Measurements of flow through the North Channel of the Irish Sea were completed. Ocean Surface Current Radar (OSCR) measurements were made from 2 July 1993 to 24 August 1994, from master and slave sites at Portpatrick and Crammag Head respectively. Surface currents were calculated every 20 minutes for 652 cells with a nominal resolution 1 km, theoretically giving complete coverage across the North Channel. Power failure at the slave site resulted in some data losses, but overall the master was 99% operational and the slave 67%; additionally, data near the Irish coast were affected by interference. A good estimate of the volume flux through the North Channel is still possible because the area affected by poor data accounts for only 15% of the cross-section. The overall mean currents showed a net outflow with significant spatial variability, the main outflow being confined to the Scottish side of the North Channel.
A 150 kHz broad-band Acoustic Doppler Current Profiler (ADCP) was maintained at 54°46 N, 5°24 W (depth 142 m) to give continuous good-quality coverage from 13 July 1993 to 28 October 1994. The data spanned a range from 13 m above the bed to 15 m below the surface, with 5 m resolution. De-tided residual currents showed one exceptional wind-driven surge (3-4 February 1994). Peak depth-averaged currents exceeded 0.5 m/s, twice as strong as all others for the year. This was more extreme than expected from probability distributions characteristic of surge elevations. The surge was caused by a deep slow-moving depression to the southwest of Ireland with associated strong winds veering slowly from easterly to southerly. This was confirmed by good agreement with results from the POL depth-averaged surge model for 3-4 February. Overall correlation between the observed and modelled flows along the North Channel was 0.78 for the 1993-94 winter. Typically, winter storms pushed water out of the Irish Sea in pulses of a few days at all depths. During the summer there was a weak flow into the Irish Sea at all depths.
Mean currents recorded by the ADCP throughout the year spiralled anti-clockwise through 180° from the surface to the bed, being directed into the Irish Sea at the surface and out of the Irish Sea near the bed.
Estimates of the net volume flux will be used to check transport model predictions of North Channel flow and its contents. The data are also being studied for the dynamics of the significant surge events and to relate the flow to forcing by winds, non-linear tides, pressure and density gradients. Surface elevations at Port Ellen, Portrush, Bangor, Portpatrick and Port Erin continue to be monitored and, together with continuing meteorological data and a flow- forcing relation, will provide a basis for continuing estimates of North Channel flow.