The Gulf Stream flows north from the Caribbean through the Gulf of Mexico, past Florida, up the U. S. east coast to Cape Hatteras where it turns eastward towards the Grand Banks and beyond to central and northern Europe. It is one of the major ocean currents in the world ocean. It also plays a major role in the global redistribution of heat from low to high latitudes, and hence a crucial role in ameliorating the climate around the northern North Atlantic, from Greenland and Iceland in the west to Scandinavia and central Europe in the east. The current has been studied extensively over the years. But with few exceptions, these studies and the concomitant estimates of its transport have been based on the dynamic method, an indirect method, which although accurate in its own way, can only obtain the velocity relative to some assumed (usually zero) reference velocity at depth. Further, given the very large scatter in these observations, it has proven difficult to accurately estimate the mean transport, let alone how it varies seasonally or from year to year.
It is possible to measure currents directly and oceanographers have used such instruments to study ocean currents in a wide variety of applications. These studies have been very helpful in learning about the dynamics of the Gulf Stream. Unfortunately, a mooring with current meters strung along it in the vertical is very costly to deploy and replace on a regular basis. Further, since the Gulf Stream may meander around more than it is wide, many such moorings would have to be maintained if the Gulf Stream is to be covered at all times. Instead, we need a method for repeatedly sampling the horizontal structure of the current so that we can obtain a better understanding of the nature of the large variability that had been observed in earlier studies. Such a technique has been available since the early 1980s, the acoustic Doppler current profiler, but it was only with the advent of the global positioning system (GPS) that navigation became precise enough that accurate current measurements from a fast moving vessel became possible.
The Bermuda Container Line (BCL) has for many years operated a
container
vessel service between Port Elizabeth, New Jersey and Bermuda. The
route
crosses the continental shelf, the Slope Sea, the Gulf Stream and
Sargasso
Sea twice a week. Since 1979 the BCL has provided very helpful support
to the National Marine Fisheries Service by letting an observer ride
along
once a month to measure upper ocean temperatures and salts across the
continental
shelf to the Gulf Stream. These observations have given oceanographers
a much clearer picture of long-term variability of the shelf and Slope
waters. When we learned in Spring 1990 that the BCL was building a new
ship, we became interested in the possibility of adding an ADCP to its
suite of instrumentation. Mr. Cor Teeuwen, the chief operations officer
of the company, was very supportive of the idea, and with the generous
support of the National Oceanic and Atmospheric Administration (NOAA),
operation of the ADCP began in late summer 1992 with the installation
of a narrow-band 150kHz RD
Instruments ADCP on Bermuda Container Line's container RO/RO ship,
the MV Oleander. Dr. C. Flagg at the
Brookhaven
National Laboratory (now at Stony Brook University) played a crucial
role in making the operation
possible. After the first five years, data collection has
proceeded with
support from ONR for a year after the initial support from NOAA ended,
and from NSF since then.
The RDI 150 kHz ADCP typically reaches down to 200-400 meters depth
depending on
weather conditions and the amount of backscatter material available. In
heavier seas bubbles get swept under the vessel blocking the acoustic
beams
from the instrument rendering it inoperative. The data acquisition
system
(DAS) software that operates the instrument, and acquires and saves the
data has been modified so that it can operate autonomously (AutoADCP).
This enables the instrument to modify its operating parameters
(settings)
to optimize the measurements when in shallow and deep waters,
respectively.
The ship's route across the continental shelf, the Slope Water, Gulf
Stream,
and part of the Sargasso Sea gives us an unprecedented opportunity to
measure
the local flow of these waters on a repeated long-term basis. The
project's
goal is to obtain a multi-year dataset that will permit detailed
studies
of seasonal and interannual variability of the upper ocean velocity and
temperature fields in the different regions of the western North
Atlantic.
At the time of this update, January 2004, we have collected 11 years of
ADCP
data.
