Seasonal Evolution of Hydrographic Fields in the Central Middle Atlantic Bight from Glider Observati

1
Seasonal Evolution of Hydrographic Fields in the
Central Middle Atlantic Bight from Glider
Observations
Coastal Ocean Observation Lab
Renato Castelao, Scott Glenn, Oscar Schofield,
Robert Chant, John Wilkin and Josh Kohut
castelao_at_marine.rutgers.edu
Introduction The hydrography over the
shelf in the Middle Atlantic Bight (MAB)
undergoes a substantial evolution throughout the
year. The majority of previous studies were based
on either mooring or relatively coarse
conductivity-temperature-depth (CTD)
observations. Here, we use very high-resolution
data to describe the seasonal evolution of
temperature and salinity in the central MAB
shelf. The use of gliders as observational
platforms allowed for shelf waters to be sampled
frequently over long periods of time, with a
total of 15902 CTD casts in 12 months. The
high-resolution observations provide a better
characterization of the spatial scales of
variability than in the past.
Cross-shelf transport During spring
and summer, freshwater lenses are found 100 km
from the coast (Fig. 4). A possible source is
advection of waters from the coastal current by
Ekman transport. As freshwater is transported
offshore, however, it is expected to mix with the
ambient water, decreasing the density anomaly of
the plume. Comparisons with a model based on
Ekman dynamics Lentz, 2004 suggest that the
density anomaly at the coast needed to explain
the occurrence of the freshwater lenses is much
larger than supported by observations.
Fig. 2 Temperature EOF along the Endurance
Line. The thermocline thickness (TT) is shown on
the top left panel.
Fig. 5 Surface velocities, drifters
trajectories and SST.
The first mode explains most of the
temperature variance in the upper layer (Fig. 2),
and is related to variations in surface heating.
Variations at depth are explained by mode 2,
being consistent with advection of colder waters
from the north during spring/summer and with
mixing during fall. The thermocline thickness
doubles in the outer shelf compared to regions
inshore of the 40 m isobath.
Methods Repeated surveys over the
shelf off New Jersey were conducted from late
October 2003 to early November 2004, as part of
the Rutgers University Glider Endurance Line
(Fig. 1). Hydrographic data were collected using
a fleet of Webb Slocum Coastal Electric Gliders.
Typical along-track resolution is about 200 m
near the shelf break improving to about 100 m
over the shallower regions.
m3s-1
Surface velocities, SST and drifters
trajectories (Fig. 5) during summer show the
persistent occurrence of a flow intensification
directed offshore and to the south, from about
the 40 m isobath near the Hudson River mouth
toward the offshore portion of the Endurance
Line. We suggest that the occurrence of this jet
following a period of high river discharge is a
possible mechanism for rapidly transporting the
freshwater across the shelf. In situ evidence of
offshore transport of freshwater along this
pathway is presented in Fig. 6.
Fig. 3 Salinity EOF along the Endurance Line.
Hudson River discharge is shown in gray on the
top right panel.
Fig. 1 Study area showing glider tracks (black
lines). Topographic contours are shown in meters.
Fig. 6 In situ evidence of the offshore
transport pathway.
Surface salinity (Fig. 3) variations
are related to the variability in the Hudson
River discharge (gray curve) and the wind
forcing. During the upwelling season (Apr-Sep), a
thin, 10 m thick surface layer of low-salinity
water extends for 100 km from the coast. A
similar widening of the region of fresher water
was observed in 2006 (Fig. 4). The widening of
the region with freshwater over the shelf is
consistent with Ekman dynamics (magenta curve).
The jet is present during summer (Fig.
7), peaking in early August. This is the same
period when the most frequent observation of the
lenses occurs. The jet transport is correlated
with upwelling winds on a scale of a few days.
Seasonal evolution of hydrographic fields
An empirical orthogonal function (EOF)
decomposition of the temperature and salinity
fields was performed in order to determine the
dominant modes of variability. For each variable,
the temporal mean at each bin was removed from
the corresponding time series.
Fig. 7 Time series of jet transport and
along-isobath wind stress.
Fig. 4 Time evolution of surface salinity and
equivalent depth of freshwater (Fs) along the
Endurance Line. The magenta curve on the right
panel is the integrated Ekman transport, which
gives the expected excursion length (Le, see
equation above) of the plume.
Further information Castelao et al. (2008),
Seasonal evolution of hydrographic fields in the
central Middle Atlantic Bight from glider
observations, GRL. Castelao et al. (2008),
Cross-shelf transport of freshwater on the New
Jersey shelf, JGR, in press. Lentz (2004), The
response of buoyant coastal plumes to
upwelling-favorable winds, JPO.

• Conclusions
• Gliders provide observations with very high
  temporal and spatial resolution.
• Freshwater lenses with large density anomaly are
  found 100 km from the coast, and they are
  inconsistent with a model based on Ekman
  dynamics.
• A jet is often found off New Jersey, which could
  be responsible for advecting the freshwater
  offshore from near the Hudson River mouth.
• The jet transport is correlated with upwelling
  winds on a scale of a few days.

Acknowledgments We thank the members of the
Rutgers University Coastal Ocean Observation
Laboratory for the highly successful glider
operations. Kipp Shearman and Jack Barth (OSU)
graciously provided some of the glider
observations, while the United States Coast Guard
Office of Search and Rescue provided the drifters
used in this study. This research was supported
by ONR and NSF. The observatory data used was
supported by ONR, NSF, NOAA, NOPP, DHS, DoD, and
the State of NJ.