EFFECTS OF SILL PROCESSES AND TIDAL FORCING ON EXCHANGE IN EASTERN LONG-ISLAND SOUND

Temperature and salinity data from Long Island Sound (LIS) show that t he pycnocline rises seaward of Mattituck sill and that the magnitude o f the longitudinal density gradient over the area of the sill remains relatively constant throughout the year. This reflects the influence o f internal hydraulic processes and vertical mixing on hydrography and volume exchange. Records of moored instruments indicate intratidal and fortnightly variations of water column stratification and current vel ocity in eastern LIS. During spring tides, thick bottom mixed layers r eveal vertically homogeneous temporal changes in temperature, salinity , and sigma-t, which arise from the oscillatory advection of a longitu dinal density gradient. Intratidal variations translate into barotropi c rectification of the residual flow and vertical homogeneity of low-p assed temperature, salinity, and sigma-t. During neap tides the flow m ay be subject to hydraulic control. Near-surface temperature and sabil ity oscillations of higher amplitude than near-bottom fluctuations are a reflection of vertical excursions of the halocline. The halocline i s raised in flood periods and depressed in ebb periods. Intratidal str atification changes reflect the corresponding low-passed temperature, salinity, and sigma-t behavior with depth. Near-bottom residual flow f avors salt intrusion as it accelerates down the salinity gradient due to the adjustment of the longitudinal density gradient established on spring tides. The residual density and flow fields reflect a change in dynamical balances from spring periods, dominated by frictional stres ses, to neap periods, when inertial and rotation effects increase.