DTIC ADA503626: Evaluation of Global Ocean Data Assimilation Experiment Products on South Florida Nested Simulations with the Hybrid Coordinate Ocean Model

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Evaluation of Global Ocean Data Assimilation Experiment Products on South
Florida nested simulations with the Hybrid Coordinate Ocean Model
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Villy Kourafalou, G. Peng, H. Kang, Patrick J. Hogan, Ole Martin Smedstad,
Robert Weisberg, Molly Baringer, Christopher Meinen
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14. ABSTRACT
The South Florida Hybrid Coordinate Ocean Model (SoFLA-HYCOM) encompasses a variety of coastal regions (the broad Southwest Florida shelf, the narrow
Atlantic Keys shelf, the shallow Florida Bay, and Biscayne Bay I and deep regions (the Straits of Florida), including Marine Protected Areas (the Florida Keys Marine
Sanctuary and the Dry Tortugas Ecological Reserve). The presence of the strong Loop Current/Florida Current system and associated eddies connects the local and
basin-wide dynamics. A multinested approach has been developed to ensure resolution of coastal-scale processes and proper interaction with the large scale Hows.
The simulations are free running and effects of data assimilation arc introduced through boundary conditions derived from Global Ocean Data Assimilation
Experiment products. The results indicate that the successful hindcasting of circulation patterns in a coastal area that is characterized by complex topography and
proximity to a large scale current system requires a dynamical.downscaling approach, with simulations that are nested in hierarchy of data assimilative outer models.
15. SUBJECT TERMS
numerical modeling, coastal dynamics, loop current, Florida current, data assimilation
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Patrick Hogan
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228-688-4537
Standard Form 298 (Rev. 8/98I
Proscribed by ANSI Std. Z39.18
Ocean Dynamics (2009) 59:47-66
DOI 10.1007/s 10236-008-0160-7
Evaluation of Global Ocean Data Assimilation Experiment
products on South Florida nested simulations
with the Hybrid Coordinate Ocean Model
Vassiliki H. Kourafalou • Ge Peng • HeeSook Kang •
Patrick J. Hogan • Ole-Martin Smedstad •
Robert H. Weisberg
Received: 11 March 2008 /Accepted: 18 October 2008 / Published online: 9 December 2008
© Springer-Verlag 2008
Abstract The South Florida Hybrid Coordinate Ocean through boundary conditions derived from Global Ocean
Model (SoFLA-HYCOM) encompasses a variety of coastal Data Assimilation Experiment products. The study evaluates
regions (the broad Southwest Florida shelf, the narrow the effects of boundary conditions on the successful hind-
Atlantic Keys shelf, the shallow Florida Bay, and Biscayne casting of circulation patterns by a nested model, applied on
a dynamically and topographically complex shelf area.
Bay) and deep regions (the Straits of Florida), including
Independent (not assimilated) observations are employed
Marine Protected Areas (the Florida Keys Marine Sanctu-
ary and the Dry Tortugas Ecological Reserve). The for a quantitative validation of the numerical results. The
presence of the strong Loop Current/Florida Current system discussion of the prevailing dynamics that arc revealed in
and associated eddies connects the local and basin-wide both modeled and observed patterns suggests the importance
dynamics. A multi-nested approach has been developed to of topography resolution and local forcing on the inner shelf
ensure resolution of coastal-scale processes and proper to middle shelf areas, while large scale processes arc found to
interaction with the large scale flows. The simulations arc dominate the outer shelf flows. The results indicate that the
free running and effects of data assimilation are introduced successful hindcasting of circulation patterns in a coastal
area that is characterized by complex topography and prox-
imity to a large scale current system requires a dynamical
Responsible editor: Brian Powell
downscaling approach, with simulations that arc nested in a
hierarchy of data assimilative outer models.
