The Interannual Variability of the Breakdown of Fall Stratification on the New Jersey Shelf

Abstract

During the seasonal evolution of stratification on the New Jersey shelf in the fall, strong thermal stratification that was established in the preceding summer is broken down through wind-driven processes and surface cooling. Ten years of output from a Regional Ocean Modeling Systems run and a one-dimensional mixed layer model is used here to examine the interannual variability in the strength of the stratification and in the processes that reduce stratification in fall. Our analysis shows that the strength of the stratification at the end of the summer is not correlated with the timing of shelf destratification. This indicates that processes that occur within the fall are more important for the timing of stratification breakdown than are the initial fall conditions. Furthermore, wind-driven processes reduce a greater fraction of the stratification in each year than does the surface cooling during the fall. Winds affect the density gradients on the shelf through both changes to the temperature and salinity fields. Processes associated with the downwelling-favorable winds are more effective than those during upwelling-favorable winds in breaking down the vertical density gradients. In the first process, cross-shelf advective fluxes during storms act to decrease stratification during downwelling-favorable winds and increase stratification during upwelling-favorable winds. Second, there is also enhanced velocity shear during downwelling-favorable winds, which allows for more shear instabilities that break down stratification via mixing. Observational data and model output from Tropical Storm Ernesto compare favorably and suggest that downwelling-favorable winds act through the mechanisms identified from the Regional Ocean Modeling Systems results.