Florida Bay Physical Science Team
Draft Summary Report
March 22, 1999 Meeting - NOAA/AOML
Purpose
The Florida Bay
Physical Science Team (PST) first met September 4th and 5th
of 1997 to compare initial results, identify deficiencies and needs of the
program, and make recommendations. A
second meeting was organized for the spring of 1998 but had to be canceled due
to schedule conflicts of the participants.
The purposes of the March 22, 1999, meeting reported here were to review
progress to date, consider how research efforts are addressing the central
question(s) of the Strategic Plan for the
Interagency Florida Bay Science Program, and discuss future
plans. In addition the group was tasked
to consider recent results of the RMA10 hydrodynamic model and FATHOM water
budget model.
The central question
of the Program most directly related to PST efforts is Central Question #1: How and at what rates do storms, changing freshwater
flows, sea level rise and local evaporation/precipitation influence circulation
and salinity patterns within Florida Bay and the outflow from the Bay to
adjacent waters.
Meeting
Results
SEAKEYS update - Sandy Vargo:
The northwest Florida
Bay weather station was installed by the Coast Guard last summer south of East
Cape and data came online October 1998.
The data include standard atmospheric variables (wind speed and
direction, air temperature., humidity, and precipitation) plus enhanced oceanographic
measurements of sea level, conductivity and water temperature. The Long Key
station has enhanced ocean measurements of water temperature, salinity, sea
level, PAR, transmissivity and fluoresence. The Sombrero station is scheduled
for the same instrument configuration as Long Key but, at present, the only
ocean sensors working are temperature, salinity, and sea level as a result of
problems related to the National Data Buoy Center’s support for basic control
units. Ocean sensors may also be added
to Sands Key station in cooperation with Charles Yentsch (NASA). Table
1 provides a description of instrumentation at each SEAKEYS
station.
It was pointed out
that the combined sea level measurements at Long Key and Sombrero provide an
estimate of the cross-Keys sea level slope, which is believed to drive net
flows through the Keys passages and with calibration could serve as a
monitoring strategy for interactions of Florida Bay and reef tract waters.
Seasonal Variation of Water Mass
Properties - Elizabeth Johns:
Using available small
boats, salinity surveys were begun in Florida Bay and adjacent areas in 1995
then expanded in September 1997 with bi-monthly interdisciplinary surveys to
include the larger Atlantic and Gulf regions involved in the transport and
dispersal of Everglades and Florida Bay waters. Results from these surveys provide boundary conditions for use in
NOAA/NOS’s Florida Shelf Domain model and the ACoE’s small-scale Florida Bay
model. During this time a typical seasonal pattern is difficult to determine
for in 1995 there was record fresh water discharge and in1997/1998 there
occurred one of the strongest El Nino's on record and La Nina is dominating
1999. The data clearly show the complexities
of the spatial and temporal variations of the low-salinity plume resulting from
fresh water discharge management, tide, wind and coastal current forcing.
Everglades fresh water
discharge from the Shark River complex into the coastal waters west of Cape
Sable forms a low-salinity plume that is advected toward the southeast by
wind-driven and Gulf-driven coastal currents.
These tend to trap the plume against the coast of Cape Sable and the
shallow boundary of western Florida Bay which provides a secondary northwestern
low-salinity end member to Florida Bay.
The largest
uncertainty affecting our understanding the plume dynamics is lack of
quantitative fresh water discharge data. The U.S. Geological Survey (USGS) is
measuring discharge in several of the larger rivers and has agreed to provide
these data in the near future.
Expansion of water
mass surveys to include the Florida Keys and the southwest Florida Shelf west
to the Dry Tortugas and north to Naples has revealed strong vertical
stratification of temperature and salinity that develops in the mid-shelf
region in the summer. This appears to
be a seasonal process due to increased heating and decreased wind mixing but is
aided by significant low-salinity input in the upper layer. At present, the source of the low-salinity input
is unknown, but due to its large volume, mid-shelf location, and northward
extension along the shelf, it appears to derive from rivers in the northeast
Gulf of Mexico, possibly even the Mississippi River. Comparison of observations with USF/ECOHAB may clarify this
question.