V. H. Kourafalou (E3) • G. Peng • H. Kang
Rosenstiel School of Marine and Atmospheric Science,
University of Miami,
Keywords Numerical modeling • Coastal dynamics •
Miami, FL, USA
Loop Current • Florida Current • Data assimilation
e-mail: [email protected]
P. J. Hogan
Naval Research Lab, Stennis Space Center,
Hancock County, MS, USA
1 Background
O.-M. Smedstad
The seas that surround the southern part of the Florida
QinetiQ North America, Technology Solutions Group—PSI,
peninsula form a unique oceanographic environment that
Stennis Space Center,
Hancock County, MS, USA
includes ecologically sensitive shallow areas (Florida Bay,
Biscayne Bay, the Dry Tortugas, and the Florida Keys Reef
R. H. Weisberg
Tract), shelf areas (the relatively broad Southwest Florida
College of Marine Science, University of South Florida,
shelf and the narrow Southeast Florida shelf extending to
St Petersburg, FL, USA
the Atlantic side of the Florida Keys), and deep areas
Present address:
(Straits of Florida); see Fig. 1. These regions are strongly
G. Peng
connected through their circulation systems and exchange
Raytheon Company,
processes on local and regional scales as well as with
Pasadena, CA, USA
£) Springer
48
Ocean Dynamics (2009) 59:47-66
Fig. 1 The South Florida
(SoFLA) model domain and
bathymetry (contours in m). Red
28N •
dots mark the ADCP mooring
sites (see Table 1). TB Tampa
Bay area, SWFS Southwest
Florida Shelf, DT Dry Tortugas,
FK Florida Keys, FB Florida
2TN-
Bay, AFKS Atlantic Florida
Keys Shelf, BB Biscayne Bay,
SEFS Southeast Florida Shelf,
CC Cape Canaveral
400 600 800 1000 1200 1400 1600 1600 2000
remote upstream regions of the Gulf of Mexico and the due to freshwater input from rivers dominate circulation
Caribbean (Lee ct al. 2002; Hamilton ct al. 2005; everywhere but in the outer shelf region, where the Loop
Current influence is important. Observational and modeling
Sponauglc ct al. 2005; Kourafalou et al. 2007). These
linkages arc a unique characteristic of the South Florida studies on the WFS (see Weisberg ct al. 2005 for a review)
coastal seas, directly connected to their proximity to a large have shown the strong variability in the WFS current
scale current system, namely the Gulf Stream, that starts as regime on several time scales and for different circulation
the Loop Current in the Gulf, extends through the Straits of governing dynamics. The inner shelf has top and bottom
Ekman layers that interact with each other, with ncarshorc
Florida as the Florida Current, and continues in the Atlantic
Ocean as the main branch of this complex current system.
currents accelerated downwind, due to the coastal constrain
The narrow Southeast Florida Shelf (SEFS) and its (no cross-shore flow near the coastal wall) that eliminates
southward extension, the Atlantic Florida Keys Shelf the Coriolis term in the along-shore momentum balance.
(AFKS), receive direct influence from the Florida Current The mid-shelf is generally in Ekman balance, while the
front and associated transient passage of meanders and
outer shelf is often dominated by flows associated with
eddies, especially near the shelf break, while subtidal basin-wide circulation.
