Development and application of a three-dimensional baroclinic model to the study of the seasonal circulation in the Celtic Sea

Abstract

A three-dimensional density-resolving model based on the Princeton Ocean Model (POM) has been developed for the simulation of tide, wind and density-driven flows in a region of the northwest European shelf extending from the Celtic Sea to the Sea of the Hebrides. Predicted co-tidal charts of the region are in good agreement with published charts based on observed tidal elevations and from previous modelling studies. Comparisons of observed and predicted M 2 and S 2 tidal elevations and currents suggest that in general, this model is of comparable or greater accuracy than previous large area models of the region, and tidally generated mixing is of the correct magnitude to enable accurate simulations of the location of tidal mixing fronts. The ability of the model to predict observed temperatures in the region was assessed by comparison with a comprehensive seasonal hydrographic data set collected in the Irish Sea in 1995. The modelled seasonal cycle of thermal stratification at a site in the western Irish Sea was in good agreement with a time series of observed temperatures. A statistical evaluation of model accuracy using all available data showed mean and root mean square (RMS) errors in near-surface temperatures of 0.25°C and 0.72°C, and of 0.05°C and 0.44°C in near-bed temperatures. A consideration of the geographical and temporal distribution of errors showed that the largest errors were near the start of the simulation when the model was consistently too warm, suggesting an inadequate representation of the initial conditions. Model predictions for the Celtic Sea in summer 1998 also compared well with a comprehensive spatial survey, with mean and RMS errors in near-surface temperatures of −0.18°C and 0.70°C, and of −0.01°C and 0.74°C in near-bed temperatures. Errors are in part due to inaccuracies in the initial conditions and the neglect of salinity variations in the model, in particular freshwater inputs in the Bristol Channel. Using the modelled flow fields to drive a particle-tracking model, the broad pattern of observed Lagrangian transport, an essentially cyclonic circulation pattern following the contours of bottom density, was successfully predicted. The baroclinic component of flow contributed 91% to the net westward flux along the frontal region in St. George's Channel, clearly demonstrating the importance of including the baroclinic component in models of the northwest European shelf. Whilst the model ably reproduces the broad scale features of the hydrodynamics observed in the Irish and Celtic Seas region, comparisons of the predicted vertical structure of thermal stratification with high frequency data sets suggest that the vertical resolution of the model is insufficient to accurately reproduce strong vertical gradients. Reproduction of the fine scale detail revealed by these data is currently the most stringent test for numerical models of the shelf seas. With increasing computer power and the development of higher resolution models, such data will act as the template against which to assess model performance.