Current Variability - Tom Lee:
The intensive
measurement period began September 1997 and consists of an array of moored
current, salinity, temperature and bottom pressure sensors maintained at the
southwest Florida shelf, the Everglades low-salinity coastal band, western
Florida Bay, and the Keys reef tract.
Advection/dispersion pathways of Shark River discharge are estimated
from surface trajectories of satellite tracked drifters. Tidal and wind forced volume transport
exchanges between Florida Bay and the Keys reef tract are estimated from
shipboard ADCP transects across the Keys passages.
Current variability on
the southwest Florida shelf seems to be primarily tide and wind driven. The
east-west flows are dominated by the semi-diurnal tides. The north-south flows
are near equally tide and wind forced. Also observed is a significant slowly
varying southward residual flow that is more persistent nearer the bottom and
further offshore. The cause of this flow is unknown at present, but it does not
appear to be related to wind forcing and may be connected to large-scale
coupling of the Gulf and Atlantic. The net effect of this residual flow may
contribute to the observed southeastward mean flow transporting Florida Bay and
west Florida shelf waters toward the reef tract. Surface drifter trajectories also show strong correlation to
local wind forcing, as well as a generally southerly net flow connecting the
Shark River discharge with the reef tract.
Subtidal flows through
the Keys passages can be either toward the reef tract or toward Florida Bay
depending on wind forcing. The
magnitude of these flows can reach 1400 m3/s from the combined
outflows through Channels 2, 5 and Long Key.
Seven Mile Bridge Channel can be of similar magnitude. Flows through the Keys passages are driven
by the sea level slope across the Keys. Measurement of sea level variations on
either side of Long Key clearly show that the subtidal (low-passed filtered)
sea level slope variations are highly correlated to the alongshore wind stress
over the reef tract. Results indicate
that wind forced coastal current dynamics in the reef tract control subtidal
sea level variability southeast of the Keys while local wind induced
setup/setdown is the primary cause of low-frequency sea level variations in
western Florida Bay. The net effect is
to cause Florida Bay waters to stand higher than the coastal waters of the reef
tract during summertime southerly winds, thus causing an outflow of Florida Bay
water toward the reef. Strong winds
from the east or northeast that occur in the fall and winter can cause the Keys
coastal waters to stand higher than Florida Bay and, thus, reverse the flow
through the Keys passages toward Florida Bay.
Water mass exchange (Western
Boundary) - Ned Smith:
Long-term current
meter measurements in Long Key Passage have been converted to volume transport
time series using current meter and Acoustic Doppler Current Profiler transects
across the channel for calibration. Average flow over a 8 month period was
toward the reef tract at 233 m3/s from current meters and 260 m3/s
from ADCP calibrations, showing very good agreement between these data and
those of Lee et al. High coherence was
found between subtidal volume transport fluctuations and winds in the direction
of 100 and 280 degrees. A next step is
to compare the volume transport time series to sea level difference and wind
time series.
Long-term current
measurements of flows across the western boundary of Florida Bay at 81E05' W longitude from stations near the
north, central, and southern parts of the boundary show inflow to the Bay in
the north and central stations and persistent outflow in the southern stations.
Similar results are shown in recent ACoE measurements along this boundary using
ADCPs. The estimated tide induced net
flow across this boundary is 1400 m3/s to the Bay, which will cause
significant tidal pumping of sea level inside the Bay which is higher than
previously reported. Several of the
Everglades National Park (ENP) tide gauges within the Bay have now been
reference leveled and these data will permit comparison of observed tidal
pumping with estimates of tide induced net flows (Dewitt Smith). A significant concern is whether the RMA10
model reproduces these pictures.
