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Application of High Frequency Radar & Okubo-Weiss Parameter to Analyze
Variability in the Florida Current
Matthew ArcherL. K. Shay, J. Martinez-Pedraja , A. B. Parks
www.rsmas.miami.edu/users/marcher
Ocean Sciences 2012
[Source: http://oceancurrents.rsmas.miami.edu]
Loop Current
Gulf Stre
am
Drifter Trajectories between 1978 - 2003
Florida Current
Florida Current
AtlanticOcean
Florida
⌐ Bahamas¬
Cuba
Florida Keys
Cal Say Bank
Santaren Channel
Gulf of Mexico
NW Providence Channel
South Atlantic
Bight • Meanders
• Cyclonic eddies; ↘ Tortugas eddies (TE)
↘ Frontal eddies (FE)
↘ Submesoscale motions…
Forcing Abrupt wind change
Length Scale 2-30 km (cross-shelf)4-40 km (along-shelf)
Translational Velocity 15-40 km/day
Rotation-Cyclonic -Dipole excited by near-inertial wave propagation
Shay et al., 1998; Shay et al., 2000 ; Parks et al., 2009 Shay et al. [1998] ; Shay et al. [2000] ; Parks et al. [2009]
Vortices in the Florida Current
Using high frequency (HF) radar as tool to monitor the evolution of the Florida Current, for:↘ Oil spill mitigation procedures in the surface layer↘ Search and Rescue (Cuban Rafters)↘ Larvae transport↘ Optimizing commercial shipping interests (i.e. going with the flow)
Objectives
FOCUSTo understand the surface current signals associated with energetic vortices to improve real time monitoring across the FC in addressing the above issues.
Radar Site
CMAN Buoy
ADCP mooring (80-m depth)
WEllen RAdar (WERA) Specifications
Frequency Modulated Continuous Wave (FMCW) System:↘ Chirp Interval: 0.26 sec↘ Operating frequency: 16.045 MHz↘ Range: 80 km↘ Horizontal resolution: 1.2 km↘ Sampling interval: 10 min
Flexible: can be altered to sample a few minutes to several hours (e.g. 128, 256, 512……samples in the averaging)
20' 80oW 40' 20' 79oW
25oN
15'
30'
45'
26oN
Accuracy
Total radial accuracy ~ 3-5 cms-1
Adapted from Parks et al. [2009]
ADCP Comparison:[14-m bin depth Vs. radar cell]
Complex correlation ~ 0.8
In Florida Current velocities often exceed 200 cm s-1
ADCP Mooring
Case Study: Submesoscale Vortex
January 2005
Peters et al., 2002:
Radius of Deformation in the Florida Straits shelf break:
~30-km
-80.02 -80 -79.98 -79.96 -79.9425
25.1
25.2
25.3
25.4
25.5
25.6
25.7
Longitude
Latit
ude
Along-Shelf≈ 17-km
-80.2 -80 -79.8 -79.6 -79.4
25.4
25.5
Longitude
Latit
ude
Cross-Shelf≈ 24-km
15' 80oW 45' 30' 15'
25oN
12'
24'
36'
48'
26oN
0 10 20 30 km
Current Velocity (cm/s)
0 20 40 60 80 100 120 140 160 180 200
15' 80oW 45' 30' 15'
25oN
12'
24'
36'
48'
26oN
0 10 20 30 km
Current Velocity (cm/s)
0 20 40 60 80 100 120 140 160 180 200
Spatial Dimensions
Slope ̴ 66 km/34 hrs
= 46 km/day
Adapted from Parks et al., JGR [2009]
Wind Stress calculation following Fairall et al., JGR [1996]
u-component velocity
Cold Front Wind Shift
• Based on the eigenvalues of the velocity gradient tensor. Physically it describes the balance between vorticity and deformation:
Okubo-Weiss (OW) Criterion
Normal Strain VorticityShear Strain
OW = Sn2 + Ss2 - ω2
Refer to: McWilliams [1984]; Isern-Fontanet et al. [2003; 2004; 2006] ; Chelton et al. [2007]; Chaigneau et al. [2008]
Application of OW
Application of OW
Application of OW
Summary: Work In progress
Jury is still out on using OW given importance of shear and strain along the western flank of the Florida Current and its impact on submesoscale structures.
↘ WERA resolves submesoscale features.↘ Marked bin to bin coherence in the surface signals.↘ Several years of data sensing the FC ↘ Regarding implementation of Okubo-Weiss …
Future Work
↘ Subsurface structure from ADCP suggests barotropic nature over the shelf break – relationship to the topography and bottom boundary layer?
↘ Relationship to the wind stress field
↘ Frequency decomposition
Parks et al., JGR [2009]