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Physical and Dynamical Oceanography of Liverpool Bay

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Ocean Dynamics (2011) 61:1421–1439 DOI 10.1007/s10236-011-0431-6 Physical and dynamical oceanography of Liverpool Bay Jeffrey A. Polton · Matthew Robert Palmer · Michael John Howarth Received: 2 December 2010 / Accepted: 28 April 2011 / Published online: 31 May 2011 © Springer-Verlag 2011 Abstract The UK National Oceanography Centre has eastward erosion of the plume during spring tides is maintained an observatory in Liverpool Bay since identified as a potentially important freshwater mixing August 2002. Over 8 years of observational measure- mechanism. Novel climatological maps of temperature, ments are used in conjunction with regional ocean salinity and density from the CTD surveys are pre- modelling data to describe the physical and dynam- sented and used to validate numerical simulations. The ical oceanography of Liverpool Bay and to validate model is found to be sensitive to the freshwater forcing the regional model, POLCOMS. Tidal dynamics and rates, temperature and salinities. The existing CTD plume buoyancy govern the fate of the fresh water survey grid is shown to not extend sufficiently near the as it enters the sea, as well as the fate of its sedi- coast to capture the near coastal and vertically mixed ment, contaminants and nutrient loads. In this con- component the plume. Instead the survey grid captures text, an overview and summary of Liverpool Bay tidal the westward spreading, shallow and transient, portion dynamics are presented. Freshwater forcing statistics of the plume. This transient plume feature is shown in are presented showing that on average the bay re- both the long-term averaged model and observational ceives 233 m3 s−1. Though the region is salinity con- data as a band of stratified fluid stretching between the trolled, river input temperature is shown to significantly mouth of the Mersey towards the Isle of Man. Finally modulate the plume buoyancy with a seasonal cycle. the residual circulation is discussed. Long-term moored Stratification strongly influences the region’s dynamics. ADCP data are favourably compared with model data, Data from long-term moored instrumentation are used showing the general northward flow of surface water to analyse the stratification statistics that are represen- and southward trajectory of bottom water. tative of the region. It is shown that for 65% of tidal cycles, the region alternates between being vertically Keywords Liverpool Bay · Climatology · ROFI · mixed and stratified. Plume dynamics are diagnosed Plume dynamics · Coastal dynamics · Coastal from the model and are presented for the region. The oceanography · Shelf sea · Model validation spring–neap modulation of the plume’s westward ex- tent, between 3.5◦W and 4◦W, is highlighted. The rapid 1 Introduction Responsible Editor: Claire Mahaffey Liverpool Bay is a shallow subsection of the semi- enclosed Irish Sea (Fig. 1). The Proudman Oceano- This article is part of the Topical Collection on the graphic Laboratory and more recently the UK Na- UK National Oceanography Centre’s Irish Sea Coastal Observatory tional Oceanography Centre (NOC) has maintained an observatory in Liverpool Bay since August 2002. B · · J. A. Polton ( ) M. R. Palmer M. J. Howarth The observatory has evolved into a multiple platform, National Oceanography Centre, 6 Brownlow St., Liverpool, L3 5DA, UK multidisciplinary ocean science undertaking with a high e-mail: [email protected] density and diverse range of partners and end-users. 1422 Ocean Dynamics (2011) 61:1421–1439 53.9 10m interval rural North Wales and the Snowdownia National Park). 10km Liverpool Bay is also important to the maritime energy 53.8 industry; the Irish Sea, and greater Liverpool Bay area, Ribble hosts numerous offshore oil and gas platforms, several 53.7 wind farms and has approved plans for significant fu- ture growth (The Crown Estate 2010). It is also the 53.6 Site A focus of a number of proposed tidal barrier installations (Burrows et al. 2009; Walkington and Burrows 2009). 53.5 Latitude Site B Sediment dynamics are of primary concern for many of the observatory stakeholders; much of the adjoining 53.4 coast is protected by natural sand dune systems that Liverpool Mersey Clywd are actively accreting or eroding and yet which also 53.3 Conwy provide a major tourist attraction and income for the Dee area. The proximity of the historic, international port 53.2 of Liverpool also provides a focus for research into -3.8 -3.6 -3.4 -3.2 -3 -2.8 -2.6 sediment transport facilitating marine shipping chan- Longitude nel management. The intensity of human demand on Liverpool Bay combined with the complexity of the Fig. 1 Map showing the Liverpool Bay region, as part of the Irish Sea, with major estuaries and long-term mooring sites A and B. local dynamics provides the focus for coastal research