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Physical and dynamical oceanography of Liverpool Bay

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Abstract

The UK National Oceanography Centre has maintained an observatory in Liverpool Bay since August 2002. Over 8 years of observational measurements are used in conjunction with regional ocean modelling data to describe the physical and dynamical oceanography of Liverpool Bay and to validate the regional model, POLCOMS. Tidal dynamics and plume buoyancy govern the fate of the fresh water as it enters the sea, as well as the fate of its sediment, contaminants and nutrient loads. In this context, an overview and summary of Liverpool Bay tidal dynamics are presented. Freshwater forcing statistics are presented showing that on average the bay receives 233 m3 s − 1. Though the region is salinity controlled, river input temperature is shown to significantly modulate the plume buoyancy with a seasonal cycle. Stratification strongly influences the region’s dynamics. Data from long-term moored instrumentation are used to analyse the stratification statistics that are representative of the region. It is shown that for 65% of tidal cycles, the region alternates between being vertically mixed and stratified. Plume dynamics are diagnosed from the model and are presented for the region. The spring–neap modulation of the plume’s westward extent, between 3.5 °W and 4°W, is highlighted. The rapid eastward erosion of the plume during spring tides is identified as a potentially important freshwater mixing mechanism. Novel climatological maps of temperature, salinity and density from the CTD surveys are presented and used to validate numerical simulations. The model is found to be sensitive to the freshwater forcing rates, temperature and salinities. The existing CTD survey grid is shown to not extend sufficiently near the coast to capture the near coastal and vertically mixed component the plume. Instead the survey grid captures the westward spreading, shallow and transient, portion of the plume. This transient plume feature is shown in both the long-term averaged model and observational data as a band of stratified fluid stretching between the mouth of the Mersey towards the Isle of Man. Finally the residual circulation is discussed. Long-term moored ADCP data are favourably compared with model data, showing the general northward flow of surface water and southward trajectory of bottom water.

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Notes

  1. Remarkably, the gross patterns in Fig. 3b can be derived from the co-tidal chart if the flow is assumed to be frictionless and that the acceleration is wholly forced by the barotropic tide.

  2. The Conway contributes to the freshwater budget of Liverpool Bay but since data have been historically unavailable its contribution is incorporated by means of scaling the Clwyd.

  3. At constant pressure and at typical salinities a change in temperature of 5°C results in a change in density of about 1 kg m − 3. Similarly, at constant pressure and at typical temperatures a change in salinity of 1 psu is required to change the density by about 1 kg m − 3.

  4. with unknown error estimates

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Acknowledgements

Wave data were provided by CEFAS as part of the WaveNet project and distributed under the Open Government Licence (http://reference.data.gov.uk/id/open-government-licence). This work was supported by NERC National Oceanography Centre national capability modelling and was partially funded under a NERC New Investigator Award. The authors are grateful for the constructive comments, received through the review processes, which have resulted in an improved manuscript.

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Authors and Affiliations

  1. National Oceanography Centre, 6 Brownlow St., Liverpool, L3 5DA, UK

    Jeffrey A. Polton, Matthew Robert Palmer & Michael John Howarth

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  1. Jeffrey A. Polton
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  2. Matthew Robert Palmer
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  3. Michael John Howarth
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Correspondence to Jeffrey A. Polton.

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Responsible Editor: Claire Mahaffey

This article is part of the Topical Collection on the UK National Oceanography Centre’s Irish Sea Coastal Observatory

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Polton, J.A., Palmer, M.R. & Howarth, M.J. Physical and dynamical oceanography of Liverpool Bay. Ocean Dynamics 61, 1421–1439 (2011). https://doi.org/10.1007/s10236-011-0431-6

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