transport in the shoreward shelf areas is wind driven (Lee A numerical model of the South Florida coastal seas and
and Williams 1999). In addition, the SEFS is directly
surrounding deep sea areas has been developed to allow the
connected to the shallow Biscayne Bay, while the AFKS study of the regional coastal processes and to serve as a tool
interacts with flows on the Southwest Florida Shelf for the support of limited area, high resolution interdisci-
(SWFS), through the narrow passages along the Florida
plinary models around Florida Bay and the Florida Keys, in
Keys island chain and along the western boundary of the
connection with the Comprehensive Everglades Restoration
shallow, mudbank-dominated Florida Bay. The broad
Project (http://www.evergladesplan.org/). Observational
SWFS represents the southern extreme of the West Florida
and modeling studies around the ecologically fragile CERP
Shelf (WFS) as it merges with the Florida Keys. The SWFS
areas have suggested that management decisions must rely
has more distinct inner (~0-30 m), middle (~30-60 m), and
on limited area models that include the proper representa-
outer (~60-100 m) domains that are influenced by different
tion of the regional South Florida circulation. Detailed
circulation forcing mechanisms. Wind stress and buoyancy
coastal dynamics and local forcing (such as local rivers) arc
fi Springer
49
Ocean Dynamics (2009) 59:47-66
real-time simulations; and the regional Gulf of Mexico
of outmost importance for restoration-related simulations
and are not present in the available large scale models. HYCOM with the Navy Coupled Ocean Data Assimilation
(GoM-NCODA), which employs cyclic boundary conditions
Furthermore, operational large scale models are not suitable
derived from a multi-year North Atlantic simulation and thus
for process-oriented studies, scenario-based simulations,
and coupling with local biogcochemical models. The is capable to perform hindcasts beyond the available ATL-
HYCOM years. A free-running simulation with the same
South Florida Hybrid Coordinate Ocean Model (SoFLA-
HYCOM, Kourafalou et al. 2005) has been developed to Gulf of Mexico model (GoM-Free) is used to better highlight
data assimilation effects prescribed to the SoFLA-HYCOM
fulfill these needs. The publicly available HYCOM code
simulations through boundary conditions.
(http://www.hycom.org) has been selected for the flexibility
on vertical discretization that offers transition from deep to
coastal flows.
Modeling in the SoFLA domain poses the challenge of 2 Model description
resolving both coastal and deep sea processes. Important
circulation forcing mechanisms governing coastal transport 2.1 The HYCOM model
processes are winds, river runoff and exchange through
The hydrodynamic model chosen for this study is the
narrow channels and around the multi-island chains, and
Hybrid Coordinate Ocean Model (HYCOM; http://www.
shallow mudbanks. The degree of linkage between the
hycom.org), named after a unique vertical layer configura-
various coastal areas depends on the strength of their
tion that is dynamically transformable from isopycnal in the
transports and the volume of water exchanged between
open stratified ocean to terrain-following (sigma) in
SoFLA subregions as well as over the whole domain. The
Loop Current/Florida Current front and associated eddies shallow coastal regions and to fixed pressure-level coor-
dinates in the surface mixed layer and unstratified seas.
dominate the oceanic influence and effectively connect the
HYCOM is a comprehensive, three-dimensional hydro-
coastal and shelf SoFLA areas to the overall Atlantic Ocean
circulation. This suggests that numerical models around the dynamic model with data assimilative capabilities and
advanced mixing schemes (Bleck 2002; Chassignct ct al.
South Florida coastal seas require a nested approach, with
boundary conditions that demonstrate an adequate repre-
2003; Halliwell 2004).
sentation of the dominant large scale flows and their An addition to previously released standard HYCOM
synoptic variability.
code employed in the SoFLA experiments involves the
The main objective of the current study is to improve our ability to add sigma and/or z-laycrs near the surface to
better resolve shallow areas (Halliwell ct al. 2008, this
understanding of the dynamics of the South Florida coastal
seas, with an emphasis on the effects of large scale forcing issue). The main task was to overcome the restriction of
preserving a thin upper isopyenic layer in the mostly
on the shelf areas. Our methodology focuses on the
evaluation of boundary condition effects on the circulation stratified subtropical areas that surround the study domains
patterns simulated by the South Florida Hybrid Coordinate and smoothly transition from the deep to the shelf areas that
Ocean Model. We employ independent (not assimilated) require the resolution of surface and bottom Ekman layers.
observations and seek to examine if free-running simu- Another application of non-standard HYCOM code
employed herein is the ability to simulate synthetic floats
lations within a topographically and dynamically complex
(either in the Lagrangian or Eulerian frame) that allow the
nested domain arc adequate to simulate reliable flows,
when aided by boundary forcing that is extracted from data
tracking of particles through the model domain and
assimilative larger scale products. This is an important step optionally sample water properties at the location of each
float (Halliwell ct al. 2003). Stationary floats (synthetic
toward a future goal of achieving forecasting capabilities
with the SoFLA-HYCOM model.
