circulation (BIAC), and will advance the basic research at the University of Bergen. During the proposed four-year research, existing historical and recent data sets as well as new observations will be exploited together with process-oriented numerical simulations and labor atory experiments. The data will be analyzed to advance our understanding of overflow mixing. New parameterizations will be explored for better representation of overflows in climate models, and will be implemented and tested through close collaboration w ith the Bjerknes Centre of Climate Research, BCCR. The specific objectives of this study are addressed in the following tasks. T1. Use gliders to infer vertical kinetic energy, vertical rate of strain, internal wave energetics, entrainment and mixing. T2 . Identify and quantify the role of internal wave-turbulence transition in mixing in the stratified dense plume-ambient interface. T3. Describe the formation mechanisms for mesoscale eddies and delineate the role of eddies in modulating the mixing and the descent rate of the plume. T4. Investigate the bifurcation of the overflow plume and a possible transverse hydraulic jump. T5. Investigate the secondary circulation and its effect on entrainment and mixing. T6. Parameterize and implement the entrainment and mixing of overflows.