1. WEST FLORIDA SHELF Physical Oceanography
Robert Weisberg
HyCODE P.I. Mtg, 1/7/02
First, I would like to thank Chris for organizing this workshop! It is a pleasure to be here.
Today I am going to give a brief introduction on the modeling study on the WFS circulation that that we conducted at USF.

2. Objectives and Applications
1) Determine how physical processes affect
material property distributions on the WFS
2) Determine the relative importance of deep ocean
forcing (by the LC and its eddies) and local forcing
by winds and buoyancy fluxes.
Applications:
Biological - Red tide, fish larvae
Chemical - Nutrients
Geological - Sediment resuspension and transport
Physical - Currents, sea level, T/S

3. Approach • Our approach combines in-situ measurements with
numerical circulation modeling
• Measurements are by moorings, ships, and satellite
• Modeling is by regional adaptations of the POM

4. Problem Definition

5. Wind Heat Flux Loop Current Desoto Canyon Mobile River
Apalachicola River
Suwannee River
Mississippi River
Wind
Hillsborough River
Heat Flux
The West Florida Continental shelf is one of the broadest continental shelves in North America. The isobaths on the WFS change very smoothly and generally parallel the coastline. The coastline in the north undergoes a right angle bend and the shelf width decreases to a minimum at the Desoto Canyon.
The shelf circulation is influenced by wind, surface fluxes, coastal river inflows (as listed in the figures) and offshore Loop Currents.
Previous research suggest that much of the effect of the Loop Current is confined in the deep water by the steep shelf break. The shelf circulation on the middle and inner shelves is determined by the local forcing (namely the wind, heat flux and rivers).
Peace River
Loop Current
Shark River

6. Accomplishments • Moored arrays: 1993-1998 • Moored array: 6/98-12/01
• Scaled down moored array: 12/01-present
• Hydrography: 6/98-12/01
• Real-time data
• Model: Mississippi River to Florida Keys
• Nowcast-Forecast Shelf Circulation Model
• Publications
Follow the order of increased time scales,
I will fist show some results on our study of tides on the WFS.
Then some results showing the response of WFS to synoptic, seasonal, and inter-annual variability.
Then an introduction of our very new nowcast/forecast system, which is only one-month old.
And finally , a summary and a slide showing what we are working on right now.

8. As Mark just mentioned, USF coastal ocean monitoring and predication system now give a real time measurement of the current, wind, surface flexes and the density on the WFS.
Beside the real-time in-situ measurement system, we also work on building a nowcast/forecast modeling system

9. Research Results

10. INNER SHELF DEFINITION
Three-dimensionality and the essential role played by stratification
Ref.
Weisberg et al. (2001) West Florida shelf response to local forcing: April Jour. Geophys. Res., in press

11. April 1998 measurement
Along-shelf velocity
Across-shelf velocity
Sea level at St. Petersburg
Wind velocity
Wind stress

12. Spring Season Transitional Circulation: The Cold Tongue and the “Green River”
Local forcing versus the LC
The dual role of winds and surface heat flux
Refs.
Weisberg et al. (1996) Geophys. Res. Lettrs., 23,
He and Weisberg (2001) Cont. Shelf Res., in press

13. • Princeton Ocean Model (2-6 km resolution, 21 layers)
• Driven by NCEP reanalysis
wind and surface heat fluxes
and river inflow
• Radiation condition along the
OB. e.g., the Loop Current is
excluded
• Comparisons are made between
the modeled and observed sea level
and current
We used the Princeton Ocean Model for our study. The figure in the upper
Panel show the curvilinear grid we used. The grid starts from west of
Mississippi in the northwest to the Florida Key West in the southeast, with
one open boundary arching in between. The grid resolution vary from 2 km
near the coast to 6 km near the Open boundary.
We forced the model by NCEP reanalysis wind and heat flux as well as the
long term monthly mean river runoff. By the way, we totally implement 7
major rivers of this region into our circulation model. Since we only consider
local forcing, radiation condition are used at the Open boundary rather to specify
the Loop Current.
Our model results are compared extensively with coastal sea level data and
in-situ ADCP current measurements that are indicated by these barely seen
blue dots. Also please remember the location of these four across-shelf transects,
as we will come back in a few minutes to show you the temperature fields along.