Figure
1 shows a 'snapshot' of upper ocean currents (at 52 meters depth)
superimposed
on a high resolution image of surface temperature. One can see very
clearly
that the warm waters of the Gulf Stream are moving much faster than the
surrounding waters. The figure also shows how the current can change
path
and direction considerably, even on time scales of a week. This
variability
has been one of the major reasons it has been so difficult for
oceanographers
to obtain an accurate estimate of the average amount of water
transported
by the current. With the hundreds of sections like this across the
upper
ocean our picture of the circulation comes into clearer focus.
By taking all these crossings and
averaging them together, we obtain an
Eulerian average (time average at a point, in this case at all points
along
the line) of the currents between the shelf break and Bermuda, Figure
2. Color represents depth where magenta (NW and SE) indicates
shallow
depths. This figure illustrates the huge advantage of making regular
observations.
It includes all data between October 1992 and November 1999. We see
quite
clearly the Gulf Stream flowing NE while to either side we see a
westward
flow. The ellipses give a measure of the variance (both magnitude and
direction)
of the currents at each site. We note the largest variance right at the
Gulf Stream with a very sharp decrease to either side. Most of this
large
variance results from the meandering of the current itself as we will
see
below. The westward flow just south of the Gulf Stream is directed
almost
to the west. Much of these waters will rejoin the current and flow
east.
Closer to Bermuda the current vectors point more to the southwest.
There
is reason to think that these waters will not rejoin the Gulf Stream,
but
eventually curve to the south and east.
Another way to look at the Gulf Stream is
to work in natural coordinates,
that is to say, in a framework defined by the current itself. We do
this
by defining a local x-y coordinate system centered where the current is
a maximum (the origin), and with the y-axis defined by the direction of
that flow. We then take all other velocity vectors to either side of
the
maximum and plot the component that is parallel to the maximum as a
function
of normal distance from the center vector. This corrects for the fact
that
the ship does not cross the current at right angles and in so doing
assumes
that the current is flowing straight and parallel, a reasonable
assumption
for points not too far from the center. By superimposing all these
sections
we obtain a statistical description of the Gulf Stream (at 52 m depth)
in natural coordinates, i.e. its own framework. What emerges is an
astonishingly
stable picture of the current, Figure 3. Of course there is
scatter,
but we also see that the current can be characterized by two decaying
exponentials,
one to either side of the velocity maximum. The one on the northern
side
drops off more rapidly with a scale width of about 27 km vs. 45 km for
the southern side. We also see that the velocity maximum is 2.04 m/s
with
a standard deviation of 0.25 m/s. The scatter increases to the sides
due
to the presence of eddy activity , primarily local recirculations as
the
meanders pass by the Oleander line. A major objective of the program is
to quantify the variability of the Gulf Stream, how much does the
transport
of the upper ocean vary between seasons and from year-to-year. An
earlier
analysis, using all data from the first five years showed almost no
variation
on either seasonal time scales or between years (Rossby and Gottlieb,
1998).
We are in the process of reexamining this question now that the data
set
is nearly twice as long (during which time there have been some major
shifts
in the atmospheric forcing of the North Atlantic).
Another valuable aspect of the ADCP program includes the repeat
sampling
of sea surface temperature (SST). The temperature sensor sits in the
instrument
at the bottom of the vessel, about 4 m deep. Even if the ADCP data
cannot
be used due to heavy weather, the temperature sensor continues to work
fine. Figure 4 shows a time-distance plot of temperature for
the
first 12 years. It shows the actual temperature record with time along
the
x-axis and distance from New Jersey along the y-axis. The 12 vertical
bands
represent the annual cycle of warm and cold SSTs. We see that the
waters
on the shelf turn much colder in winter than those in the Sargasso Sea
(>600 km). We also see that the Gulf Stream is warmer than any of
the other
waters all the time, particularly in winter, reflecting the fact that
the
high speed of the current brings warm waters from the Caribbean much
faster
than they can be cooled off by the cold air masses coming off the North
American continent. The pairs of thin vertical black lines in this and
the next
figure delineate gaps in the data.
If we now remove the observed annual cycle at each distance point
from
New Jersey, we can get an idea of how much the SSTs vary between years.
