Modelling temperature and salinity in Liverpool Bay and the Irish Sea: sensitivity to model type and surface forcing

C K O'Neill,J A Polton,E J O'Dea,J T Holt

doi:10.5194/os-8-903-2012

Abstract

Abstract. Three shelf sea models are compared against observed surface temperature and salinity in Liverpool Bay and the Irish Sea: a 7 km NEMO (Nucleus for European Modelling of the Ocean) model, and 12 km and 1.8 km POLCOMS (Proudman Oceanographic Laboratory Coastal Ocean Modelling System) models. Each model is run with two different surface forcing datasets of different resolutions. Comparisons with a variety of observations from the Liverpool Bay Coastal Observatory show that increasing the surface forcing resolution improves the modelled surface temperature in all the models, in particular reducing the summer warm bias and winter cool bias. The response of surface salinity is more varied with improvements in some areas and deterioration in others. The 7 km NEMO model performs as well as the 1.8 km POLCOMS model when measured by overall skill scores, although the sources of error in the models are different. NEMO is too weakly stratified in Liverpool Bay, whereas POLCOMS is too strongly stratified. The horizontal salinity gradient, which is too strong in POLCOMS, is better reproduced by NEMO which uses a more diffusive horizontal advection scheme. This leads to improved semi-diurnal variability in salinity in NEMO at a mooring site located in the Liverpool Bay ROFI (region of freshwater influence) area.

Highlights

The Irish Sea is a semi-enclosed shelf sea located between Great Britain and Ireland; Liverpool Bay is an area of the eastern Irish Sea, bordered by the North Wales and Lancashire coasts
This spatial pattern remains in the lowest resolution POLCOMS-Atlantic Margin Model (AMM) model with HI forcing, whereas the POLCOMS-Irish Sea Model (IRS) and NEMO errors are more homogeneous across the observed region
RMS errors for each CTD station (Fig. 5) show a less clear spatial signal than could be seen in the ferry comparison, though the POLCOMS-AMM model appears to be generally worse in the east of Liverpool Bay, along the coast

Summary

Introduction

The Irish Sea is a semi-enclosed shelf sea located between Great Britain and Ireland; Liverpool Bay is an area of the eastern Irish Sea, bordered by the North Wales and Lancashire coasts. Liverpool Bay in particular is considered to be a region of freshwater influence, or ROFI (Simpson, 1997), and is subject to input from several major river systems including the Conwy, Dee, Mersey, and Ribble estuaries (Fig. 1d). The freshwater input to Liverpool Bay is estimated to be 7.3×109 m3 annually (Polton et al, 2011). The interaction of the resulting horizontal salinity gradient and strong tides leads to a cycle of stratified and mixed conditions known as strain induced periodic stratification (SIPS) (Simpson et al, 1990; Howlett et al, 2011; Polton et al, 2011). On the ebb tide vertical shear in the tidal current (due to bed friction) means that surface fresh water is advected over saline water, creating stratified conditions. Excluding non-linear effects, on the flood tide the return vertical shear flow restores the stratification to its original profile

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