The spectrum of subtidal variability in Chesapeake Bay circulation

The advent of long, continuous time series records of circulation in Chesapeake Bay has revealed the existence of large amplitude fluctuations within the subtidal range 0·03–0·6 cycles day^?1. These fluctuations represent direct and indirect response of the estuary to variations in wind stress, fresh water inflow, and coastal sea level. The fluctuations in circulation are accompanied by synchronous fluctuations in transportable properties such as salinity and temperature. A quantitative model is presented to explain this variability within the main stem of Chesapeake Bay in terms of a linear reponse to irregular, time-varying meteorological forcing. The model calculates transfer functions and energy spectra of laterally averaged transport, surface elevation, and salinity in two layers separated by a halocline, over the frequency band 0·03–0·6 cycles day^?1. Transfer functions between volume transport and wind stress obtained from one-month-long field experiments at three different cross sections in Chesapeake Bay are used to constrain model friction parameters. Using existing estimates for wind stress and coastal sea level energy spectra, energy spectra for volume transport and surface elevation are calculated as a function of longitudinal position. It is found that the observed volume transport spectrum at the mouth of the Bay can be explained quantitatively as the combined response to statistically indepentdent wind stress and sea level fluctuations. Variations in sea level account for 90% of the volume transport variance at the Bay mouth and dominate the volume transport spectrum below 0·375 cycles day^?1. In the upper Bay, longitudinal wind stress accounts for most of the variance. A maximum in the volume transport spectrum at 0·4 cycles day^?1, caused by a local maximum in the wind spectrum, is found at all upper Bay cross sections.