Meridional Exchanges and Mixing at the Iceland-Faroe Ridge

Abstract

The Greenland-Scotland Ridge influences the exchange of warm and cold water masses between the North Atlantic and the Nordic Sea, a circulation with significant impact on global ocean circulation and climate. This study examines aspects of the exchange east of Iceland. Data from three years of intense Seaglider hydrographic surveys of the region are used. The dense overflows across the Iceland-Faroe Ridge (IFR) and Faroe Bank Channel (FBC) are investigated in Chapter 2. The flow of dense overflow on the Atlantic side of the IFR is found to be directed along-slope with speeds averaging 21.5 cm/s. The overflow/ambient interface lays parallel to the IFR topography. Mesoscale eddy deformation of the interface is generally small compared with the topographically induced slope. The mean FBC overflow transport is estimated at 1.8 +/- 0.2 Sv, and an indirect calculation produces a lower bound estimate of IFR overflow of 0.8 Sv. The dominant IFR overflow branch (mean 0.48 Sv) is at the northwestern end of the ridge, where a highly variable narrow jet along the Iceland shelf break transports dense water to the southwest. Chapter 3 outlines a method to infer dissipation of turbulent kinetic energy using Seaglider observations of finescale vertical velocity and density. The resulting Seaglider-inferred dissipation estimates compare well with observations made by a standard microstructure shear probe. Application of the method to the full set of Seaglider dives from the field campaign reveals regions of elevated dissipation and mixing which are crucial to the development of North Atlantic Deep Water (NADW) and the lower branch of the Atlantic Meridional Overturning Circulation (AMOC). A seasonal flux of fresh water across the IFR is explored in Chapter 4. Low salinity thermohaline intrusions are subducted in winter time at the Iceland Faroe Front, injecting cold, fresh water into the large volume of Atlantic Subpolar Mode Water south of the ridge. The associated heat and salt flux are comparable to previous estimates of eddy induced and intrusion driven fluxes at the front. The heat and salt flux is inconsequential relative to the total heat and salt budgets of the Nordic Seas, but is important locally.