mooring instruments) were launched in SoFLA simulations
The Global Ocean Data Assimilation Experiment (GODAE; to compare simulated currents with ADCP mooring data.
http://www.godac.org) has been selected as the source of
2.2 The outer North Atlantic and Gulf of Mexico
suitable products for boundary conditions. Ongoing simula-
tions and prediction with GODAE global and basin-scale
HYCOM models
models have fulfilled the main GODAE objectives of
developing state-of-the-art models and assimilation methods One of the challenging tasks for nested models of coastal
to produce boundary conditions to extend predictability of
and shelf seas is the choice of appropriate boundary
coastal and regional subsystems (International GODAE Steer-
conditions. Nested models have to rely on available
ing Team 2000). Around South Florida, two data assimilative simulations from larger scale models, which often lack the
models are directly affiliated with GODAE: the basin-scale resolution and topographic and forcing details that are
North Atlantic (ATL) HYCOM, which provides archives of
necessary for the representation of coastal dynamics. It is
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50
Ocean Dynamics (2009) 59:47-66
often necessary to develop an intermediate model, nested
would allow GoM-HYCOM simulations over any time
within a global or basin-scale model, which, in turn, will frame, without having to rely on the availability of large
provide boundary conditions for the coastal model. Such a
scale model archives. The daily ATL-HYCOM boundary
multi-nested, downscaling approach has been followed forcing fields were binned into monthly values to create
herein, from the Atlantic to the Gulf of Mexico and then
perpetual year forcing archives. The cyclic boundary
to the South Florida model (SoFLA-HYCOM, described in
conditions have been successfully used to force GoM
Section 2.3). Below, we give a brief description of three
simulations forward in time, during the 2004-2005 simu-
outer models employed in SoFLA-HYCOM simulations lation and beyond.
and we examine if the boundary conditions provided by the
Two GoM-HYCOM simulations have been carried out: a
intermediate GoM model are an improvement over the
free-running one (GOM-Frec) and a data assimilative one,
coarser ATL model and if the data assimilative GoM-
employing the Navy Coupled Ocean Data Assimilation
NCODA run is a better choice as outer model than the
(NCODA, Cummings 2005) system (GoM-NCODA). The
twin free-running GoM-Free model. The SoFLA-HYCOM NCODA is an oceanographic version of the multivariatc
model is being developed as a future regional model itself,
optimum interpolation (MVOI) technique widely used in
to further support the boundary needs of higher resolution,
operational atmospheric forecasting systems. A description
limited area interdisciplinary local models.
of the MVOI technique can be found in Daley (1991).
The ATL-HYCOM model (sec description in Chassignct
Figure 2 shows an evaluation of the two GODAE
et al. 2007) is the US contribution to the international GODAE
products that are used for boundary conditions in this
project. The ATL-HYCOM model simulation that has been
study, namely the ATL-OI and the GoM-NCODA. Snap-
directly employed in this study will be referred to as ATL-OI, as
shots of Sea Surface Height on the same day (February 15,
the data assimilation scheme is using the Optimal Interpolation
2005) are plotted with superimposed tracking of the Loop
based Modular Ocean Data Assimilation System (MODAS).
Current, based on the operational IR (infrared) frontal
This system consists of daily operational 1/4° Sea Surface
analyses performed by the Naval Oceanographic Office.
Height (SSH) analysis (gridded data) of available real-time
This is a routine model validation procedure performed at
satellite altimeter observations (Fox et al. 2002). The Cooper
NRL, employing independent (not assimilated) satellite-
and Haines (1996) technique is used to project the surface derived high resolution SST observations. The cyclic
information from altimetry SSH to the interior of the ocean.
