Salt and heat trends in the shelf waters of the southern Middle-Atlantic Bight

Abstract

This work reports on the evolution of water masses within the southern portion of the Middle-Atlantic Bight (MAB) and their exchange with the slope waters based upon the Ocean Margins Program hydrographic dataset (February–October 1996). Water mass distributions were quantified in terms of their content of freshwater and of Gulf Stream Water, with the Cold Pool Water (CPW) as the core shelf water. The CPW entered the area during the early spring, migrated offshore during the early summer and disappeared by early fall. An analysis of the salt and heat balance was estimated from the observed data, extended over an annual cycle. Surface heat and evaporative exchanges were obtained using an atmospheric Eta-coordinate model. This analysis supported the concept that the southernmost portion of the MAB circulates more as a positive estuary, exchanging with the slope waters, than as a shelf conduit exporting MAB waters to the Southern Atlantic Bight. Thus, the primary disposition of the shelf waters, entering the SMAB from the north, is in an offshore surface flow. This offshore flow requires, in turn, a sub-surface onshore flow. Significantly, the type of estuarine circulation switches during the year. During the stratified period, the circulation was analogous to that of a ‘highly stratified’ estuary; during the unstratified period, it resembled that of a ‘well-mixed’ estuary. The important difference between these two modes is that the winter exchange is enhanced, relative to that during the summer period; for example, the offshore flow increased from 2.8 to 3.4 10⁵ m s⁻¹ and the onshore flow from 0.44 to 1.0 10⁵ m s⁻¹, respectively. It is suggested that the enhanced exchange is linked to the reduced density gradients, in both the vertical and horizontal, characteristic of the winter convective season. The slower exchange during summer favored freshwater retention in the SMAB volume, with salinities decreasing by 2 psu over the period; the shorter, more intense exchange during the winter favored freshwater loss (salting). In addition to providing salt, the onshore flow brought enough heat to dominate the heat budgets; for example, the advective heat gain increased from 46 to 135 W m⁻², between the summer and winter periods. With regard to the OMP objectives, these results suggest a significantly enhanced potential for carbon loss off the shelf during the winter period, compared to that of summer.

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.