Three-dimensional Modelling of the Summer Circulation in the Celtic Sea

摘要

A three-dimensional density-resolving model based on the Princeton Ocean Model (POM) has been developed for the prediction of tide, wind and density-driven flows in a region of the north west 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 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 predictions 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 dataset collected in the Irish Sea in 1995. The predicted 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 0.30°C and 0.75°C, and of 0.08°C and 0.50°C in near-bed temperatures. Model predictions for the Celtic Sea in 1998 also compared well with a more limited dataset, although predicted temperatures were generally about 0.5°C lower than observed. This is probably due to the neglect of salinity variations in the model, both as freshwater inputs in the Bristol Channel and inadequate representation of inflowing Atlantic water at the southern open boundary. Using the predicted 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 was predicted to contribute 91% to the net westward flux along the frontal region in St George's Channel, clearly demonstrating the importance of its inclusion in models of the north west European shelf.