boundary conditions used for GoM-NCODA perform well
Relaxation to the MODAS 1/8° Sea Surface Temperature (SST)
and maintain a robust Loop Current that is actually closer to
analysis derived from the five-channel Advanced Very High
the observed track, as compared to the ATL-OI. Both
Resolution Radiometer (AVHRR) is also included. The vertical
simulations agree very well with the satellite-derived frontal
mixing is represented through the K-Profile Parameterization position, while the cyclonic eddy east of the front is absent
(KPP scheme, Large et al. 1994, 1997). As this model focuses
in the course resolution (1/12°) ATL-OI and well resolved
on large scale processes and for computational efficiency, the in the 1/25° GoM-NCODA.
coastline has been moved to 20 m, which effectively makes it Both outer models use atmospheric forcing from the
unsuitable for representing circulation in coastal areas.
coupled ocean-atmosphere FNMOC (Fleet Numerical
The GOM-HYCOM model has resolution of 1/25° (-3.5
Meteorology and Oceanography Center) Navy Operational
to 4 km) and set-up similar to Prasad and Hogan (2007). It
Global Atmospheric Prediction System (NOGAPS, Hogan
extends from 77.36°W to 98.0°W and from 18.09°N to
and Rosemond 1991; Roscmond 1992; Hogan and Brody
30.71°N and has 20 hybrid layers in the vertical. The model 1993), which provides three-hourly wind and daily thermal
has used actual coastline with the minimum depth of 2 m, an forcing (interpolated to three-hourly values).
improvement over the ATL-HYCOM in the Gulf of Mexico
region. The NASA-GISS (Goddard Institute for Space 2.3 The nested South Florida HYCOM model
Studies) Level 2 turbulence closure (Canuto et al. 2001,
2002) is employed for vertical mixing parameterization.
The SoFLA-HYCOM is the regional model of the South
The intermediate GoM-HYCOM model employs a
Florida seas (Kourafalou et al. 2005), extending from
GODAE product at the open boundaries. A cyclic boundary
22.78°N to 28.61°N and from 77.36°W to 83.76°W and
condition has been developed from a synoptically forced
with a horizontal resolution of 1/25°, i.e., about 3.5 to
simulation (1999-2002) of the ATL-HYCOM, to represent
4 km. It includes the full extent of the Straits of Florida (to
"climatology" in the Loop Current inflow through the
Cuba in the South and to the Bahamas in the East), the
Yucatan peninsula and the Florida Current outflow through
inner, middle, and part of the outer Southwest Florida Shelf
the Straits of Florida and adjacent passages between Cuba (south of ~28.5°N, from the Florida Keys to the Tampa Bay
and the Bahamas. The goal was to develop an intermediate area) and the full Southeast Florida Shelf from Biscaync
model product closely related to GODAE objectives that Bay to Cape Canaveral (Fig. 1).
*J Springer
51
Ocean Dynamics (2009) 59:47-66
sea surf, height 20050216 [09.1]
Fig. 2 Evaluation of GoM-
HYCOM boundary conditions
30N
derived from ATL-HYCOM-
based climatology. Sea Surface
Height on February 15, 2005
computed by the ATL-OI (up-
per) and the GoM-NCODA
(lower). The continuous white/
black line marks the boundaries
of the Loop Current and a
cyclonic eddy, based on inde-
pendent frontal analysis of high
24N
resolution SST data; black lines
are segments based on data
more than 4 days old
20N
86W HW 82W 80W 78W
8 I
8 8
sea surf, height 20050216 [30.3]
30N
24N
20N
98W
s §
S 8
The topography is derived from the 2-min NAVO/NRL
extrapolate initial and boundary conditions provided by the
DBDB2 global data set with true coastline and a minimum outer model into the SoFLA water grid points.
depth of 2 m. As is the case for coastal and regional nested The model is driven by fields of wind stress, air
simulations, outer models are typically "large scale", basin- temperature, atmospheric humidity, heat fluxes (surface
shortwave and long wave), and salt flux (precipitation
wide or global models that have coarser resolution and large
minimum coastal depths. To address discrepancies between only). Surface latent heat and sensible heat fluxes, along
desired topographic details in shallow South Florida areas with evaporation, are calculated using bulk formulae during
and larger scale models with deeper coastal minimum
the model run time using the model Sea Surface Temper-
depths, a capability was developed to modify topography ature (Kara ct al. 2005). All sources of land-based
in the nested model to include shallower water regions and freshwater runoff have been included, both as point sources
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Ocean Dynamics (2009) 59:47-66
for the major rivers and as a line source for neighboring for comparison to modeled currents and will be employed
in the evaluation of different boundary conditions for the
small rivers.