Hurricane Georges' wind fields and
precipitation over Florida Bay - Craig Mattocks:
The atmospheric data
for Hurricane Georges is being compiled for the period of influence over the
Keys and Florida Bay. Wind fields are being gridded for use with Bay
Models. The South Florida Water
Management District (SFWMD) precipitation grid over south Florida has now been
merged with NEXRAD derived precipitation fields over Florida Bay to produce a
grid over the Bay and Everglades with a two mile resolution that will be useful
for models. This was done for a single atmospheric event but will be expanded
over a longer period for model use. Local weather services are now putting wind
and precipitation maps through LAPS. The Miami Weather Service should be
approached as to their activities or plans for LAPS. However, these data are insufficient to provide wind and
precipitation fields for model runs.
Evaporation computations and
predictions - Craig Mattocks:
The Advanced Regional
Prediction System (ARPS) mesoscale meteorological model has been initialized
with surface property data on a 2 mile grid to produce 2 mile grid
evaporation/precipitation fields over South Florida and Florida Bay. The model clearly shows the influence of
agricultural activities in shifting hot spots (compared to runs without man's
influence) and divergence in wind and rainfall patterns. These gridded fields
will be supplied to the ACoE for use in their Bay modeling effort.
Evaporation/precipitation fields show considerable north-south gradients
between the Everglades and Florida Bay and within the Bay that are not
incorporated in ACoE's hydrodynamic model.
Smaller-scale, process-oriented
studies of advection/dispersion (inner basins) - Ned Smith:
Long-term current
meter deployments in three channels into Rabbit Key Basin all indicate a steady
mean outflow with low-frequency variability that appears wind induced. The
steady mean outflow suggests that the inflow occurs over the surrounding
shallow banks on incoming tides. These waters are trapped within the basin on
falling tides and forced out the deeper channels. More measurements of this type are necessary, however, in other
representative basins. In addition, the
Intracoastal Waterway appears to be an important conduit for water exchange in
the eastern Bay, but not much is known about this process at present.
In general it was
agreed that application of a 2-dimensional hydrodynamic model to the Bay was
appropriate. 3-D water mass structures and circulations may occur at times on
small spatial scales near banks or fresh water outflows or near the western
extreme of the domain. The effects are not considered to be long-lasting, nor
to generate significant baroclinicity.
FATHOM Mass Balance Model - Jack
Cosby:
FATHOM is a
sophisticated mass balance model that divides Florida Bay into 44 basins
separated by line segments paralleling the shallow banks. The model estimates
salinity in each of the basins or representative groups of basins on a monthly
basis to look at seasonal, annual, and interannual changes. The approach also
assumes each basin is well-mixed.
FATHOM is driven
by freshwater inflow and sea level setup and assumes salinity in the Gulf and
Atlantic is constant at 35 psu. It
applies tide and wind induced sea level at the open boundaries to the Key West
annual cycle of sea level and computes flow across segments (banks) from
Mannings equation. However, it ignores
flow through channels. Measured
rainfall estimates are used and considered uniform across the Bay. Seasonal and annual evaporation rates are
derived from a combination of data sources and assumed to be constant across
the Bay. While salinities were estimated for the period 1973 - 1995 and
explained approximately 50% of the observed variance, modeled salinities were
too high during wet season.
Uncertainties in the model may be the result of poorly known freshwater
discharge, unresolved evaporation and precipitation patterns, and mixing and
freshwater input along the western boundary.
There are also plans
to use the model to estimate residence times and initial estimates indicate
residence times for the northeast portion of the Bay of about 7-8 months. FATHOM is run on a PC and can be used to
evaluate different water delivery strategies for the Bay.
Since freshwater runoff that occurs in the northeast
portion of the Bay is a small fraction (about 10%) of the precipitation that
falls over the Bay as a whole, in FATHOM it affects only the northeast corner
of the Bay.
Discussion of models - Bill Nuttle:
While estimates of
annual evaporation rates in the south Florida region range from 75 to 207
cm/year, evaporation in Florida Bay is still poorly resolved and spatial and
temporal scales are uncertain. Precipitation is also poorly resolved but better
than evaporation. Accurate estimates of Bay salinities resulting from future
water delivery actions are needed to advise managers. Both the ACoE hydrodynamic model and FATHOM require better
resolution of evaporation and precipitation to make reliable forecasts of Bay
salinity. Approaches to obtaining
better evaporation/precipitation estimates are planned. These include:
1.
