Modulation of residual currents in Rhode Island Sound by stratification and the spring-neap cycle

Circulation near estuarine-shelf interfaces controls important physical, chemical and biological exchange processes. A component of residual flow, or tidal rectification, can occur due to sloping bathymetry in these coastal areas due to the transfer of momentum from tidal frequencies to subtidal frequencies. Factors controlling rectification include available tidal kinetic energy and summer stratification. Many inner costal areas have regions where stratification balances tidal mixing creating a mixing front and strong residual flow. In addition to solar insolation and freshwater input, tidal mixing can modulate the position of this front. Rhode Island Sound (RIS) located south of Narragansett Bay and open to continental shelf waters is a convenient area to study different forcing of subtidal residual flow. We analyze data from moored Acoustic Doppler Current Profilers, chains of moored thermistors and conductivity-temperature-depth (CTD) instruments to study hydrography in this area from late 2009 to late 2011. Seasonal differences in the residual flow are observed with an intensification of a surface cyclonic flow around the periphery of RIS in the spring and summer, concurrent with an increase in stratification. Tidal kinetic energy is positively correlated with residual velocities at stations located in RIS. Along the periphery of RIS, residual velocities increase from neap tides to spring tides with the most significant velocity increase occurring in the top 25 % of the water. High amplitude (Spring) tides generate more vertical mixing at the near-shore stations than during neap cycles, corresponding to a significantly stronger RIS periphery current. We hypothesize that an increase in tidal kinetic energy over the spring-neap cycle changes both local hydrography and residual velocities through modification of tidal rectification and tidal mixing. Such changes in the periphery current will influence how, for example, nutrients and larvae from central RIS enter Narragansett Bay and how the semi-diurnal outflow from the Bay is either advected from or re-entrained into the system.