SUB-TIDAL VARIABILITY IN THE HUDSON RIVER PLUME AS A RESULT OF HIGH FREQUENCY FORCING

1
SUB-TIDAL VARIABILITY IN THE HUDSON RIVER PLUME
AS A RESULT OF HIGH FREQUENCY FORCING
Session 050,543
Hunter, E.J., Rutgers University, Chant, R.J.,
Rutgers University, Wilkin, J., Rutgers University
Abstract
Observations of Sea/Land Breeze Variability
Ocean Model
Model cases
During the course of Lagrangian Transport and
Transformation Experiment (LaTTE) in 2004, 2005,
and 2006, the structure of the Hudson River plume
was highly variable. Although forcing due to
variations in discharge, low-frequency winds, and
ambient shelf circulation are important,
high-frequency forcing and sub-tidal response of
the plume is apparent in the observations. Tidal
mixing in the estuary manifests as fortnightly
variability in plume stratification in the 2005
and 2006 mooring records. Diurnal wind
variability related to the sea-land breeze system
(SLBS), while episodic, accounts for up to 50 of
the energy in surface currents in the New York
Bight Apex at times. Regional Ocean Modeling
System (ROMS) simulations demonstrate a subtidal
response to this forcing. Spring-neap variability
in tidal mixing modifies the estuary outflow
Rossby and Froude numbers, resulting in increased
freshwater transport in the coastal current
during spring tides with lower freshwater
transport during neap tides. SLBS variability
resulted in greater storage of fluid in the bulge
region of the Hudson River plume and freshwater
transport in the coastal current as low as 30
percent of the total river discharge.

• Grid
• 1 km horizontal resolution.
• 30 vertically stretched levels.
• Model setup
• Mellor and Yamada level 2.5 turbulence closure.
• Orlanski-type radiation Conditions.
• Tidal harmonics specified at the open boundary.
• Initial Conditions
• Constant temperature (5 degrees C).
• Constant Salinity (32 PPT).
• Quiescent fluid.
• Forcing
• No surface heat flux.
• Zero wind stress, idealized diurnal wind.
• Constant discharge (500 m3/s,5 C,0 PPT)

M2 Tidal Range Wind Regime
1 Decreased No wind
2 Normal No wind
3 Increased No wind
4 Decreased NS SLBS
5 Normal NS SLBS
6 Decreased NS SLBS
7 Mixed No Wind
Fig. 9) ROMS output. Representative surface
salinity and surface velocity for a) No wind,
normal M2 tide and b) SLBS, normal M2 tide.
The Regional Ocean Modeling System (ROMS
information online at http//www.myroms.org) is a
three-dimensional, free-surface, hydrostatic,
split-explicit, primitive-equation ocean model.
Background
Model Results
There is a dramatic increase in diurnal energy
near the mouth of the Hudson river beginning in
April. The diurnal major axis doubles with the
onset of the SLBS. (Hunter et al., 2007)
Estuary Mouth Results
Fig. 6) Time series of diurnal wind energy at
Ambrose Tower and the percent of total energy in
the diurnal band in the CODAR domain shown in
fig. 5). Grey areas show sea-breeze days
determined independently.
The objective of the Lagrangian Transport and
Transformation Experiment (LaTTE) is to
characterize the evolution of physical, chemical
and biological properties of the Hudson River
Plume (Fig. 1). The Hudson River plume is
typically considered to be a surfaced-advected
plume ((Yankovsky and Chapman, 1997) such as that
shown in Fig. 2).
The onset of the SLBS in April results in
enhanced diurnal band energy in the coastal ocean
near the Hudson river mouth. As much as 50 of
the total kinetic energy (at times) in the
surface water is the result of SLBS energy.
(Hunter et al., 2007)
Parameters of the Hudson River outflow vary with
tidal range. Increased tidal range leads to
increased outflow depth, decreased outflow Rossby
number and increased cross channel asymmetry.
With the exception of decreased tidal range
cases, the SLBS has little effect on outflow.
Coastal Current Results
River plumes in nature are rarely as well defined
as Fig. 2). Fig. 3) illustrates this variability
with three realizations of the Hudson river
plume. It is well-established that low-frequency
wind, river discharge, and ambient shelf
circulation contribute to plume variability. The
impact of forcing such as tides and high
frequency winds on low-frequency plume
variability is less understood.
The coastal current is dramatically modified by
the onset of the SLBS. Although the coastal
current is wider and deeper during SLBS events
(fig. 12), there is a decrease in freshwater
transport (fig. 13) in the SLBS cases, suggesting
enhanced storage of freshwater in the
recirculating bulge region due to the SLBS. This
is evident in the surface signature of the plume
(fig. 14) and consistent with Chant et al. 2008.
Observations of Fortnightly Variability
Fig. 7) a) Time series of CODAR radial velocity
along a single range cell (Fig. 1) positive is
away from the Hudson river mouth. North-South
wind (positive north) is overlayed. b) Sea level
height at Sandy Hook.
One net effect of the SLBS on the Hudson River
outflow is the redirection of the outflow at each
ebb tide. When on the northerly phase of the SLBS
the ebb moves along long island and while on the
southerly phases it moves into the coastal
current.
Conclusions
b)
a)

• SLBS variability and, to a lesser extent,
  spring-neap variability are clear signals in the
  LaTTE dataset.
• Changes in mixing over the spring-neap cycle in
  the estuary modify parameters of the outflow
  (depth, velocity, etc.) and hence the development
  of the plume.
• Spring tides tend to decreases stratification
  and increased freshwater transport in the coastal
  current and vice versa for neap tides.
• The SLBS acts directly on the Hudson river plume
  through both advective momentum flux and enhanced
  mixing .
• Freshwater transport in the coastal current
  decreases during SLBS events, with freshwater
  flux dropping to lt40 of river transport.
• The SLBS combined with the geometry of the Hudson
  River mouth provides a secondary freshwater
  pathway along Long Island.

Plume Results
Fig. 4) Time series of low-pass filtered
stratification at mooring locations in Fig. 1
along with tidal range at Sandy Hook.
References Hunter, E., R. Chant, L. Bowers, S.
Glenn, and J. Kohut (2007), Spatial and temporal
variability of diurnal wind forcing in the
coastal ocean, Geophysical Research Letters, 34,
L03607, doi10.1029/2006GL028945. Yankovsky, A.
E., and D. C. Chapman (1997), A simple theory for
the fate of buoyant coastal discharges, Journal
of Physical Oceanography, 27, 1386-1401. Chant,
R. J., S. M. Glenn, E. Hunter, J. Kohut, R. F.
Chen, R. W. Houghton, J. Bosch, and O. Schofield
(2008), Bulge Formation of a Buoyant River
Outflow, Journal Geophysical Research 113,
C01017, doi10.1029/2007JC004100. Contact Eli
Hunter, Institute for Marine and Coastal
Sciences, 71 Dudley Rd, New Brunswick, NJ, 08648,
hunter_at_marine.rutgers.edu
Fig. 8) Climatology of a) Hudson River discharge
and SLBS frequency and b) sea surface temperature
in April.
The mooring deployments in 2005 and 2006 show
spring-neap variability in the low frequency
stratification time series at all mooring
locations. Tidal currents outside of the estuary
are small, suggesting fortnightly mixing
variability in the estuary is related to
stratification in the plume. The Spring/Neap
variability in 2005 is somewhat obscured by a
large discharge event in early April.
Climatologies of river discharge, SLBS, and SST
show the onset of the SLBS with increased river
discharge and plume signature.