A box model of the seasonal exchange and mixing in Regions of Restricted Exchange: Application to two contrasting Scottish inlets

We present a time-dependent box model of exchange and mixing processes in Regions of Restricted Exchange (RREs). The model is applied to two contrasting Scottish inlets, but can potentially be applied to a wide range of stratified inshore water bodies. The model represents the vertical structure of the inlet using up to three horizontally uniform layers, representing surface, intermediate and deep basin water respectively, and calculates the daily volume, thickness, salinity and temperature of each layer over an annual cycle. The model is forced by observed time series of daily-mean wind stress, river discharge, surface heat flux, and depth-profiles of the external coastal temperature and salinity. Advective and diffusive fluxes between the layers, and between the inlet and adjacent coastal ocean, are calculated from parameterisations of known physical processes in inshore stratified waters, utilising published analytical solutions and empirically-based formulae. The goal is for the model to be generally applicable to RREs to support improved resource management. As such, the number of free parameters within the model has been kept to a minimum.The model predictions of layer temperature and salinity are quantitatively compared to observations from year-long sampling programmes in two contrasting Scottish inlets, Loch Creran and Loch Etive. The model showed good agreement with the observed data, reproducing the broad scale seasonal cycle and other shorter-term fluctuations. RMS errors for temperature were less than 1 °C, and for salinity typically of the order 1. The model results suggest that Loch Creran behaved as a well-mixed box, with weak stratification maintained by strong vertical diffusion, and volume exchange between the inlet and coastal waters dominated by tidal forcing. In contrast, Loch Etive exhibited classical fjordic three-layer hydrography and dynamics, with strong stratification, weak vertical diffusion and a density-driven circulation comparable in strength to tidal exchange. The effects of a deep water renewal event on water properties were successfully reproduced by the model. The potential for wider application of the model to resource management issues and to other types of RRE is discussed.

► We present a box model of mixing and exchange in Regions of Restricted Exchange.
► We apply the model to two, dynamically contrasting, Scottish inlets.
► The model reproduces observations, and gives insights into exchange mechanisms.
► Time-varying exchange rates and flushing times for each model layer are calculated.
► The model provides a useful tool for effective resource management in RREs.