By Glen Gawarkiewicz
Woods Hole Oceanographic Institution
One of the main science goals for PEACH is to understand how waters from the shallow continental shelf intermingle with the waters of the deep ocean. The Cape Hatteras region is particularly complicated because there is southward flow that comes down from New Jersey and Maryland and northward flow that comes up from Florida and Georgia over the continental shelf. The collision of these two shelf currents results in a convergence of the shallow water flow. There can be large temperature differences where these northern and southern waters meet. Ultimately, all of these shallow waters are driven offshore near the Gulf Stream and become embedded beneath the surface waters of the Gulf Stream.
The Gulf Stream exerts a big influence on shallow waters both north and south of Cape Hatteras. The Gulf Stream separates from the continental slope in this area, and meanders of the Gulf Stream push and pull waters from the outer continental shelf, resulting in very complex patterns of temperature, salinity, and currents. Because the currents vary on time scales ranging from hours to days and can be strongly affected by the winds, it is a constant challenge while we are out on this cruise to interpret the data that is flowing into the ship from various instruments on the ship as well as the telemetered data from surface buoys, gliders, and satellites.
Our first cross-shelf transect north of Cape Hatteras revealed some interesting surprises. From historical data, we expected to see water temperatures of 10°C (50°F) or less and salinities of about 33 parts per thousand throughout the water column. However, the majority of the water over the continental shelf north of Cape Hatteras was 16°C or warmer (61°F). Similarly, salinities were 34.5 parts per thousand or greater, a substantial difference from what we expected.
A look at the near surface currents from the shipboard Acoustic Doppler Current Profiler (ADCP) suggests why the shelf might be so warm and salty. North of our section along 35.75°N, there is a strong onshore flow that likely is carrying the warm, salty water onshore. To the southeast of our section, we can see the beginnings of the strong northeastward flow associated with the Gulf Stream. That northeastward flow will carry all of the shallow waters away, and eventually out to the central region of the North Atlantic Ocean.
For more information on the PEACH project, please click HERE.