|Last update: Tuesday, 30-Sep-2008 16:11:54 CEST|
Surface currents and waves in Liverpool Bay
byJohn Howarth, Rose Player
Proudman Oceanographic Laboratory, Joseph Proudman Building ,6 Brownlow St, Liverpool, L3 5DA
A Coastal Observatory has been established in Liverpool Bay, part of the semi-enclosed Irish Sea. The region is relatively shallow, with depths less than 40 m. Tides are the dominant physical process (currents are up to 1 m s-1) and there is a well defined circulation driven by density gradients resulting from the freshwater input from the rivers Dee, Mersey and Ribble, and by winds. The water column can be stratified in both temperature and salinity, stratification varying tidally and seasonally. The Coastal Observatory (http://coastobs.pol.ac.uk) involves extensive measurements, and modelling with grid sizes ranging from 12 km (reaching into the ocean) down to 200 m.
The Observatory includes a phased array HF radar system measuring surface currents and waves every 20 minutes with a 4 km resolution, established in spring 2004. These measurements are complemented by, and have been compared with, results from two Acoustic Doppler Current Profilers mounted on sea bed frames (measuring the full current profile and waves) and by an in situ directional wave buoy. The radar current data were subsampled to hourly, analysed harmonically for tides and tidal residuals calculated. The residuals contained no large (storm) events. Principal component analysis was applied to the residuals, showing just a few significant modes the first accounting for 41% of the variance. Although the coherent tides had been removed when calculating the residuals, incoherent energy at tidal frequencies is still prominent in the spectra of the residuals (seen also in the ADCP records). At low frequencies clockwise rotating energy dominates, although there is no energy at the inertial frequency.
Waves are mostly locally generated, with recorded significant wave heights of up to 5 m. Since the region is shallow with some extensive sandbanks significant wave-current interactions occur and the measurements provide a good test of coupled 3-D hydrodynamic spectral wave (3-G WAM) models.