This study was a part of the Coastal Mixing and Optics experiment (CMO),
which is an Accelerated Research Initiative (ARI) funded by the Office of
Naval Research (ONR). The objective of the CMO experiment was to study the
mixing of ocean water on the continental shelf and the effect of mixing on the
transmission of light through the water.
Some of the specific objectives that were met by the Ocean Physics
Laboratory (OPL) with the passage of the two hurricanes were to:
1) Make distinctions among particle types, and
2) Relate optical and particle variability near the ocean bottom to
physical processes affecting sediment resuspension.
The significance of this study is:
1) It provided us with a unique set of physical and optical observations
during the passage of two hurricanes within a two week period,
2) The observations provide a data set for future modeling of near bottom
physical, geological, chemical, and biological processes, and
3) The modeling enables the study of the fate of bottom materials,
including sediment, organisms, and pollutants.
The CMO site is located in the Middle Atlantic Bight at 40.5 N
latitude, and 70.5 W longitude. It is about 100 km south of Cape Cod,
Massachusetts in approximately 70 m water depth. The site is in the Mud Patch,
which is a 2-14 m thick lens of fine-grained material consisting of up to 95%
silt plus clay. The first mooring deployment was from July 8 through
September 25, 1996. The tripod was first deployed from August 9 through
September 25, 1996. This presentation will cover the period from August 27
through September 21.
Mooring and tripod diagram
Several physical and optical instruments were deployed on a mooring and
two tripods. The tripods were ~400 m from the mooring. Instruments were
placed in Bio-Optical Systems (BIOPS) at 13.5, 37, and 52 m on the mooring,
and at about 2 m above the bottom on the optical tripod. BIOPS instruments
used in this study are the transmissometer, temperature sensor, WET Labs
ac-9, which measures absorption and attenuation concurrently at nine wavelengths, and the WETStar fluorometer.
BASS current meters, deployed at 7 depths
above the bottom by Sandy
Williams at WHOI, were also used in this study. We are concerned with the one
at 0.38 meters above the bottom (mab). The sampling rate for the temperature
sensors, fluorometers, and transmissometers was eight times per hour. The
ac-9 sampling rate was once per hour, and the BASS current meters sampled at 2
Hz for 28 minutes and 49 seconds. Additional instruments used in this study
include those on meteorological National Data Buoy Center (NDBC) buoy 44008,
which was located about 90 km east of the CMO site.
Hurricane Edouard reached category 4 status on the Saffir-Simpson
hurricane scale with 140 mph winds on August 29, 1996 when it was about 700 km
south of the CMO site. It maintained category 4 status for seven days, then
decreased in intensity, and moved towards the north, where the eye reached within
110 km of the CMO site on September 2, 1996.
Hurricane Hortense also reached category 4 status with 120 mph winds on
September 13, 1996 when it was approximately 800 km south of the CMO site. It
dropped in intensity to category 3 the same day. The eye was nearest the CMO
site on September 14, 1996, when it was within 350 km away. The distance of
Hortense to the CMO site was more than three times greater than that of
NDBC Buoy 44008 data
This plot shows meteorological data taken by instruments on NDBC buoy
44008. Barometric pressure dropped by about 45 mb with the passage of
Hurricane Edouard, and about 25 mb with Hortense. Wind speed evaluated at 5 m
above the sea surface increased from near zero to greater than 25 m/s with
Edouard, and from about 4 m/s to 15 m/s with the passage of Hortense. The
winds were more sporadic with Hortense due to the greater distance from the
CMO site. Wave height increased dramatically with Edouard, increasing from
less than 1 m to greater than 9 m. Hortense resulted in wave height increasing to
about 4 m. The significant wave period was much longer for Hurricane Hortense
due to the greater distance from the CMO site.
These plots show physical and optical time series data taken by BIOPS
instruments. The top panel shows temperature at four depths, with the
legend shown on the right. Summer stratification, which is common in this
area, can be seen during the first several days of the time series.
Stratification breaks down with the passage of Edouard, with mixed layer
depths (MLD) increasing from about 12 m to greater than 65 m. This was
calculated using a 1 degree temperature criterium. The water column remained
mixed for several days, and then began to restratify on September 9. A few
days following restratification, Hurricane Hortense passed over the mooring,
remixing the water column with MLD increasing from ~20 m to ~50 m.
