Salt and heat trends in the shelf waters of the southern Middle-Atlantic Bight
Introduction
The Ocean Margins Program (OMP) was the most recent of a sequence of oceanographic experiments sponsored by the Department of Energy (DOE), focused upon the Middle-Atlantic Bight (MAB) region. An objective of this program was to quantify the role of the ocean margins, in the net removal of atmospheric CO2 over the shelf; likewise, its subsequent transport and burial in the deep waters of the bordering ocean. Previous DOE projects, such as the Shelf Edge Exchange Processes (SEEP) I and II, have covered the MAB from Cape Cod to the Delaware and Chesapeake estuaries (Walsh et al., 1988; Biscaye et al., 1994). The OMP 96 was concerned with the southern end of the MAB, or the shelf region inshore of the 75 m isobath and between Cape Henry of Chesapeake Bay and Cape Hatteras (Fig. 1); these will be referred to as the SMAB.
Between Chesapeake Bay and Hatteras, the shelf width to the 75 m isobath narrows by about 75% (from 110 to 30 km). Bigelow (1933) and Bigelow and Sears (1935) were the first to note the importance of this restriction on the prevailing southward flow, within the MAB (Fig. 1). This flow is sustained largely by the intermittent freshwater inputs along the coast, that maintain the offshore diabathic, sea-level gradient, driving geostrophically the coastal flow to the south. Despite the fact that the SMAB is supplied with runoff from Chesapeake Bay and by local sources, observations do not indicate any substantial continuation of the MAB flow past Cape Hatteras and into the South Atlantic Bight. Pietrafesa et al. (1994) estimated a flow of 0.25×105 m3 s−1, leaving the SMAB to the south; in contrast, Beardsley et al. (1976) determined 2.6×105 m3 s−1 entering the SMAB, off Cape Henry. We note that the magnitude of this convergence north of Hatteras appears to be substantiated by companion works from the OMP 96 data sets (Pietrafesa and Weatherly, 2000; Vetrano and Hopkins, 2002), although the seasonal and interannual variability are less well described. The primary route of the southward-drifting MAB waters has been considered, therefore, to be off-shelf (Fig. 1); thence, to the northeast, as reported first by Ford et al. (1952).
The effect of the bathymetric constriction at Cape Hatteras is a geostrophic flow convergence; this raises the sea level downstream, forcing the prevailing southwestward flow to veer offshore (Hopkins, 1982). Other dominant mechanisms, responsible for this offshore transport, have been attributed to meteorological forcing, entrainment processes and frontal instabilities, induced by the proximity of the Gulf Stream (GS) (cf. the reviews by Hopkins and Garfield, 1977; Beardsley and Boicourt, 1981). The region between the shelf break and the GS has been referred to as the “Slope Sea”, by Csanady and Hamilton (1988); it acts as a water mass buffer between the fresh MAB shelf waters and the salty Gulf Stream Waters (GSWs). Diabathic exchange (Fig. 1) is considered to be dominated by filaments of shelf water being entrained offshore (Ford et al., 1952; Fisher 1972; Lillibridge et al., 1990; Churchill et al., 1993; Churchill and Berger, 1998) as well as slope and/or GSWs being discharged onto the shelf (Gawarkiewicz et al., 1990; Churchill and Cornillon (1991a), Churchill and Cornillon (1991b); Gawarkiewicz et al., 1992; Churchill et al., 1993; Churchill and Berger, 1998). The position of the GS influences also the circulation on the SMAB (Böhm et al., 1998).
The primary emphasis of this work is on the seasonal evolution of the SMAB shelf waters, as observed in 1996, and the associated water mass exchange through the shelf-break section. The SMAB waters freshened and warmed strongly, during the observational period from February to October; this has led us to conclude that the subsequent winter period must serve as a period of strong salting and cooling, in order that the annual heat and salt contents approximately balance. In Section 4, the dominance of advection over atmospheric forcing, in closing this thermohaline cycle, is described; the calculation of cross-shelf exchange volume is made and compared with historical values.
Section snippets
Hydrographic data
The hydrographic data derived from the OMP cruises, undertaken during 1996. Four of these were carried out on the R.V. Oceanus, whilst it was deploying or recovering the OMP moorings (Fig. 1); this will be referred to as the February Cruise (First Deployment, 1–13 February 96), the May Cruise (First Recovery, 5–17 May 96), the June Cruise (Second Deployment, 22 June–1 July 96) and the October Cruise (Second Recovery, 7–17 October 96). An additional cruise was carried out on the R.V. Oregon II
Cold Pool Water
The Cold Pool Water (CPW) was defined firstly by Bigelow (1933); subsequently, it has been described by a number of investigators (Bigelow and Sears, 1935; Ketchum and Corwin, 1964; Myers, 1974; Hopkins and Garfield, 1977; Beardsley and Boicourt, 1981, Houghton et al., 1982; Ou and Houghton, 1982; Burrage and Garvine, 1988). The CPW is formed by winter convection over the northern portions of the MAB, Gulf of Maine and Scotian Shelf (Hopkins and Garfield (1979), Hopkins and Garfield (1981);
Annual salt and heat budgets
The freshening trend observed throughout the stratified season suggests that an opposing salting trend must operate during the unstratified season, to sustain a salt balance in the SMAB region. As an attempt to understand this quantitatively, we have constructed salt and heat budgets, based upon the four primary assumptions, as outlined below.
- (1)
The shelf water type of February 1997 can be represented by the mean February 1996 shelf water type (T=7.10°C and S=33.43 psu). To check this assumption we
Summary and conclusions
During 1996, a number of oceanographic cruises were carried out, covering the shelf and slope region between the Chesapeake estuary and Cape Hatteras (February, March, May, June, July, August and October). Four of these cruises were dedicated principally to the deployment and recovery of moored current meters and scalar sensors (February, May, June and October); the remainder were dedicated to CTD/Rosette hydrography, including chemical and biological parameters. These data provided an
Acknowledgements
Funding was obtained from the Department of Energy Grant No. DE-FG05-95ER62053 to which goes our sincere appreciation, as well as to the principal investigator of the project, Dr. L. J. Pietrafesa. Our thanks go to chief scientists Dr. Georges Weatherly of Florida State University (R/V Oceanus cruises) and Dr. Jeff Govoni of the Beaufort NOAA labs (R/V Oregon II cruise), to the masters and crews of the two mentioned vessels and to all the scientists and technicians who participated in the
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