Lagrangian Transport and Transformation Experiment LaTTE

1
Donglai Gong , Jennifer Bosch, Robert Chant, Josh
Kohut, Hugh Roarty, Scott Glenn Institute of
Marine and Coastal Sciences, Rutgers University,
New Brunswick, NJ USA
The behavior of the buoyant coastal plume at the
mouth of the Hudson River and shelf wide
circulation on the NY Bight are studied using
high resolution HF Radar current maps, SST, and
satellite ocean color data products provided by
the Rutgers Universitys Coastal Ocean
Observatory Lab. Coastal plumes and shelf
circulation can exhibit complex behaviors that
depend on a variety of physical forcings such as
topography, river discharge, wind, tide and
background flow. 2-D time series of surface
current and temperature allow us to capture
several dominating features of plume and shelf
dynamics. LaTTE is a process study in a research
observatory, which together provide the context
and framework to study these shelf processes in
an interdisciplinary manner. The 2005 field
experiment took place in the middle of April soon
after the main spring discharge event.
Two high discharge events occurred at the end of
March and beginning of April. LaTTE 2005 took
place a week after peak discharge.
Tidally driven eddies at the mouth of Hudson
River were observed at the beginning of April.
The river plume shoots out southeastward on
ebbing tide. It then turns southwest and west
due to a combination of wind and Coriolis effect.
The flow impacts the NJ coast near Shewsbury
Rock. The flooding tide then drives the flow
back up towards the harbor mouth. This
recirculating pattern repeats over many tidal
cycles. A few days later when strong upwelling
wind is established, the flow heads eastward
along Long Island instead of southward.
Recirculation presumably stops. Above figures
show strong convergence zone just south of NY
Harbor entrance and near perpendicular impact
angle with the coast. A Lagrangian drifter study
based on CODAR vectors shows a circulating
trajectory.
Hudson River discharge at the mouth if NY/NJ
Harbor is highly tidal driven, occurring in
pulses at tidal intervals. The exit angle of
each ebbing tide discharge depends on the wind.
How the plume exits can affect the flow dynamics
at the bight apex. Above figures show three
typical scenarios. Some conditions could lead to
the formation of a bulge as seen in laboratory
experiments and numerical simulations.
Wind during April can be divided into three
regimes. From April 2 to April 8, the wind at
Ambrose Light is generally upwelling favorable.
This leads to a clear mean offshore flow across
the entire shelf. River discharge also peaks
during this time. The surface current shows an
eastward advection of the Hudson plume along the
Long Island coast. Satellite SST shows a fresh
river plume coming out of the Hudson.

• Lagrangian Transport and Transformation
  Experiment (LaTTE)
• Participants
• Bob Chant (Rutgers), Bernie Gardner (U. Mass),
  Scott Glenn (Rutgers), Bob Houghton, (Lamont) ,
  John Wilkin (Rutgers), John Reindfelder
  (Rutgers), Bob Chen (U. Mass), Paul Bissett
  (FERI), Tom Frazer (U. Florida), Mark Moline
  (Cal-Poly), Oscar Schofield (Rutgers), Meng Zhou
  (U. Mass)
• Objectives
• To quantify the intensity and spatial structure
  of diapycnal mixing rates in a buoyant plume and
  its dependence on along-shore wind stress.
• To quantify the strength and structure of
  secondary flows in the plume, their dependence on
  along-shore wind stress, and to characterize
  their role in delivering buoyant fluid to regions
  of enhanced diapycnal mixing.
• To quantify the transformation and loss processes
  of estuarine contaminants and dissolved organic
  matter within the plume (i.e. CDOM
  photodegradation rates, contaminant metal uptake
  and remineralization rates, contaminant metal
  redox and speciation changes).
• To access the growth and species composition of
  the phytoplankton community in the evolving plume
  and the response of the zooplankton community.
• To relate variations in the chemical and
  biological processes of objectives three and four
  to changes in the plumes circulation, structure
  and mixing processes due to variations in the
  along-shore wind stress.
• To develop methods to assimilate tracers into
  coastal circulation models to test and improve
  current biological and chemical models.