The vertical mixing is represented through the K-Profile SoFLA-HYCOM simulations. Mooring C18 is very close
Parameterization (KPP scheme, Large et al. 1994, 1997). No to the model boundary, so it will not be used for quan-
titative evaluation, but it will be employed in the discussion
data assimilation has been applied within the SoFLA domain.
The nesting technique follows the standard and robust of certain processes. The mooring locations and the
capability for nesting HYCOM within a larger HYCOM observing periods for each data record used in this study
run, by employing boundary conditions that implement
are given in Table I. The longest record is for mooring C19
archived variables (off-line) from the coarse outer grid to (full 2004-2005 period), followed by C17 that spans
the fine inner grid. The barotropic boundary conditions 15 months from May 2004 to August 2005.
along the nested interface use the method of characteristics
(Browning and Kreiss 1982, 1986). The baroclinic fields
4 Numerical simulations during 2004-2005
for temperature, salinity, pressure, and velocity in the
nested model are relaxed toward the outer model solution
within buffer zones and over c-folding times that were 4.1 Model set-up and forcing
dictated by the archive frequency.
The objective of evaluating nested simulations in this study
is to develop a regional model around the South Florida
coastal seas that will properly account for coastal to offshore
3 The data
interactions, while being capable to simulate coastal flows.
Although the focus is on the effect of the boundary
Early studies on continental shelf dynamics have estab-
conditions, certain improvements that are expected to be
lished that the primary subtidal current response to wind
forcing on wide, shallow continental shelves, such as the important for future simulations of wind-driven and
West Florida Shelf, is a strong along-shore current directly buoyancy-driven coastal circulation and connectivity among
shallow environments (such as Florida Bay and the Florida
forced by coherent, synoptic-scale along-shore winds and
opposing cross-shelf currents in upper and lower Ekman
Keys) are also explored. These include the following.
layers (Csanady 1978; Beardsley and Butman 1974;
- Certain topographic details in the SoFLA-HYCOM
Mitchum and Sturgcs 1982; Lee et al. 1985). The WFS
model are absent in the outer models: corrections in
exhibits the typical shelf dynamics with the additional
shallow depths around Florida Bay and the Florida
complexity of flows associated with a north to south
Keys passages and the model minimum depth (2 m for
pressure gradient imposed on the Gulf of Mexico by the
SoFLA, same as in GoM, but much smaller than the
Loop Current, as well as shelf break processes associated
ATL 20 m minimum depth which excludes the inner
with direct Loop Current influence (Huh et al. 1981;
shelf and hence coastal currents).
Paluszkiewicz et al. 1983; Weisberg et al. 1996, 2001;
- SoFLA-HYCOM has higher vertical resolution in the
Weisberg and He 2003). Therefore, middle to outer shelf
shelf areas, which is important for the proper represen-
observations on the WFS are essential for the evaluation
tation of near surface buoyant plumes that arc critical
of SoFLA-HYCOM model simulations under different
for inner shelf circulation, CERP restoration scenarios,
boundary conditions.
