Inertial oscillations in the Irish Sea

Abstract

An inertial current with a maximum speed of about 0.20 m s 1 was observed in June 1981, by a current meter 17 m below the surface of the thermally stratified waters of the western Irish Sea. The dynamics of the surface waters have been modelled using contemporary wind data recorded at the site to drive a horizontally unbounded surface slab 20 m deep with a velocity decay time of about three inertial periods (1.86 days). From thermistor chain data, inertial frequency potential energy was observed to radiate beneath the thermocline, descending at a rate of about 0.06 cm s 1. The energy available for internal mixing from the observed oscillation may have been as much as 80 J m 2, compared with 150 J m 2 from wind-induced surface mixing. The mean surface mixing power in summer is about7.1 × 10 4W m 2, while the mean shear-induced mixing power could be as much as2.0 × 10 4W m 2. The likely occurrence of strong inertial events has been estimated using the wind record fcr 10 successive summers at Dublin airport. Large oscillations with speeds 0.20m s 1, or mixed layer kinetic energy(MLKE) > 400 J m 2, have an average duration of about 28 h. They occur about once every 2 months between mid-May and mid-August but seem to be more frequent from then until mid-September. Moderate oscillations (MLKE > 200 J m 2) are more evenly distributed, occurring about once every 2 weeks. Small oscillations (MLKE > 40 J m 2) occur nearly 50% of the time.