Scientists from the University of Maryland Center for Environmental Science (UMCES), Woods Hole Oceanographic Institution (WHOI), and the Virginia Institute of Marine Sciences (VIMS) recently carried out field experiments in the Chesapeake to explore how Bay waters mix. The Gooses Reef CBIBS buoy provided important longer-term data the researchers incorporated into their work.
Researchers conducted two sets of experiments, each lasting three months, to test the hypothesis that wind is the dominant driver for mixing in the Chesapeake Bay and other similar estuaries. In estuaries—where the river meets the sea—fresh water is driven at the surface toward the sea and the oceanic salty water moves landward underneath. The energy that drives this slow but steady circulation is thought to come from turbulent mixing generated by tidal currents as they rub across the bottom during their ebb and flow excursions.
But new evidence, especially for large estuaries such as the Chesapeake Bay, Delaware Bay, or Long Island Sound that have a low-to-moderate tidal range, shows that wind is at least or perhaps even more important than the tides in creating this mixing. Improving understanding of the circulation of these important water bodies will facilitate restoration and protection of their water quality.
The research team deployed 17 moorings and bottom-lander platforms in the middle part of the Chesapeake, stretching over 50 miles, to continuously monitor conditions. Each set of installations had both meteorological and oceanographic sensors, including Acoustic Doppler Current Profiler (ADCP) sonars that provided profiles of currents from top to bottom of the water column.
In addition, one highly instrumented research tower stretched from the bottom to above the water and was held stiffly vertical by four guy wires attached to anchors arrayed in a square. The high-frequency, high-resolution sensors on the tower measured wind fluctuations, surface waves, water turbulence, internal waves, helical flows called Langmuir cells, sharp current gradients called shears, and cross-estuary oscillations of the temperature and salinity structure.
The team of scientists also did work throughout the array of moorings and bottom-landers from on board the University of Delaware’s R/V Hugh R. Sharp, towing an instrumented flying wing called the Scanfish that tracked turbulence intensity, temperature, and salinity. These roving measurements filled in the spaces between the fixed instruments with high-resolution sampling.
CBIBS data played an important role, too, as the project incorporated the CBIBS Gooses Reef buoy into the instrument array. It also served as a long-term reference station that monitored Bay conditions between the first and second field experiments.
The science team enjoyed calm weather for launching and retrieving instruments and widely varying weather during the study, which provided a range of conditions and information for analysis.
Initial review of the data has revealed an unprecedented look at a rich set of circulation and mixing processes. While a definitive test of the wind hypothesis will have to await detailed analyses, the investigators believe sufficient results are in to convince them that wind mixing will prove to be the dominant mode of mixing in the Chesapeake Bay and similar estuaries.