Intrusions of outer shelf and slope water within the nearshore zone off Long Island, New York1,2

Abstract

From 1974 through 1978 studies were made of temperature, salinity, and water velocity, by hydrographic surveys and from moored instruments, in the waters within 15 km of the southern Long Island coastline. Data of late summer reveal frequent intrusions of anomalous water which appear to be of slope or outer shelf origin. Three intrusions of notably differing character are described. Two occurred at middepth, within the pycnocline. Concurrent hydrographic and current meter data suggest that these water masses were transported shoreward due to circulation forced by wind stress and by longshore density gradients. The third intrusion was initially observed near the surface and was of anomalously high temperature. Its T/S relation was similar to a warm and salty bolus detected 12 days earlier in the midshelf region. Hydrographic data suggest that this bolus may have been a detached parcel of slope water. Such intrusions may commonly occur during summer and fall and may be related to the appearance of tropical fish in the Long Island -vicinity during these seasons.

A climatology of salty intrusions over the continental shelf from Georges Bank to Cape Hatteras

“…Moored observations and a few surveys suggest intrusions last at least a few days [ Flagg et al , 1994]. The presence of slope water intrusions near the coast [ Churchill , 1985] (also Figure 2) suggests that at least some intrusions must persist for more than 10 days, given the shelf width of about 100 km and assuming sustained, middepth cross‐shelf velocities do not exceed 10 km d −1 .…”

“…(There are also intrusions of relatively fresh shelf water over the slope Gordon and Aikman [1981] that are not discussed here.) These intrusions are often observed in hydrographic surveys and occasionally in moored observations (e.g., Figure 1) as lenses or tongues of anomalously salty water that are order 10 m thick and 10 km in horizontal extent [ Bigelow and Sears , 1935; Boicourt and Hacker , 1976; Voorhis et al , 1976; Gordon and Aikman , 1981; Houghton and Marra , 1983; Welch , 1981; Churchill , 1985; Burrage and Garvine , 1988; Gawarkiewicz et al , 1990; Flagg et al , 1994; Gawarkiewicz et al , 1996]. The tendency in these observations is for salty intrusions to occur during the spring and summer near the seasonal pycnocline.…”

“…The primary generating mechanism also remains uncertain. A number of different mechanisms have been proposed, including wind forcing [ Boicourt and Hacker , 1976; Churchill , 1985], warm‐core rings [ Gawarkiewicz et al , 1990, 1996; J. H. Churchill et al, Slope water intrusions onto Georges Bank, submitted to Journal of Geophysical Research , 2003], baroclinic pressure gradients [ Posmentier and Houghton , 1981; Aikman , 1984], interfacial drag [ Welch , 1981], and double diffusion [ Voorhis et al , 1976; Houghton and Marra , 1983].…”

The Mean Along-Isobath Heat and Salt Balances over the Middle Atlantic Bight Continental Shelf

“…4). Temperature increases more rapidly between Chesapeake Bay and Cape Hatteras because the 70-m isobath is over the slope where there is warmer slope and Gulf Stream water (e.g., Churchill and Cornillon 1991;Gawarkiewicz et al 1992;Savidge and Bane 2001;Flagg et al 2002). Standard deviations of the depth-averaged temperature (100-km along-isobath bins) are relatively constant (18-1.58C) from Georges Bank to Delaware Bay and then increase as the 70-m isobath gets near the shelf break and encounters both shelf water and the warmer slope and Gulf Stream water.…”

“…Standard deviations of the depth-averaged temperature (100-km along-isobath bins) are relatively constant (18-1.58C) from Georges Bank to Delaware Bay and then increase as the 70-m isobath gets near the shelf break and encounters both shelf water and the warmer slope and Gulf Stream water. Occasional, relatively warm, individual depthaveraged temperatures (.158C) evident throughout the MAB are probably warm salty intrusions of slope water (e.g., Gordon and Aikman 1981;Churchill 1985;Flagg et al 1994;Lentz et al 2003c). These general features of the along-isobath distribution of depth-averaged temperatures are also evident along the 30-, 50-, and 90-m isobaths.…”