Spatial and temporal variability of turbulent mixing in an estuary

Observations with a microstructure/CTD proŽ ler and an ADCP during Ž ve cruises in 1994/95 provide a glance at the turbulent mixing climate of a partially mixed estuary, the Hudson River. The cruises took place under widely varying conditions of river discharge and external forcing by water level  uctuations in New York Bight. Tidal and fortnightly variability patterns of stratiŽ cation, shear and mixing characteristicswere qualitatively repeatable, largely following the description of Peters (1997). (i) Longer-term variations of buoyancy frequency and Richardson number (Ri) during  ood were weakly correlated with the river discharge. (ii) All  ood tides had substantial mixing. Cruise-to-cruisevariationsof turbulentdissipation rate, salt  ux, and stress were large and correlated with variations in Ri. (iii) During ebb tides, mixing was weak in response to stable stratiŽ cation during neaps, and it increaseddramatically toward spring tides parallelwith the occurrenceof low Ri. Cruise-to-cruise variations of mixing became small toward spring tide. (iv) The largest vertical salt  ux occurred during spring ebbs, which provided about 30% of the total fortnightly salt  ux;  oods throughout the fortnightly cycle providing most of the remainder. (v) The observed ‘‘level’’ of small-scale mixing, i.e. stress and vertical salt  ux, was consistentwith the integral momentum and salt budgets in the uniform segment of the Hudson where the bulk of the measurements were taken. (vi) One topographically forced  ow feature examined, lee waves behind a trench across the river, was associatedwith elevatedmixing.