Craig Mattocks’ 2 km ARPS model using relative
humidity, temperature and wind measured at CMAN and Everglades National Park
sites for evaporation and NEXRAD supplemented by Everglades National Park
gauges while available (see #4 below) for precipitation.
2.
Eric Swain of USGS running regressions with soil
evaporation - correlation method.
3.
Peter Swart’s isotope method was discussed but was
felt to be of marginal utility.
4.
DeWitt Smith’s enhanced measurement network i.e.,
Dewitt is planning to install an additional evaporation/precipitation station
in the central Bay east of Whipray Basin.
Also Ned Smith is installing a relative humidity sensor on the Long Key
SEAKEYS station. These stations together with the Everglades National Park’s
Joe Bay station and the new SEAKEYS Northwest Florida Bay station south of East
Cape forms more of a triangle through the Bay allowing better north-south and
east-west coverage. Each station will measure precipitation, radiation,
humidity, temperature and winds for estimating evaporation. Consideration
should also be given to more sophisticated methods of estimating evaporation,
possibly using an eddy-correlation method.
Feedback to Modeling Groups - Bill Nuttle:
The two modeling efforts underway: the ACoE/WES hydrodynamic model and
FATHOM have agreed to estimate salinity in Florida Bay as a result of different
management scenarios: 1) changing fresh water inflows; and 2) removing the
railroad causeway at Lignumvitae.
The ACoE has issued a
formal report showing results of the RMA10 model evaluation and verification
and discussing possible impacts from changing fresh water deliveries to Florida
Bay. A similar report will soon be out for the FATHOM model.
Only a few members of
the PST have reviewed the ACoE model report dated Oct. 1998. However it was
pointed out that although the hydrodynamic model show very promising results in
comparisons of modeled and observed sea level, currents, volume transports and
salinity at several stations at the boundaries and interior of the model, there
were also many stations where the agreement was poor. This indicates that the
model calibration and verification has not been completed and there needs to be
a continued effort of model adjustments (roughness coefficients for example) to
improve the comparisons with observations. The model calibration effort must be
conducted as a cooperative effort between the modelers and observationalists to
avoid problems.
It was pointed out
that the magnitudes of the fresh water inflows to Taylor Slough and Shark River
Slough that were used in the ACoE model verification runs were a factor of 10
or more larger than observed. This caused salinities in the northeast portion
of the Bay to be much fresher than ever observed.
It was agreed that the
PST would review both the COE/WES Florida Bay model report and the FATHOM
report and send comments to Bill Nuttle for synthesis and transmittal to COE
and FATHOM investigators.
Appendix I
PHYSICAL SCIENCE TEAM
MEETING AGENDA
March 22, 1999
Attendees:
Peter Ortner, Dawn Welcher, Tom Lee, Libby Johns, Joe Boyer, Bill
Nuttle, Jack Cosby, Frank Aikman, Ned Smith, Ed Patino, Dewitt Smith, Peter
Swart, Sandy Vargo, John Wang, Hugh Willoughby, Craig Mattocks, Eric Swain,
Dave Gruber, and Joan Browder.
0930 Introduction (Ortner and Lee)
0945 SEAKEYS update (Vargo)
1000 Current variability (Lee)
1020 Water mass exchange - Western Boundary (Smith)
1040 Seasonal variation of water mass properties
- SW Florida Shelf and the Keys (Johns)
1100 Smaller-scale, process-oriented studies of
advection/dispersion (Smith [inner basins] and Lee [Shark River discharge])
1130 Hurricane Georges' wind fields and
precipitation over Florida Bay (Mattocks)
1150 Questions/Comments
1200 Working Lunch
1230 ACoE's modeling status
1300 FATHOM Mass Balance Model (Cosby)
1315 Evaporation computations and predictions
(Mattocks)
1330 Discussion of models (Nuttle)
*Water budget differences (FATHOM
and ACoE)
*MEG comments
1500 Final comments
*Data assimilation/exchange
*Next meeting
1600 Close