Summertime Thermocline Salinity Maximum Intrusions in the Mid-Atlantic Bight

Mooring data with high vertical resolution have been combined with hydrographic data to investigate the character of high salinity thermocline intrusions in the shelf water-slope water frontal zone in the southern Mid-Atlantic Bight. The coincidence of current, temperature and salinity data has allowed a detailed investigation of how the intrusion process is initiated. The data show that the time scale for both the onset and duration of the intrusions is very short. Most intrusions at a single location seem to last no more than a day, while the onshore flow associated with many, but not all, intrusions accelerates very quickly attaining amplitudes of 10-20 cm s(-1). As has been noted in earlier studies, there is a correlation between the intrusions and upwelling favorable winds, but the magnitude of the wind stress is not sufficient to account for the onshore transport of the intrusions. The combination of current measurements with density profiles has allowed the calculation of gradient Richardson numbers associated with the intrusions. These calculations show that during the strong onshore flow periods there is significant mixing at the boundaries of the intrusions. At other times when the high salinity water is being passively advected along the shelf, there appears to be little shear turbulence at the intrusion boundaries. Continuous data coverage during the summer suggests that the intrusion process is actually fairly infrequent, apparently requiring an offshore preconditioning in which high salinity waters of the correct density are located just offshore in addition to upwelling favorable winds.