Long range CODAR and SST imagery taken near the
time of the first and second peak river discharge
shows a coastal plume traveling down the NJ
shore. Because of the weak wind during this
time, the coastal plume seen above is likely
buoyancy driven. ADCP data (right most figure)
collected by mooring array (3/15/05 5/19/05)
shows strong along coast flow in the middle and
southern most moorings. We see almost no current
with a hint of northward flow in the northern
mooring. This is suggestive of a curved exit
flow at the NJ/NY harbor. Radius of curvature of
such flow can be calculated along with the Rossby
Number Ro v/fR (35 cm/s)/(0.9e-5 15 km)
2.6.
RGB OCM with CODAR overlay04 / 09 / 2005 409
P.M. GMT
RGB Terra with Codar overlay 04 / 18 / 2005 338
P.M. GMT
Exceptionally clear skies during the weeks of
LaTTE in April allowed nearly continuous
satellite coverage. RGB images from Terra and
Aqua satellites (U.S.) as well as Oceansat
(India) clearly show where the river plume water
is. The shape and size of the plume seen in RGB
correspond well with surface current measurements
at the same time (see overlay figures above). A
bulge seems to have formed off Sandy Hook by the
time the LaTTE 2005 began. A combination of
variable wind, high river discharge, and strong
spring tidal pumping contributes to the formation
of a bulge like structure. Detailed analysis of
the data along with a modeling effort is underway
to examine the possible bulge formation at apex
of the NY Bight.
From April 8 to April 17, the wind direction
becomes more variable. The mean shelf flow is in
the southwest direction. The surface current at
the bight apex exits the harbor, curves to the
right and continues along the NJ coast. A shelf
flow in the direction opposite the coastal plume
coincides with the location of a topographic
feature just off the northern coast of NJ.
Satellite SST shows the plume widens near the
mouth of the harbor. A relative warm piece of
water also coincides with the topographic feature
mentioned above, suggesting interaction with the
Hudson Shelf Valley.
The wide continental shelf of the coast of NJ and
NY exhibit complex dynamics on a variety of
temporal and spatial scales. HF Radar, satellite
remote sensing, moorings and shipboard sampling
are used during the LaTTE experiment to resolve
these events. High resolution CODAR surface maps
at the mouth of the NJ/NY Harbor show a pulsating
Hudson plume. Depending on wind, tide and river
discharge, the plume can flow either along the
Long Island coast, come down the NJ coast or
maybe something in between. We believe
conditions favorable for the formation of bulge
were observed during LaTTE 2005. Understanding
the underlying processes and quantifying the
events remain open research topics. At the widest
section of the NY Bight we have the Hudson Shelf
Valley. This topographical feature has been
shown to affect water mass distribution and flow
dynamics, as seen in SST and long range CODAR.
The HSV is a potentially important pathway for
nutrients/pollutants exiting the NJ/NY Harbor.
While we know some of the main factors
contributing to the transport along the highway,
how the highway is turned on and off and what is
the governing physics remain active areas of
research. During LaTTE 2004 and 2005 we observed
different aspects of the Hudson/NY Bight coastal
plume. While the majority of the time, the plume
does travel down the NJ coast, strong upwelling
wind can push the plume up by the Long Island
coast as seen in LaTTE 2004. Combinations of
variable wind, high discharge and strong spring
tide during LaTTE 2005 led to interesting flow
dynamics both at the bight apex and on the shelf.
Detailed analysis of high resolution remote
sensing and in-situ data as well as a numerical
modeling effort are being undertaken aimed at
unraveling the underlying physical and ecological
dynamics.
The Hudson Shelf Valley is a valley carved out by
the prehistoric Hudson River during the previous
ice age when the sea level was significantly
lower. It runs across one of the widest sections
of the Mid-Atlantic continental shelf. Past
studies using current meters suggested a
transport pathway. Surface current maps above
clearly show significant transport along the
Hudson Shelf Valley. The physics driving the
cross shelf transport along the valley is being
studied. The mechanism is not yet clear. This
transport highway appears often and is highly
dynamic. It can be turned on or off within a
matter of hours. Wind and tide are expected to
be the major factors affecting the flow. The
highway can be seen in both surface current data
as well as sea surface temperature data. The
transport highway (we fondly call it the
Turnpike!) can be a significant pathway for
nutrients and pollutants leaving the NJ/NY
Harbor. There is evidence that at least a
portion of the river plume water seen during
LaTTE 2005 was transported out of the NY Bight
apex through the highway. The ecological
implication of such transport is potentially
significant. Coupled physical and ecological
models such as ROMS/EcoSim will be used to study
the cross shelf transport and its impact on
marine ecology. Sampling strategy for LaTTE 2006
will undoubtedly take transport along the
turnpike into consideration.
X-Band (installed 2003)
L-Band
During the last interval from April 17 to April
27, the wind is mostly upwelling favorable with a
brief interlude of downwelling favorable wind.
The mean shelf flow is weak except at the Hudson
Shelf Valley (HSV), where there is a clear
offshore flow, indicating the HSV is a potential
transport highway across the NY Bight. The sea
surface temperature is warmer throughout the
region. Warm plume seems to accumulate near the
bight apex. Upwelling wind causes offshore
transport along the coast.
COOL Room Operational Research Oceanography
Wind clearly plays a major role in the behavior
of the plume at the NY Bight apex and shelf
circulation but it is not always the dominating
driver. The surface current varies greatly
within each tidal cycle. When the wind is weak
or variable, the flow could be driven by
geostrophy and ambient circulation. We are
investigating the physical forcing mechanisms
dominating the behavior of the plume and shelf
circulation at a range of temporal and spatial
scales.
Rutgers operates nine CODAR sites (6 long range,
2 standard range, 1 medium range) as part of the
North East Observing System (NEOS). Three
standard range CODAR systems, two owned by
Rutgers and one by Stevens Institute of
Technology provided high-resolution nested
coverage for the NY Harbor area during LaTTE 2004
and 2005. L-Band and X-Band satellite dish
receive sea surface temperature, chlorophyll and
ocean color products from polar orbiting
satellites from U.S., India and China. COOLRoom
is where all data is acquired and processed.