Abstract :
Current meter data obtained off the mouth of Delaware Bay in the spring and autumn of 1984 are used to characterize the seasonal and subtidal variability in the source region of the Delaware coastal current. There are several major features of the coastal current which are common between the two seasons. At the mooring site the structure of the mean current indicates estuarine outflow from surface-to-bottom. The magnitude of the mean flow decreases and the direction of the mean flow veers further downshelf to the south and SSW with increasing depth. The water column is not vertically well-mixed, and a surface-to-bottom salinity difference of about 2 exists for both seasons. Local wind does not appear to be the dominant factor responsible for producing the subtidal current fluctuations, especially at very low frequencies (<0•1 cycles day−1). The current vectors are only coherent with the clockwise rotating part of the wind vector. In addition to these major features which occur in both seasons, there are major differences as well. The magnitude of the mean flow decreases by a factor of two and the mean salinity increases by about 2 from spring to autumn, probably in response to the seasonal cycle in the Delaware River discharge. During autumn, the subtidal currents show coherent fluctuations throughout the water column, and these currents are essentially rectilinear and aligned in the same direction. During spring, the current variance along the semiminor axis is more significant and there is an appreciable difference in the principal axis direction between the surface and the bottom. In spring, the current and salinity show coherent fluctuations at frequencies lower than 0•1 cycles day−1. These low frequency fluctuations are apparently caused by earlier river discharge events. The lag between the major river discharge events and the very low-frequency variations in salinity is about 3 weeks. In autumn, the current and salinity are significantly coherent at subtidal frequencies higher than 0•2 cycles day−1. The intratidal salinity variability is substantially larger in spring than in autumn. The strength of the buoyant outflow in spring produces an enhanced horizontal salinity gradient, which in turn results in an increase in the intratidal salinity variability relative to that in autumn. This result suggests that the tidal variability in salinity can be strongly influenced by the non-linear interactions with the subtidal variability in the Delaware coastal current.