The given location for site B was maintained from 5th April 2005 from both directive driven and blues skies researchers until 26th January 2010. The major estuaries are labelled and in order to better understand the coastal ocean. the tributaries are shown. The bathymetric contours are at 10-m In this article, data are presented from two fixed intervals showing that site A is in about 20 m of water mooring sites (the full specifications are detailed in Howarth and Palmer (The Liverpool Bay Coastal While current plans are underway to extend the focus Observatory, resubmitted)): (1) site A is located at of the observatory to a wider Irish Sea perspective there 53◦31.8 N, 3◦21.6 W at the Mersey Bar. The site remains a requirement from the user community to has been maintained continuously since August 7th provide sustained monitoring of Liverpool Bay. This 2002. The site is strongly influenced by the Mersey paper draws on over 8 years of measurements, in con- freshwater outflow and to a lesser extent by the river junction with model data, to describe the forcing mech- Dee. (2) Site B is located 21 km to the west of site anisms and the physical oceanography of Liverpool Aat53◦27 N, 3◦38.4 W. The site was maintained Bay. between 5th April 2005 and 26th January 2010, fol- With a spring tidal range in excess of 10 m, the lowing which the moored instrumentation was moved region experiences one of the largest tidal ranges on 9 km north to the same latitude as site A. The new Earth, only exceeded in the UK by the Bristol Channel. location of site B was chosen in order to better meet Subsequently, the bay experiences strong tidal cur- scientific demands and to avoid future wind farm de- rents which interact with the seabed and horizontal velopment. In this paper only data from the original density gradients to produce complex dynamics which site B location is presented. Data from each moor- have important consequences on the fate of freshwater ing site and the other instrument platforms, which and biogeochemical pathways (Greenwood et al. (Spa- combine to make the Irish Sea Observatory, will be tial and temporal variability in nutrient concentrations compared with the three-dimensional hydrodynamical in Liverpool Bay, a temperate latitude region of fresh- model, POLCOMS (Holt and James 2001;Holtetal. water influence, submitted); Yamashita et al. 2011). 2005). The simulation data presented here are from Liverpool Bay is subject to many of the modern pres- the 1.8-km horizontal resolution configuration (Fig. 19 sures on our coastal systems; it is in close proximity to shows this resolution in context with the Liverpool a number of economically important cities and encom- Bay subregion), with 32 evenly spaced vertical levels passes the national boundaries of England, Wales and at each location. The atmosphere is forced by 1◦ and the Isle of Man. The Bay receives freshwater input from six hourly ECMWF winds and surface fluxes are com- a number of large English and Welsh rivers includ- puted using bulk formulae. The combination of ob- ing the Mersey, Ribble and Dee (having large catch- servational and simulation results are used to provide ment areas covering heavily industrialised and highly a synoptic overview of the physical oceanography of populated regions) and the rivers Clwyd and Conwy Liverpool Bay and to describe the fate of the riverine (which have the comparatively pristine catchments of freshwater. Following dynamical insights of Yankovsky Ocean Dynamics (2011) 61:1421–1439 1423 and Chapman (1997), the freshwater, once it is in the the earth, under the Sun and Moon, results in semi- bay, will be referred to as a plume. diurnal components of the total tidal signal that explain The dynamics of Liverpool Bay are governed by the most of the Irish Sea’s tidal variability. These are the tides. The tides determine the fate of the fresh water M2 and S2 constituents, with a 12.4- and 12.0-h period, plume, the sediment, nutrient and contaminant riverine and are associated with the gravitational pull of the loads as they enter the sea. Therefore, to understand Moon and Sun, respectively. Similarly, the orbit of the the dynamical effects on chemical or biological cycles, Moon around the Earth results in a twice monthly it is necessary to understand the horizontal and vertical alignment of the Earth, Moon and Sun. The alignment structure of the tidal currents, how they vary in time, of the gravitational pull between the bodies modulates and to understand the associated mixing processes. the tidal amplitudes with a 2-week period. This cycle Studying the dispersal of the fresh water plume as it is known as the spring–neap cycle. Spring tides occur enters the sea provides useful insight into how the on the alternating weeks around the time when the riverine chemical, biological or sediment load might Moon is either full or new, and are associated with the also disperse.
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