subtidal hydrodynamics at cape-associated shoals · pdf filematlab wavelet package...
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IV. Conclusion
III. Discussion
Currents:
• Geostrophic balance existed
between FC and sea levels at
weekly intervals, in an inverse
relation by the 180°phase (Fig 2).
• Currents were consistent at all
locations, with along-shelf currents
dominating (Fig 3).
• Stronger semidiurnal and weaker
diurnal signals in currents and sea
level (Fig 4).
Subtidal Hydrodynamics at Cape-Associated ShoalsSabrina Marie Parra1*, Arnoldo Valle-Levinson2, Peter N. Adams3, Juan Felipe Paniagua-Arroyave3
1American Society of Engineering Education Research Associate at Naval Research Laboratory, Stennis Space Center, MS; 2Department of
Civil and Coastal Engineering, University of Florida, Gainesville, FL; 3Department of Geological Sciences, University of Florida, Gainesville, FL.
Inner-shelf subtidal currents at Cape Canaveral were dominated by the along-shelf component, which
was controlled by the geostrophic balance between FC and sea levels, with secondary effects from
weekly north winds.
The inner-shelf is mainly affected by wind and surface gravity waves
with secondary forcing from offshelf currents [1]. The influence of winds,
waves and the Florida Current (FC) on subtidal currents were
investigated at the Cape Canaveral inner-shelf in Florida. The study
focused on two cape-associated shoals: Chester Shoal at False Cape
and Canaveral II Shoal at Cape Canaveral (Fig 1).
I. Introduction
II. Data
Acoustic Doppler current profilers (ADCPs) were moored on either side
(seaward and landward) of Canaveral (Can.) and Chester (Che.) shoals
for 52 days in 2013 (Fig 1). The following variables were filtered to
remove tides and the inertial period:
• ADCP depth, and depth-averaged currents and backscatter
• NOAA buoy wind velocity and wave parameters
• Florida Current transport
• Sea level at Trident Pier south of Cape Canaveral
Fig. 1: Bathymetric map of Cape Canaveral with the four moorings (circles), National
Oceanographic and Atmospheric Administration (NOAA) buoy #41009 (diamond), the NOAA
Trident Pier tidal gauge location (black square) and FC transport obtained from cable
measurements over the green line in the inset Florida map.
Fig 3: Cape Canaveral subtidal parameters. All vectors are in oceanographic convention. Currents and echo anomaly were depth-averaged. Fig. 4: Spectra of depth-averaged currents,
sea levels and depth-averaged echo intensity anomaly from each mooring. Black lines represent the 95% confidence interval. Fig. 5: CEOF of currents. A) CEOF mode 1 time series for the
cross-shelf (blue) and along-shelf (green) velocity components. Mode 1 vertical profiles for the B) cross and C) along shelf velocities for each mooring. Fig. 6: Wavelet coherences between
CEOF mode 1 of the cross-shelf (first column) and along-shelf (second column) currents and forcing parameters.
Fig. 2: Geostrophic balance. A) Subtidal sea level at Trident Pier (blue) and FC transport (black).
B) Wavelet coherence between blue and black lines. Contours show high (red) and low (blue)
coherences. Areas enclosed by black contours represent coherence >95% confidence limit.
Shaded areas are the cone of influence where values are less reliable. Coherence phase
angles are represented by the vectors [2, 3].
VI. References
[1] Lentz & Fewings (2012) Annu. Rev. Marine. Sci. 4.
[2] Torrence & Compo (1998) Bull. Am. Meteorol. Soc. 79.
[3] Grinsted et al. (2004) Nonlinear. Proc. Geoph. 11.
[4] Bjornsson & Venegas (1997) CCGCR Report 97.
Fig. 2: Geostrophic Balance: Florida Current
Complex Empirical Orthogonal Function (CEOF):
• CEOF shows dominant spatial and temporal
structures [4].
• Mode 1 accounted for 92% of the variance, with
unidirectional vertical profiles throughout (Fig 5B,C).
• Along-shelf current vertical structures had same
direction and magnitude, while cross-shelf currents
varied with Chester weaker than Canaveral.
• Cross-shelf vertical structure at Chester east was
opposite the rest, showing convergence/divergence at
Chester Shoal.
Wavelet Coherence: EOF1 vs Forcings
• Fortnightly and weekly north winds
played a role in shelf currents,
suggesting passage of weather
systems (Fig 6).
• Sea levels dominated the along-shelf
currents at almost all periods (Fig 6).
• Weekly FC oscillations dominated
both current components during the
first half when FC variability was high.
V. Acknowledgements
This work was supported by a Bureau of Ocean Energy Management grant. Special thanks to Viktor Adams,
Michael Dickson, Lauren Ross, Jackie Branyon and others that helped with fieldwork. The FC transport is freely
available from the Atlantic Oceanographic and Meteorological Laboratory
(www.aoml.noaa.gov/phod/floridacurrent/). Thanks to Aslak Grinsted, John Moore and Svetlana Jevrejeva for their
MATLAB wavelet package (noc.ac.uk/using-science/crosswavelet-wavelet-coherence).
Fig. 3: Parameters
Fig. 5: Complex Empirical Orthogonal Function - Mode 1
Fig. 4: Spectral Analysis
Fig. 6: Wavelet Coherence Analysis
Fig. 1: Study Area
B
A
A
B C
Cross-Shelf Along-Shelf
Along-Shelf
Cross-Shelf
Chester East
Chester West
Canaveral East
Canaveral West
Trident Pier
FCFlorida
La
titu
de
, °
N
Longitude, °E
Along-Shelf Currents
Cross-Shelf Currents
Sea Level
Chester East
Chester West
Canaveral East
Canaveral West
Cross-Shelf EOF vs … Along-Shelf EOF vs …