To determine the properties of the Long Island Sound residual exchange flow
at the two ferry transects. |
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| Determining the characteristics of the mean water circulation of the Long Island Sound is important as it provides information on the time a given water parcel, or the chemical and hydrographic properties it may possess, may reside in the Sound before being exchanged with the surrounding ocean. However, determining this mean water circulation for the Long Island Sound is a significant challenge due to the lack of observations of ocean currents in the sound and due to the large "signal to noise" ratio associated with these measurements. In regards to the latter, it is important to note the mean circulation values are typically on the order of a few centimeters per second. However, the Sound experiences large tidal fluctuations which make this smaller value hard to detect. For example, the figure above shows the current profile over the course of three days in 1990 from an Acoustic Doppler Current Profiler (ADCP) that was mounted on the bottom of the Sound in a region close to the Bridgeport - Port Jefferson ferry track. Evident are the very strong east-west current fluctuations which given their (near) twice-a-day period are known to be associated with tides. From this figure it is difficult to determine if the current in this region has a mean, non-zero residual value. In order to determine the mean value of the current from this sort of data, it is necessary to have quite long records (~weeks to months) in order that the tidal fluctuations can be averaged out of the record, leaving only the mean "residual" circulation. For this project, ADCPs are mounted on each particpating ferry. Data from these instruments/ferries will thus provide valuable information on the current structure across the the central and eastern end Sound. Having relatively long records of current variability at these two ferry transects will allow a greatly improved estimate of the residual circulation of the Sound and the associated flushing/exchange times with the surrounding ocean. |
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