Abstract
Abstract. Bardsey Island is located at the western end of the Llŷn Peninsula in northwestern Wales. Separated from the mainland by a channel that is some 3 km wide, it is surrounded by reversing tidal streams of up to 4 m s−1 during spring tides. These local hydrodynamic details and their consequences are unresolved by satellite altimetry and are not represented in regional tidal models. Here we look at the effects of the island on the strong tidal stream in terms of the budgets for tidal energy dissipation and the formation and shedding of eddies. We show, using local observations and a satellite-altimetry-constrained product (TPXO9), that the island has a large impact on the tidal stream and that even in this latest altimetry-constrained product the derived tidal stream is under-represented due to the island not being resolved. The effect of the island leads to an underestimate of the current speed in the TPXO9 data in the channel of up to a factor of 2.5, depending on the timing in the spring–neap cycle, and the average tidal energy resource is underestimated by a factor up to 14. The observed tidal amplitudes are higher at the mainland than at the island, and there is a detectable phase lag in the tide across the island; this effect is not seen in the TPXO9 data. The underestimate of the tide in the TPXO9 data has consequences for tidal dissipation and wake effect computation and shows that local observations are key to correctly estimating tidal energetics around small-scale coastal topography.
Highlights
Scientific understanding of global tidal dynamics is well established
This can mean that the tidal energetics in the products, and in other numerical models with insufficient resolution, can be biased because unresolved wakes downstream of the topography can act as a large energy sink (McCabe et al, 2006; Stigebrandt, 1980; Warner and MacCready, 2014)
The overestimate in TPXO9 can lead to the tidal energetics being biased high in the region if they are based on the that data alone
Summary
Scientific understanding of global tidal dynamics is well established. Following the advent of satellite observations, up to 15 tidal constituents have been mapped using altimetryconstrained numerical models, and the resulting products are verified and constrained further using in situ tidal data; see Stammer et al (2014) for details. Gebco.net/, last access: 2 November 2020; Jakobsson et al, 2020) This can mean that the tidal energetics in the products, and in other numerical models with insufficient resolution, can be biased because unresolved wakes downstream of the topography can act as a large energy sink (McCabe et al, 2006; Stigebrandt, 1980; Warner and MacCready, 2014). Because many of the altimetry-constrained tidal databases are models, and not altimeter databases, they provide tidal currents as well as elevations This is true for TPXO9 (see Egbert and Erofeeva, 2002, and https://www.tpxo.net/, last access: 2 November 2020, for details), the altimetryconstrained product used here. Bardsey Island is a rocky melange of sedimentary and igneous rocks, including some granites, located 3.1 km off the Llyn Peninsula in northern Wales, UK (Fig. 1a) It is approximately 1 km wide (though it is only 300 m wide at the narrowest point) and 1.6 km long. On the other hand, the tidal frequency is larger than f , only one wake eddy will be shed on each tidal cycle, if it has time to form at all
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