OBSERVATIONS OF THE TRANSVERSE STRUCTURE AND DYNAMICS OF THE LOW-FREQUENCY FLOW-THROUGH THE NORTH CHANNEL OF THE IRISH SEA

During March-June 1985 an array of moored current meters was maintained across the North Channel of the Irish Sea, providing the first comprehensive cross-channel measurements of the low frequency flow (<0.4 cph). This flow was at up to 0.2 m s-1 and orientated parallel to the axis of the channel. The mean volume transport was northward at 0.11 Sv (1 Sv = 1 x 10(6) m3 s-1) prior to servicing of the array in late April and 0.14 Sv subsequently. Underlying this was a partitioning of residual flow pattern across the channel, with persistent southward flow (0.05 and 0.03 Sv) on the west of the channel and northward flow in the east. An examination of the momentum balance indicated the large scale wind field to be a principal control of the along-channel flow variability, both as a consequence of the generation of an along-channel pressure gradient and direct forcing. On the western side, an additional relationship was observed between along-channel wind stress and Coriolis. In the east, along-channel flow was associated with a cross-channel surface slope, implying a geostrophic balance. However, the unaccounted momentum balance resembled the pressure gradient time series, implying the balance was not truly geostrophic. Application of a simple correlation model suggested that the mean current structure could not be fully described in terms of wind. The partitioning of the mean flow regime is consistent with linearized bottom friction balancing the large scale horizontal density differential between the Irish Sea and Atlantic waters. The regional significance of this persistent flow regime is considered.