Seasonal and interannual variability in the circulation of Puget Sound, Washington: A box model study

A prognostic, time-dependent box model of circulation in Puget Sound, Washington is used to study seasonal and interannual variations in residence times and interbasin transports. The model is capable of reproducing salinity variability in the Sound at seasonal timescales, and is shown to have hindcast skill at interannual timescales. Modelled transports vary as much between as years as between seasons. The largest seasonal feature is a sharp transport drop in late autumn into the deep Main Basin of the Sound, which is shown to be caused by increased river flow into Whidbey Basin. The high degree of transport variability leads to large interannual differences in residence times; for instance, for Whidbey Basin the residence time varies from 33 to 44 days in the period between 1992 and 2001 and for southern Hood Canal it varies from 64 to 121 days. This indicates that residence time estimates based on a year or less of data may not yield representative values. A forcing sensitivity study shows that in all basins except the South Sound, salinity variability in the Strait of Juan de Fuca accounts for more of the seasonal variability than river variabilitly does. However, year-to-year variability in river discharge charges affects interannual variability in transports as much as the Strait of Juan de Fuca salinity does. The model demonstrates poorest skill in the basins most affected by the Strait of Juan de Fuca salinity, indicating that the sparse data available for the Strait may not provide adequate boundary conditions for the model.