Estuarine Exchange Model of the Pamlico and Albemarle Sounds

Abstract

The thermohaline dynamics of the Croatan-Albemarle-Pamlico Estuarine System (CAPES) controls its estuarine circulation and exchange with the Atlantic Ocean. The exchange with the ocean controls the physical flushing of the freshwater input to the estuarine system and thereby has great influence on its biological, chemical, and physical environment. An improved effort in quantifying the exchange will assist in better defining the biological response of the CAPES to its external forcing. The present study demonstrates the utility of a new approach proposed by Hopkins (1999) for determining the exchange based on the internal potential energy difference between the system and the ocean. The method has the advantage of using easily observable data, i.e., the internal water balance and oceanic salinity. A model of the CAPES exchange, or the Albemarle Pamlico Sound Estuarine Exchange (APSEEx) Model, was constructed using simulation modeling software (EXTEND™). Models developed in this fashion are very economical and facilitate duplication, modification and transfer among users since EXTEND™ is cross-platform compatible (i.e., the software runs in both Windows and Macintosh systems). The major assumptions of the model are that the Sounds are laterally wellmixed to the depth of the sill, and that the exchange with the ocean occurs only through a single inlet having a combined cross-sectional area of the three barrier island inlets (i.e., Oregon, Hatteras, and Ocracoke Inlets). For lack of data, two important processes in the inlet, the vertical mixing (based on the Richardson number) and the bottom friction (based on Ekman dynamics), were calibrated by obtaining a best fit between the model and observed salinity in the Pamlico Sound. The calibrated model (APSEEx 1998-2000) was able to follow the trend of the internal salinity with R2=85.1%. By applying a mean water balance (1998-1999) and a mean ocean salinity (=29.5 ppt) to the year 2000 instead of their in-situ values, the model was able to predict salinity in the Pamlico Sound for a specific period, i.e., on the order of months. Time-dependent flushing times were calculated for the period 1998-2000. Freshwater replacement of the freshwater volume by river discharge takes ˜9 months, while the flushing of the PS volume by the inflowing ocean water is ˜3 months. To achieve a reliable simulation model, better time series salinity data from the ocean and sound would be needed, together with calibration data in the inlets for the salinity-mixing and bottom friction coefficients. Possible applications of the APSEEx Model to several scientific and management issues related to the CAPES are discussed. Suggested improvements to the model are also described.