and biophysical applications, to be explored in ancil-
The Coastal Ocean Monitoring and Prediction System
lary studies. The bottom 20 SoFLA layers arc identical
(COMPS; http://comps.marinc.usf.edu) of the University of
to the GoM layers (total number of GoM layers is 20)
South Florida (USF) has four relevant mooring sites on the
and to the top 20 layers of the 26-layer ATL-OI, whose
Southwest Florida Shelf: C13, C17, C18, and C19; see
bottom five layers were discarded because their
Fig. 1 and also Halliwell et al. (2008, this issue). Data from
densities do not exist in the regional SoFLA-HYCOM
Acoustic Doppler Current Profilers (ADCP) will be used
Table 1 West Florida Shelf buoy and model buoy attributes
Data buoy ID Lai Lon Water
depth (m) Missing data during Model buoy sampling depth
range starting:interval:ending
2004-2005
83.07
C13 26.07 50 02/08/05-09/14/05 3:1:45
82.22 25
C17 25.25 03/10/04-05/06/04; 09/13/05-10/12/05 4:1:21
C18 25.00 X2
83.71 07/14/04-12/07/04 10:5:74
24.62 82.72 25
C19 none 3:1:24
£) Springer
Ocean Dynamics (2009) 59:47-66
53
Table 2 SoFLA-HYCOM simulations attributes
performed for a 2-year simulation (2004-2005): cxpl is
nested in the GoM-Free, exp2 is nested in the GoM-
Forcing
Exp# Boundary conditions Vertical layers
NCODA, and exp3 is nested in the ATL-OI. All three
experiments have the attributes described in Section 4.1 and
GoM-Free COAMPS 26
1
2 26
GoM-NCODA COAMPS
were initialized from outer model fields on January 1, 2004.
26
ATL
COAMPS
3
In order to evaluate the effect of atmospheric forcing in the
l.l
GoM-Free NOGAPS 20
numerical simulations, identical experiments were per-
20
2.1 GoM-NCODA NOGAPS
formed, but with NOGAPS atmospheric forcing employed
.VI 20
ATL NOGAPS
in the SoFLA runs, similar to the forcing of the outer model
simulations. These are: expl.l, exp2.1, exp3.1, nested on
GoM-Free, GoM-NCODA, and ATL-OI, respectively. To
further highlight the effects of boundary conditions, experi-
model. SoFLA has a total of 26 hybrid vertical layers
ments 1.1 and 2.1 have also the same vertical structure as
ranging from sigma or z-coordinate in shelf regions to
the outer Gulf of Mexico simulations and exp3.1 has the
isopyenic coordinate in deep waters. The number and
same number of layers as the outer ATL model within the
thickness of the upper model layers have been chosen
SoFLA domain. The attributes of all model simulations arc
to adequately resolve the entire water column in the
given in Table 2.
shelf areas, especially where stratification due to
At the open boundaries, 13 grid point-wide "buffer"
riverine freshwater inputs prevails.
(or boundary relaxation) zones with e-folding time of 0.1
- The nested simulations employ the best available
to 24 days (outer to inner grid) are used to relax the baro-
forcing, namely three-hourly archives of the 27 km
clinic mode temperature, salinity, pressure, and velocity
horizontal resolution Coupled Ocean/Atmosphere
components.
Mcsoscale Prediction System (COAMPS, Hodur 1997;
Hodur ct al. 2002), as compared to the 1° NOGAPS
4.3 Simulation of the circulation in the South Florida region
forcing of the outer models (see Section 2.2).
The synoptic variability in the South Florida flow field is
4.2 Numerical experiments
characterized by periods of wind-driven versus buoyancy-
driven and eddy-driven currents. Frequent eddy passages
In order to evaluate the effect of boundary conditions in
dominate the current field in the Straits of Florida, with
South Florida nested simulations, three experiments were
substantial influence on the hydrodynamics around the
Fig. 3 Seven-day composite
turt.htQht M»y2»,2004 0OZ pO-OH]
(May 24 to 31, 2004) from the
Aqua-satellite chl-a data (upper
left); Sea Surface Height fields
from GoM-NCODA on May 29,
2004 (upper right); June 4, 2004
(lower left); and July 8, 2004
(lower right)
•W Springer
54
Ocean Dynamics (2009) 59:47-66
pressure gradient imposed by the LC in the Gulf. An
narrow Southeast Florida shelf and the Florida Keys Reef
Tract. The broad Southwest Florida Shelf is mostly example of LC variability is shown in Fig. 3. A weekly
Acqua-satellite chl-a image composite for May 24—31,
influenced by the passage of weather systems and by the
2004 shows the Loop Current under the tendency to shed a
freshwater inputs from local rivers.
