The response of the circulation in the Faroe-Shetland Channel to the North Atlantic Oscillation

This study, based on satellite-derived sea-surface heights and temperatures as well as hydrographic data, attempts to shed some light on the role of the extreme phases of the North Atlantic Oscillation (NAO) for the local dynamics of the Faroe-Shetland Channel (FSC). During the low-NAO event 2009-10 the Shetland-slope current showed a significant deflection from its usual path above the maximal gradient of the bathymetry, ultimately resulting in an anticyclone. This led to an accumulation of North Atlantic Water (NAW) over the deeper parts of the channel, manifested as a pronounced deepening of the halocline. Leading this deflection of the slope current by around 2 weeks, a cyclonic eddy associated with a doming of the halocline and originating from north of the Faroes (and hence constituted by Modified North Atlantic Waters) had moved southwards in the channel, coming to rest at its southern entrance. Assessing the influence of the NAO on these regional dynamics using 1992-2010 altimetric data, it was found that for positive phases of the NAO, the surface circulation tended to be strongly bathymetrically constrained and thus resembles the mean regional circulation. The negative phases of the NAO are associated with a regional weakening of the wind-stress curl, which leads to a contraction of the Norwegian-Sea gyre and a linked northward migration of the FSC recirculation involving a deflected path of the Shetland-slope current. This change in the circulation under negative NAO conditions may have an impact on the regional ocean climate through the accumulation of saline NAW in the channel.