14. Sea Level Comparisons Data (thin line) Model (thick line)
The coastal sea level
responds primarily to
local, shelf-wide forcing
The model is initialized with the density field observed from a hydrographic
survey at the beginning of March, 99 and run for three months from March to
May, The figure show the sea level comparison between the model and data.
From the top to bottom are the time series of the wind, observed and modeled
sea level at Pensocola, apapachico, St. Petersburg and Naples.
The observed sea levels have been low-passed filtered to
remove the tides and inertial oscillation and are shown by the thin line. The model
results are shown with thick line. We see the agreements are very well with r
Square values all above 0.8. We conclude that the coastal sea level respond
Primarily to the local forcing.

15. Current time series from Model and data
at surface, mid-depth and near bottom
Seasonal Mean Current
Vectors and variance ellipses
wind
Surface
current
Mid-depth
current
We also compared the modeled current with 6 ADCP current data for that
period. As an example, the figure on the right show 3 month time series of
the wind, and the observed and modeled currents at surface, mid-depth and
bottom. We found that the model is able to pick up the observed synoptic
variability nicely. The complex correlation coefficient squared between the
modeled and observed currents at all three levels are about 0.8.
The overall comparisons with all 6 current measurements are shown
in the figure on the left, where we give the seasonal mean vector and the
variance ellipses at different mooring locations.
We can see the model driven by the local forcing is able to capture the seasonal
feature of observed current reasonable well too.
Based on these agreement between the model and data, we use the model to
discuss the seasonal circulation in spring 99
Bottom
current
Black (Model); Red (Data)

16. Model Sea Surface Temperature May 31
Seasonal mean depth
average current
Middle
Shelf jet
Model Sea Surface Temperature
May 31
“Cold
Tongue”
Present here are the spring mean depth average currents, modeled SST and modeled
sea surface salinity on May 31. We see the seasonal mean circulation field are
upwelling type with a southeastward jet located on the mid-shelf.
Associated with the southward jet are the annually occurring low temperature
Tongue that is due to the effect of surface heat a low salinity tongues that is caused
by the current advection of the river water.
In addition, Biologist also report an annual-occurring spring chlorophyll plume
on the mid shelf/ The plume is also called “green river” extend southeastward from
Cape San Blas. Our result show that it is the mean circulation transport
nutrient rich Mississippi , Mobile and Apalachicola river water to the mid-shelf
and produce the aggregation of chlorophyll.
Since all the feature can be produced by the model without Loop Current, we can
rule out the Loop Current as the reason for these unique temperature, salinity and
biological features.
Modeled Sea Surface Salinity
May 31
“Green
River “

17. Evolution of monthly mean flow fields
Early Spring
Evolution of monthly mean flow fields
Heat flux
cold
colder
warm
Late Spring
Heat flux
In addition to the seasonal mean, we produced the monthly mean current fields
to show the evolution of the shelf circulation. March mean is featured with the
coastal jet. In April, this jet migrate offshore to the mid and outer
shelf and reach its maximum in May.
The cartoon on the right show how this happen.
In the early spring, the heat flux is out of the ocean, cooling effect
make the shallow water much colder. Thermal gradient supports
southeastward baroclinic current, which enhance the wind driven part, results
a strong coastal jet. As time advance, the effect of surface fluxes changes
from cooling to warming, heats up the shallow water very quickly. The resulting
cold tongue then supports a cyclonic broclinic current. This current helps to
decrease the southeastward Current at the coast and increase the southeastward
current on the mid shelf.
Therefore, we see Surface Heat fluxes is the key component in this evolution.
Long tem current observation on the WFS oconfirms the existence of mid-shelf jet.
Surface heat flux, is needed to combine with the wind to produce this mid-shelf jet.
cold
warm
warm