The result is quite interesting, and has led to the development of new
ideas about what causes these fluctuations and how they affect the Gulf
Stream, Figure 5. Again, the transformation from figure 4 to 5
removes
the observed annual cycle and nothing else. We now see how the waters
north
of the Gulf Stream exhibit warm and cool periods, regardless of time of
year. These can last for several years, such as the 1994-1995 warm
period
and the cool period the following two and a half years. Further, when
the
waters north of the Stream are cooler than average, the Gulf Stream
assumes
a more southerly path (dark blue near 500 km), and vice versa (red).
(Note: an acoustic window was installed Sept. 2002, introducing
likely thermal shielding of the transducer.)
The lateral shifting of the stream can be seen more clearly in Figure 6 which shows the position of the northern edge of the Gulf Stream (defined as where the temperature drops 2°C below the maximum in the center of the current). The three lines show the observations (green), and the three month and one year lowpass filtered versions (black and red, resp.). We think the location of the current and the surface temperature anomalies are linked and result from a time-varying transport of cold waters along the Canadian shelf from Labrador. When this transport increases, it leads to a stronger cooling of the shelf and slope waters and at the same time forces the current a bit farther to the south and vice versa (Rossby, 1999; Rossby and Benway, 2000).
Significantly, the SST anomalies south of the Gulf Stream vary
independently
of those north of the Stream and generally have a smaller amplitude.
This
suggests that SST anomalies for the two regions may be forced by
different
mechanisms. Although we have not yet tested this, we think that SST
anomalies
south of the Gulf Stream most likely reflect variations in cooling by
the
atmosphere.
In summary, the repeat measurements of upper ocean velocity and
temperature
from the CMV Oleander are helping us to address many scientific issues.
The beauty of this measurement program is that it collects absolute
velocity
data with good accuracy and spatial resolution (every 2.7 km) in the
dimension
(or direction) that is normally inaccessible, namely repeatedly along
the
same horizontal path. Only a few other programs like this exist in the
world, namely across the northern North Atlantic and in the western
Pacific.
In our case, the sampling rate is high, twice a week, and traverses
several
different oceanic regimes from the shelf, across the Gulf Stream and
the
western Sargasso Sea. The ADCP reaches down through the wind-driven
mixed
layer and the seasonal thermocline. With the data obtained to date we
have
found the Gulf Stream to have a very well-defined and largely invariant
velocity structure, and that the bi-exponential structure of the
current
extends to at least 250 m depth. The transport by the core of the
current
remained remarkably stable independent of season during the first
several
years. We are looking at the more recent data to assess to what degree
this remains true on longer time scales. The reason is that during the
1990s there have been some major shifts in the wind systems over the
North
Atlantic, and we want determine to what extent these have induced
changes
in transport by the Gulf Stream. We have found a striking correlation
between
position of the Gulf Stream and sea surface temperature and salinity
anomalies
north of the Gulf Stream suggesting a new thermohaline rather than
winddriven
mechanism for governing the mean path of the current.
Cornillon, P. and K-A Park, 2001. Warm core ring velocities inferred
from NSCAT. Geophysical Research Letters, 28, 4,
575-578.
Dong, S, S. Hautala, and K. Kelly, 2007. Interannual variations in
upper-ocean heat content and heat transport convergence in the western
North Atlantic. J. Phys. Oceano., 37, 2682-2697.
Flagg, C., G. Schwartze, E. Gottlieb, and T. Rossby, 1998. Operating
an acoustic
Doppler current profiler (ADCP) aboard a container vessel. J.
Atmos.
Oc. Tech., 15, 257-271.
Flagg, C. N., M. Dunn, D-P. Wang, T. Rossby, and R. Benway, 2006. A
study of the currents of the outer shelf and upper slope from a
decade of shipboard ADCP observations in the Middle Atlantic Bight. J.
Geophy. Res., 111, C06003, doi:10.1029/2005JC003116.
Luce, David L. and Tom Rossby, 2008. On the size and distribution of
Rings and coherent Vortices in the Sargasso Sea. J. Geophys. Res. In press.
Rossby, T. and E. Gottlieb, 1998. The Oleander Project: Monitoring the variability of the Gulf Stream and adjacent waters between New Jersey and Bermuda. Bull. Amer. Meteor. Soc. 79, 5-18.
