Numerical investigation into the effect of freshwater inputs on the circulation in Pelorus Sound, New Zealand

Observations collected in Beatrix Bay, a small bay within Pelorus Sound, New Zealand, show that although the bay is thermally stratified for much of the year, salinity stratification as a result of intermittent rainfall and river inflow plays a dominant role in the density structure and circulation within the Bay. A 3‐dimensional baroclinic model of Pelorus Sound is developed and applied to the simulation of idealised pulses of rainfall and river inflow to investigate their impact on the density structure and circulation within Pelorus Sound, focusing on Beatrix Bay. The model shows that the rainfall causes near‐surface salinity stratification of 1–2 psu, sufficient to introduce phase differences of c. 2 h between surface and bottom M 2 tidal currents. Following the river inflow, a surface tongue of fresher water is seen to propagate towards the mouth of the Sound, taking c. 4.5 days to reach Beatrix Bay, by which time tidal stirring in the Sound has caused the fresh water to become almost well‐mixed.