18. Local change
Advection
Ocean circulation
Vertical and Horizontal Diffusion
primarily surface heat flux
We have also studied the shelf water temperature balance of the shelf water for the
spring season. Consider the temperature equation, the change of temperature is
caused by the advction and vertical and horizontal diffusion. The temperature
advection is due to the ocean circulation, while the temperature diffusion is
primarily due to the surface heat flux.
We performed a term-by-term diagnose of temperature at different depth on
The shelf. Each term after depth integration provide information on how much
it contributes to the temperature change of the water column.
Here as an example, we show the time series of depth integrals of advetion,
diffusion and their sum, which equal the local change of temperature for
a point on the shelf. Mean and std are also calculated for each series to provide
better idea on how things really works. We see for the seasonal mean, the ocean
advection produces cooling effect. The diffusion related to the Surface fluxes being
one positive and bigger, provides warming effect and largely controls water
temperature increases. However, STD of ocean advection is much bigger than
That of diffusion, indicating that in synoptic scale, temperature variation is primarily
controlled by ocean advection.
=local change
Mean (diffusion) > Mean (advection); Std (diffusion) < Std (advection)

20. Tides on the WFS Barotropic tides
Four major tidal constituents (M2, S2, K1 and O1)
Ref:
He and Weisberg, 2002, Tides on the West Florida Shelf
Submitted to Journal of Physical Oceanography

21. Modeled
Tidal current
ellipses
Observed
Tidal current
ellipses

22. Loop Current vs Local Forcing Influences
The effect of Loop Current forcing on the shelf superimposed upon
the effect of Local wind forcing on the shelf.
Refs:
He and Weisberg, 2002, A Loop Current intrusion case study on the West
Florida Shelf, manuscript submitted JPO.
Li and Weisberg, 1999a, West Florida shelf response to upwelling favorable wind forcing, Part I. Kinematic description. Jour. Geophys. Res., 104,
Li and Weisberg, 1999b, West Florida shelf response to upwelling favorable wind forcing, Part II. Dynamics analyses. Jour. of Geophys. Res., 104,

23. Daily SST June 1 – July 6, 2000

24. Southward
Geostrophic Current

25. Daily subsurface Current map June 1 – July 6, 2000

26. “Mature “Young Phase” Phase “ LC only LC only LC + Upwelling LC + wind
Downwelling
Wind
LC +
Downwelling
wind

27. Inter-annual variability of the WFS circulation
Inter-annual variability in the circulation
as accounted for by
Local and offshore forcing
Ref:
Weisberg and He (2002). Anomalous circulation on the west Florida shelf: Local and offshore influences.

28. Inter-annual variability
In the local wind forcing

29. Time Series of Topex SSH Anomaly Along Track 26
ADCP Current Measurements
(PA 30m, CM 50m and
LB 20m)

30. Local Forcing
Only
Spring 1998
Local Forcing
Only
Spring 1999

31. Local Forcing
Only
Local Forcing
Only +
Loop Current
Spring 1998
Spring 1998

32. Current at 30 m in Spring and Summer 1998
Local forcing only
Local forcing +LC

33. Current at 50 m in Spring and Summer 1998
Local forcing only
Local forcing+LC

34. Current at 20 m in Spring and Summer 1998
Local forcing only
Local forcing +LC

35. Temperature Along Mote, USF Sarasota, Tampa Transects between March
8/5
3/30
200m
USF
30m (Mote)
9/9
50m USF
5/18
30m (Mote)
11/9
200m
USF
6/10
200m
USF
Temperature Along Mote,
USF Sarasota, Tampa
Transects between March
And November 1998
7/7
200m
USF

36. Summary
• Collected in-situ data: currents, sea level, hydrography, surface
met., including a real time network (COMPS).
• Developed a regional circulation model (Miss. River to FL. Keys)
for hindcast studies and for coupling with a biological model.
• Developed a regional nowcast-forecast circulation model.
• Performed model-data comparisons for tidal, synoptic,
seasonal, and inter-annual time scales.
• Local forcing, independent of the Loop Current and eddies,
accounts for much of the WFS variability, including the spring
cold tongue and “Green River”.
• The Loop Current modifies the effects of local forcing, primarily
by setting the height of material surfaces at the shelf break.
Occasionally, when impinging at the southern edge, the LC also
sets the inner-shelf in motion.
• The circulation, and its control on material property distributions,
is fully three-dimensional.
• An asymmetry occurs between upwelling and downwelling wind
responses due to inner-shelf control by the bottom Ekman layer.