Massachusetts Bay represents an exceptional natural environment to study ISWs. The bay is bounded to the east by Stellwagen Bank, an underwater extension of Cape Cod, poiting NNE (see Fig. 1). The western side of the bank drops quickly (60 m over about 5 Km) into Stellwagen Basin, while the eastern side presents a more gentle slope towards the Gulf of Maine.
Several observations have been made of ISWs originating
from the bank and propagating onshore (westward) (Halpern,
1971, Chereskin, 1983) during the summer and early fall,
when a strong pycnocline (
, about 23
cpy)
located about 20 m below the surface
separates cold and salty waters from water warmed by solar heating and
freshened by river runoff.
In August 1998, researchers from the United States Geological Survey
field office in Woods Hole (USGS) and the Woods Hole Oceanographic
Institution (WHOI) conducted a joint experiment to characterize the
ISW
field and its impact on sediment transport within Stellwagen Basin.
Heavily instrumented moorings were deployed at three sites (Figure 1,
labelled A, B, C and D) along a cross-basin transect in order to observe
the evolution of the ISWs as they propagate westward from their
generation area near Stellwagen Bank.
During the one month deployment, two hydrographic
cruises collected CTD data along the line joining the
moorings, and a 200-Khz echosounder
was used to map the ISW field
acoustically.
A preliminary analysis of the data shows the existence of a very
regular ISW field, in sync with the semidiurnal tide
(Butman et al., 1998).
After being generated over the Bank, the waves propagate westward
towards the mainland. The packets arrive at mooring A
on average 
hrs
after low tide in Boston (the water elevation is essentially in phase
in the Massachussets Bay/ Bost Harbor system) and are spaced on
average 
hrs, which coincides with the frequency of M2,
the semidurnal component which is dominant in this region. The tidal
origin is therefore well established.