The Oleander Project: Tracking Ocean Currents from a Volunteer Observing Ship
H. Thomas Rossby, Professor of Oceanography
George Schwartze, Jr., Marine Research Specialist
Graduate School of Oceanography
H. Thomas Rossby is a professor of oceanography at the
University of Rhode Island's Graduate School of Oceanography. His interests
include the study of ocean currents and their variability and the development
of ocean instrmentation. He likes to go to sea..
George Schwartze,
Jr. received his training in electronics and computers while serving in
the US Navy for six years. He has been at GSO since 1976, working first with
the atmospheric chemistry group and now with the physical oceanography group.
In the early 1980s, a remarkable instrument that can profile upper ocean currents
from a ship came into widespread use. The acoustic Doppler current profiler
(ADCP) works by transmitting sound down from the ship and recording the return
echoes. The transmitted sound consists of short acoustic pulses that are beamed
in four directions: fore, aft, port, and starboard. The echoes result from the
sound bouncing off layers of zooplankton that normally inhabit the upper ocean.
Due to the relative motion between the waters below (on average the zooplankton
move with the water) and the ship, the reflected signal's frequency will be
shifted a small but precisely measurable amount. By listening to this frequency
shift (Doppler) as a function of time delay (the surface water echoes return
first and deeper waters later), one can construct a profile of Doppler shifts
as a function of depth. These can then be converted into a profile of currents
relative to the ship to of 200m to 400m depth. In years past, the lack of precision
navigation limited the system's usefulness to shallow waters where the ship's
motion could be tracked over the bottom or to coastal waters with good LORAN-C
navigation. With the advent of the Global Positioning System (GPS), a ship's
movement can be accurately tracked everywhere, and by subtracting the ship's
speed and heading (obtained with GPS) we can estimate the currents within a
range of a few centimeters per second.
In 1992, we installed the ADCP profiling instrument
the in the hull of the MVOleander and have operated it continuously since
then. The freighter Oleander (and its predecessors) operates between
New Jersey and Bermuda on a weekly schedule. The ship has been reporting weather
observations, temperature, and salinities for decades. The six years of current
measurements offer an interesting view of seasonal and annual variability. Figure
1 shows the ship's track and velocity vectors at 52m
depth on one such transit from Bermuda, with sea surface temperature obtained
one day later from an orbiting satellite. The figure shows the coincidence of
the warm waters and high velocities that characterize the Gulf Stream. Indeed,
it is these high speeds that bring the warm waters north from the Caribbean
Sea. By contrast, the velocity field in the surrounding waters is quite variable,
and only by making many trips can one obtain enough information to construct
an accurate estimate of the mean field of currents. Figure 2
shows the mean field along the Oleander line from the first six years
of operation. The long vectors pointing northeast between 37°N and 38°N
indicate the mean position and strength of the Gulf Stream. The ellipses provide
a measure of the variability of the currents: greater in the direction of elongation
of the ellipse and less in the normal direction. Their orientation in the direction
of the current suggests that most of the variability results from the meandering
of the current and not from a large-scale turbulent eddy field. Away from the
stream, both north and south, we observe a weak, rather uniform mean flow to
the west that cannot be discerned from a single transit as in Figure 1. Also,
the nearly circular variance ellipses indicate that the currents fluctuate with
equal likelihood in all directions. The westward flowing waters between the
Gulf Stream and 34°N look like they will rejoin the stream west of the line,
whereas the waters south of 34°N have a more southwesterly heading. The
34°N latitude seems to separate two regimes: a recirculating body of water
to the north, and a flow to the south.
Much of the focus of this program to date has
centered on the Gulf Stream and how it varies in space and time. In Figure 3,
we show a composite plot of all crossings of the current but plotted relative
to the velocity maximum of each crossing. That is each crossing, such as the
one in Figure 1, is plotted so that only the downstream components, or those
parallel to the velocity maximum, are retained. These are then plotted as a
function of cross-stream or normal distance from the velocity maximum, which
we position at the origin in the figure. Notice the general similarity of the
crossings and tight scatter around the velocity maximum: 2.05 plus or minus
0.25ms-1. This figure tells us that the Gulf Stream maintains a well-defined
structure regardless of its position and direction of flow. Significantly, the
maximum velocity shows virtually no variation with season or from year to year.
Further, if we sum the velocities from each transit across the Gulf Stream,
we find that downstream transport at this depth is relatively constant, with
little variation between seasons or from year to year.
The stability and steadiness of the structure
of the Gulf Stream means that its mass transport remains quite stable. This
observed steadiness of Gulf Stream transport appears to contradict other studies
that suggest that the Gulf Stream can undergo significant variations in transport.
During the 1990s, major changes in the wind forcing of the North Atlantic circulation
have taken place. We have noticed these indirectly through the very mild winters
that New England has experienced in the last few years. Will these large-scale
changes show up and transport changes in the Gulf Stream or be manifested in
some other way? Theoretical arguments suggest a delay of about three years,
the time required for a change or disturbance in the large-scale ocean circulation
to propagate across the ocean and manifest itself in the Gulf Stream. The Gulf
Stream also transports enormous amounts of thermal energy which is, needless
to say, of tremendous interest in terms of climate. Were the current to slow
down or its average temperature to drop, the amount of heat carried into the
northern North Atlantic would decrease, potentially causing very serious consequences
to the climate of Europe and perhaps other parts of the northern hemisphere.
By combining these observations with many other data sets, both concurrent and
historical, we hope to learn more about the stability of the Gulf Stream and
its mass and heat transport.
Curiously, while the structure of the Gulf Stream
has remained stable, it has shifted position substantially during this period.
In 1994-1995, it crossed the Oleander transit line almost 100km farther
north and, since early 1996, it has shifted back south. These shifts show a
strong correlation with major changes in temperature and salinity of the Slope
Waters. This correlation is evident in Figure 4, which
shows the anomaly of sea surface temperature of the waters between the Gulf
Stream and the shelf break as a function of time. An anomaly signifies that
the mean and annual cycles of temperature have been removed so that only the
departures from the annual cycle remain. (The actual sea surface temperature
varies from approximately 24°C in summer to approximately 11°C at the
end of winter.) Note the very cold waters in early 1997 and 1998 despite the
fact that those were very mild winters (especially 1998) along the east coast
of the United States. The second panel shows the position of the Gulf Stream
during the same period. Notice its southerly position when the waters are cold
and more northerly position when the waters are warm. A similar pattern of variability
can also be found in the 21-year long record of surface temperature and salinity
along the same line. Although that sampling program did not extend south across
the Gulf Stream, we can tell from the data that when the Slope Waters turn exceptionally
warm and saline, the Gulf Stream shifts north roughly 100km. What causes the
Stream to migrate north or south on these long, interannual time scales, and
why do the surface water properties change so dramatically? Our working hypothesis
is that the cold waters (associated with a southward displacement of the current)
result from a stronger flow of cold waters from the Labrador shelf.
We are now looking for other data sets to help
clarify this question. These figures illustrate how repeat transits enable us
to investigate questions that would be extremely difficult to pursue by other
means.
Our most sincere thanks and appreciation go to
the operators of the Bermuda Container Line for their strong and continuing
interest and support of these measurement programs on the MV Oleander
.