The bottom three panels show beam attenuation (or beam c) without the
effects of water measured by the ac-9. (Notice the differences in scales of
the y-axes) We obtained good spectral quality
throughout the time series; the wavelengths are shown in the legend box on the
right. Beam c is a measure of turbidity, which is an indication of sediment
resuspension. Looking at the botom panel for Hurricane Edouard, sediment resuspension was greatest
at the 68 m depth, with ac-9 beam c increasing from mean levels of 1 m^-1 to
greater than 30 m^-1. The resuspension event can be seen in the 52 and 37 m
depths as well, with a half day time lag in resuspension between each depth.
The resuspension event was significant with Hortense, despite the great
distance from the mooring. Beam c values increased to greater than 20 m^-1,
with a time lag of three quarters of a day between each depth, and little
resuspension at the 37 m depth.
What materials were resuspended?
These plots are chlorophyll concentration measured by the WETStar
fluorometer vs. beam c measured by the transmissometer.
Notice the differences in scales of both the x and y-axes. These give us a
rough indication of what materials may have been in the water column by the
following method. High concentrations of chlorophyll and relatively low
values of beam c are indicative of phytoplankton in the water. Conversely,
high values of beam c with relatively low chlorophyll concentrations are
indicative of detrital matter.
If we look at the top panel, which was for
calm conditions before the passage of Hurricane Edouard, we see that there is
mostly phytoplankton at the 37 m depth, with little detrital matter. At the
52 m depth, there was about half phytoplankton and half detritus. The
material at the 68 m depth was mostly detrital matter. During the passage of
Hurricane Edouard (middle panel), beam c values increase by greater than 10-fold. There is
no longer any phytoplankton at any depth due to intense mixing of the water
column. We suspect that the high chlorophyll concentrations at the 68 m depth
were due to resuspended fluorescing matter off of the seafloor. After the passage of Edouard (bottom panel), the values of beam c and
chlorophyll concentration are almost back to calm condition values. There is
no phytoplankton at the 52 or 68 m depths, and some phytoplankton returned to
the 37 m depth.
What were the mechanisms of resuspension?
These plots are time series data measured or derived from the BASS current meters. The top panel is the
magnitude of mean current velocity measured by
the BASS current meter at 0.38 mab. The second panel is wave orbital velocity
estimated by integrating the energy under the wave peak in the velocity
spectra. The third panel shows mean current bottom shear stress computed
using low-pass filtered current velocity and the law of the wall formulation.
Combined wave and current bottom shear stress, computed using wave-orbital
velocity, mean current velocity, and the model presented in Christoffersen and
Jonsson (1985), is shown in the fourth panel. The bottom panel is dissipation
rate, computed using shear velocities estimated from the Christoffersen and
Jonsson model. The dissipation rate was near zero at calm conditions,
increasing greatly during the passages of the two hurricanes.
Sediment resuspends when bottom stresses exceed a critical value. We found
that the resuspension event forced by Edouard was induced by currents and
waves, while the resuspension during Hortense was induced by waves, with
little or no influence by the currents.
How do we know this?
If we look at the sediment resuspension events shown in the 68 m beam c
record (green) and overlay it with the mean current bottom shear stress (red),
we see that there is a high correlation between mean current shear stress
and bottom resuspension with Edouard; the R^2 is 0.68. But, there is no
correlation between mean current bottom shear stress and the resuspension
event with Hurricane Hortense (R^2=0.007).
If we do the same thing for the 68 m beam c record (green) and combined wave and
current bottom shear stress (purple),
we see a high correlation between combined wave and current bottom shear
stress and the resuspension events for both hurricanes. The R^2 for Edouard
is 0.80, and for Hortense, it is 0.75. This is strong evidence that the
resuspension event associated with Hurricane Edouard was forced by currents
and waves, and that of Hortense was forced by waves, with little or no
influence by the currents.
This is not unexpected because of the greater distance from Hortense to the
CMO site, and boundary shear stresses associated with waves can be of order
magnitude greater than those with a mean currents. This is due to the
formation of a wave boundary layer within the current boundary layer which
causes waves to resuspend material when the currents are too weak. The
currents are then present to transport the material along or across-shelf.
In conclusion, we captured two consecutive hurricanes within a two week
period during the CMO experiment. We found that beam c values increased from
1 m^-1 to greater than 30 m^-1 with Edouard, and to greater than 20 m^-1 with
the passage of Hortense at the 68 m depth. The sediment resuspension event
can be seen in the 37 m optical data as well. The resuspended material
consisted of mostly detritus as seen in the chlorophyll concentration vs. beam c plots and
supported by partitioned ac-9 absorption data, which will be
presented later. Hurricane Edouard resuspension was locally driven by
currents and waves, and Hurricane Hortense resuspension was remotely driven by
waves, with little or no influence by the currents.