Nearshore currents off Long Island

Abstract

Some new and interesting current meter observations have recently been reported by Scott and Csanady [1976]. One of their results is the finding of a linear relationship between longshore wind stress and currents observed 2 m above the bottom. Using a frictional model, they are able to draw inferences about the longshore slope of sea level in the absence of wind. The purpose of the present note is to expand on their interpretation of the longshore slope and to offer some cautionary remarks concerning the comparison with geodetic leveling.The model of Scott and Csanady (their equation (3a)) becomes rwx/p = rt2b + gh ( O •'/ O x )where the x direction is along the coast, rw is the wind stress, and r is a resistance coefficient; the other symbols have traditional meanings. By careful analysis and prudent averaging they arrive at their Figure 8, which allows them to determine a value of 0.158 cm/s for r and an intercept of 0.46 cm"/s" for the wind stress (divided by p) when the longshore current goes to zero, 2 m above the bottom. From the intercept, Scott and Csanady calculate a longshore slope of the sea surface of 1.4 X 10-*, or about 7 cm in 500 km. They conclude that sea level falls about 7 cm from Boston, Massachusetts, to Atlantic City, New Jersey. Such slopes of sea level, on the coastal side, are found with every known current; they are consistent with the observed density distributions and are dynamically in balance with the slope of the sea surface across the coastal flows as the currents change latitude. These effects have been discussed elsewhere [Sturges, 1974].The density distribution off the northeastern coast of the United States suggests that sea level slopes down from north to south about 10 cm along the 2000-m isobath, relative to the 2000-dbar surface, from Cape Cod to the Chesapeake Bay. The data of Scott and Csanady gave much the same longshore slope close to shore, suggesting that the longshore gradient changes little with distance from the shore, at least out to the 2000-m isobath. One interesting and perhaps surprising aspect of this resultis to see how the longshore sea level slope changes with season. Figure ! shows the annual trend of sea level at Boston and Atlantic City. The means between the two curves have been offset by 7 cm in the sense suggested by Scott and Csanady, which is in reasonable agreement with the value obtained from the large-scale density field just offshore discussed above [see Sturges, 1974, Figure 1]. It appears that the longshore slope is a minimum in the fall of the year when the longshore wind stress is also a minimum. The yearly average winds are out of the west southwest; the longshore stress is approximately 0.5 dyn cm" [Hellerman, 1967]. In the summer they are lighter and Copyright ¸ 1977 by the American Geophysical Union. Paper number 6C0919. more variable; in September they are weak and shift around from SSW to the north or northeast [Marine Climatic Atlas, 1956]. Whether this change in longshore slope is associated with a change in the mean flow field is a...

Cross-shelf circulation and momentum and heat balances over the inner continental shelf near Martha's Vineyard, Massachusetts

“…Over the inner shelf, the balance tends to be between three terms: along-shelf wind stress and bottom stress and pressure gradient (Scott and Csanady, 1976;Pettigrew, 1981;Lentz and Winant, 1986;Masse, 1988;. In the surfzone, the wave radiation stress gradient again becomes a dominant term and is balanced by bottom stress (Thornton and Guza, 1986;Svendsen and Putrevu, 1994;Feddersen et al, 1998;.…”

The dynamics of mean circulation on the continental shelf

“…From the vorticity tendency balance, it was shown that the assumption of vanishing cross-shore transport at all distances from the coast implied a constant longshore pressure gradient. The magnitude of this gradient was viewed as a parameter expressing the influence of the rest of the ocean on the shelf region considered, and was determined from the analysis of observational evidence (Scott and Csanady, 1976). Key features of the observations, such as the presence of a "line of divergence" in the cross-shelf flow or the offshore increase in the magnitude of the longshore velocity, were reproduced by his model.…”

Temporal evolution of the momentum balance terms and frictional adjustment observed over the inner shelf during a storm

“…Early investigations (e.g., Scott and Csanady, 1976) pointed out that tidally driven fluctuations hinder the analysis of the wind-induced fluctuations over relatively shallow depths, where the frictional adjustment time may be similar or shorter than the tidal period. In these environments, for typical wind events, the wind-forcing, tidal and frictional periods are all comparable; in particular, the semidiurnal and frictional times become similar at depths of about 60 m (Csanady, 1982).…”