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]