Rossby, T., 1999. On Gyre Interactions. Deep Sea Res. II., 46, 139-164.
Rossby, T., 1999. Guest editor of special issue of Maritimes devoted to Oceanography from Volunteer Observing Ships. University of Rhode Island.
Rossby, T. and R. Benway, 2000. Slow variations in mean path of the Gulf Stream east of Cape Hatteras. Geophysical Research Letters, 27, 117-120.
Rossby, T. and H.-M. Zhang, 2001. The near-surface velocity and potential vorticity structure of the Gulf Stream. J. Mar. Res., 59, 949-975.
Rossby, T., 2001. Sustained ocean observations from merchant marine
vessels. MTS Journal, 35, 38-42.
Rossby, T., C. N. Flagg and K. Donohue, 2005. Interannual variations
in upper-ocean transport by the Gulf Stream and adjacent waters between
New Jersey and Bermuda. J. Mar.
Res., 63, 203-226.
Ryan, J. P., J. A. Yoder, P. C. Cornillon, and J. A. Barth. 1999. Chlorophyll enhancement and mixing associated with meanders of the shelfbreak front in the Mid-Atlantic Bight. J. Geophys. Res., 104, 23, 479-493.
Sato, O. and T. Rossby, 1995. Seasonal and secular variations in dynamic height anomaly and transport of the Gulf Stream. Deep Sea Res. , 42, 149-164.
Sato, O. and T. Rossby, 2000. Seasonal and low-frequency variability of the meridional heat flux at 36°N in the North Atlantic. J. Phys. Oc., 30, 606-621.
Schollaert, S. E., T. Rossby and J. A. Yoder, 2003. Inter-annual
variability of phytoplankton chlorophyll: Monitoring slope waters
linked to the Gulf Stream. Earth
System Monitor, 13(3).
Schollaert, S. E., T. Rossby and J. A. Yoder, 2004. Gulf Stream cross-frontal exchange: possible mechanisms to explain inter-annual variations in phytoplankton chlorophyll in the Slope Sea during the SeaWiFS years. Deep-Sea Res. II, 51, 173-188.
Stammer, D. and J. Theiss, 2004. Velocity Statistics inferred from
the TOPEX/Poseidon-Jason-1 Tandem Mission Data. J. Marine Geodesy, 27,
doi:10.1080/01490410490902052.
Stoermer, S., 2002. Ekman flow and transports in the Northwest
Atlantic
from acoustic Doppler current profiler data. Master of Science thesis,
Graduate School of Oceanography, University of Rhode Island.
Wei J. and D.-P. Wang, 2008. A numerical study of stability and
propagation of warm core rings in a continuously stratified ocean. Continental Shelf Research. Submitted.
Wei J., D.-P Wang and C. Flagg, 2008. Mapping Gulf Stream warm
core rings from shipboard ADCP transects. J. Geophys. Res. Accepted.
Technical Issues
The technical aspects of operating an ADCP on a container vessel and
addressing the special problems we had to solve are discussed in Flagg
et al. (J. Atmos. Oceanic Tech., 1998 ). Combining data from the
attitudinal-GPS
(AGPS, a GPS-based compass) and ADCP `bottom tracking' mode has yielded
transducer misalignment angle and speed scaling factor to within
0.07°
and 0.3%, respectively. The ADCP data are processed using the publicly
available CODAS (Common Oceanographic Data Access System) data
processing
software developed at the University of Hawaii by Dr. Eric Firing,
Julie
Ranada, Ramon Cabrera and others.
People and Support
The Oleander project began in 1991 as a cooperative effort involving
Dr.
Tom Rossby,
Dr.
Peter Cornillon, Dr. Erik Gottlieb, Dr.
Charlie Flagg, George
Schwartze, and Sandra
Anderson-Fontana, and the CMV Oleander operators (Bermuda
Container Lines) and crew. It operated under a contract from the
National
Oceanic and Atmosperic Administration with additional support from the
Office of Naval Research through 1998. Starting in 1999 the project is
operating under 4-year grants from the National Science Foundation.