Abstract
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.
Highlights
The Faroe-Shetland Channel (FSC), shown in Fig. 1, is delimited to the east by the Shetland shelf and to the west by the Faroe plateau
The present investigation could serve as a guideline for the FSC dynamic activity to be expected during extreme North Atlantic Oscillation (NAO) periods, which should be seen in the light of previous NAO-related investigations primarily limited to how the magnitude of the Atlanticwater inflow in the FSC is affected by the NAO
It is suggested that well-organised leakage of Modified North Atlantic Waters (MNAW) into the FSC in the form of a cold-core eddy may play a role for the onset of the slope-current deflection
Summary
The Faroe-Shetland Channel (FSC), shown in Fig. 1, is delimited to the east by the Shetland shelf and to the west by the Faroe plateau. Helland-Hansen and Nansen (1909) noted that this passage is an area of intense dynamic activity, a feature that later has been confirmed by, for example, Tait (1957) and Poulain et al (1996) This is due to the North Atlantic current and its two-branch extension in. Winds are of particular interest, their strength frequently measured in terms of the meridional sea-level pressure difference between the Icelandic low and the Azores high, an approach pioneered by Walker (1924) This pressure dipole is regarded as one of the large-scale modes of climate variability and is, in one or another of the forms whereby it can be quantified as an index, commonly referred to as the North Atlantic Oscillation (NAO). The present investigation could serve as a guideline for the FSC dynamic activity to be expected during extreme NAO periods, which should be seen in the light of previous NAO-related investigations primarily limited to how the magnitude of the Atlanticwater inflow in the FSC is affected by the NAO (see Nilsen et al, 2003)
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