Variability in the Loop Current has also been shown to large ring, while anticyclonic eddy activity at the eastward
impact circulation on the West Florida Shelf (Paluszkiewicz periphery of the ring (near the WFS) was quite strong. The
ct al. 1983; Wcisbcrg and He 2003; Weisberg et al. 2005). GoM-NCODA SSH fields around the same period (May 29
and July 4) are in very good agreement, as this simulation
Based on several years of measurements, a mean southward
flow has been found in the Southwest Florida Shelf (Lee et assimilates satellite-derived SSH fields. The ocean color
al. 2002) and has been attributed to a north to south data are not assimilated in the model. Also shown in Fig. 3
Fig. 4 Observed and simulated
Observed/Simulated V5m at C17 (82.2°W & 25.3°N) For 2004
hourly values of near-surface (at
5-m depth) along-shore velocity
(in cm s ) at the mooring
station C17 (position marked on
Fig. 1) for year 2004 (see
Table 1 for data attributes),
computed from COM PS data
(red lines) and SoFLA model
simulations (blue lines): a exp 1;
b exp2; c exp3. The along-shore
mean:-2.1
40
wind stress component (in N
Jan04 std :5.7
2
m ) from the three-hourly
30 60 90 120 150 180 210 240 270 300 330 360
COAMPS-27 km forcing is
ii*ifc**J I I I I I I I I I I I I I il il I i il I A^^-faX* X^ilI^Ai^^lA^a^^i^^AMlrl I III I l.M.>*itlt*. *i *•!•
given in (d). Time series mean
and standard deviations (std) are
b exp2
also given. Maximum winds in
the summer-fall are associated
with hurricanes Charlie
(day 226, August 13, 2004) and
Ivan (day 259, September 15,
2004). The straight red line
marks a data gap
mean:-1.8
-40 •
Jan04
std :6.1
1
' i i i i i i i i i i i • • ' • • i
30 60 90 120 150 180 210 240 270 300 330 360
i
••'•--'• l.....i.....l.....t>^ ^.l... >l.^ .>l.i>^.l.. >^l. ...l
J J J J J
mean:0.0
Std :6.0
Jan04
30 60 90 120 150 180 210 240 270 300 330 360
mean:l.O
d coamps-27km wind-V
0.2
std :1.8
^jyvv
J 0.0
z
-0.1
mean:-0.0l 1
-0.2
Jan04 std :0.052
-0.3
30
60 90 120 150 180 210 240 270 300 330 360
time (day)
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55
Ocean Dynamics (2009) 59:47-66
is the SSH field from the GoM-NCODA for July 8. The LC with the findings of Weisberg and He (2003) and adds
stayed in a relatively "young" (limited northward exten- support in the hypothesis by Hctland ct al. (1999) that a
sion) position through the beginning of June, bending mature Loop Current (which is fully extended into the Gulf
eastward and approaching the shelf break. This was of Mexico) can cause trapping of the cross-shelf pressure
gradient and a southward flow confined over the shelf
associated with a southward flow event that spanned across
break region, while a young Loop Current (following a
the WFS (sec Section 5.1). As the ring reattached itself, the
recent eddy shedding) located near the southwest portion of
LC changed to an extended configuration by the beginning
of July and a westward bending, while the southward flow the shelf will cause a larger along-isobath sea level slope
gradually became confined in the outer shelf. This agrees and the onshore penetration of the southward flow.
Fig. 5 Same as in Fig. 4, but for
Observed/Simulated V5m at C13 (83.1°W & 26.1°N) For 2004
mooring station C13. The
• «• • • - • • i
vertical lines mark a shelf flow
event associated with oceanic
influence (days 140-180, May
19-June 28, 2004)
d coamps-27km wind-V
02
01
J%mJk*kJbfBlL
00
-0.1
-0.2
Jan04
-03
30 60 90 120
150 180 210
time (day)
£)spi
ringer