Mesoscale Variability on the Continental Slope of the Faroe-Shetland Channel

Data acquired from two moorings situated on the continental slope of the Faroe-Shetland Channel are analysed with a view to assessing the sub-tidal variability within the region. Data are resolved into orthogonal long-slope and cross-slope components of flow, and divided into 4 Phases corresponding to equipment deployment periods. Furthermore, four depths are selected at each Site for analysis so as to sample the surface, intermediate and bottom waters present in the channel. Spectral analysis is performed using the Welch averaged periodogram method. Peak variance throughout the year is in excess of 4(m/s)2/cpd, in the low frequency band, occurring during Phase 3 at Site 2 at a depth of 40m. Energy in the low frequency band was seen to decrease with depth, whilst energy in the semi-diurnal band increased with depth at Site 1, a trend not repeated at Site 2. The data are then low-pass filtered using a 4th order elliptic filter in the time domain, with a cut-off frequency specified as the spectral peak between the diurnal and low frequency bands, which generally equates to 0.55-0.65 cpd. The filtered time series for the study period illustrate the thermocline fluctuating with dept, in particular at Site 1. Shoaling of the thermocline may be seen to occur during episodes of strong up-slope flow, corresponding to an intensification of the slope current and the onset of stratification. Contrastingly, a deepening of the therrnocline was seen during periods of flow reversal in the upper layers. The mechanism for this warming of the slope waters is postulated to be a result of meanders in the slope current. The rotary motion described by the feature is not consistent with that of an eddy, whilst the existence of a meander provides the fluctuating flow in the cross-slope direction in addition to the flow reversal in the upper layer. The meanders may be due to atmospheric forcing, but the lack of appropriate data precludes a positive identification of the source region of the disturbance. A link is found between stronger currents at neap tides due to the reduction in friction with the sea bed, and would benefit from further study. It is concluded that whilst the two moorings used in this report provided useful data regarding the dynamics of the slope current and slope region in general, a more elaborate array of instruments would be required to fully resolve the mesoscale variability of the channel