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1. Dynamics of the Loop Current Experiment
Upper-‐lower layer coupling in Loop Current Eddies Ekman and Franklin Kathleen A. Donohue1, Robert Leben2, Peter Hamilton3 and D. Randolph Wa=s1 1Graduate School of Oceanography, University of Rhode Island, Narraganse=, [email protected] 2University of Colorado, Boulder 3SAIC, Raleigh, North Carolina
B1331
1. Dynamics of the Loop Current Experiment
Goals: Increase dynamical understanding of Loop Current, eddy-‐shedding mechanisms, and genesis of lower-‐layer flows.
Elements: Moored arrays of current and temperature and bo=om-‐mounted pressure equipped inverted echo sounders (PIES) together with remote-‐sensing and numerical-‐modeling approaches.
Deployment: April 2009 RotaYon: July & November 2010 Recovery: November 2011
Here we show results from the first 15 months: May 2009 to July 2010.
Temperature and velocity measurements from the nine tall moorings resolve the full water column structure.
Meanders (~ 7 day period) along the Loop Current periphery precede Loop Current Eddy formaYon.
Upper-‐layer Loop Current flows are not visually coherent with lower-‐layer flows.
2. CirculaCon dominated by interacCon between the Loop Current and Eddies Ekman and Franklin
3. Upper and Deep StaCsCcs Principal axes of the standard deviaYon ellipses and the mean vectors are not aligned with those of the upper layer (~200 m depth).
In lower layer, mean circulaYon shows an west-‐east pa=ern of anYcyclone-‐cyclone pair.
Deep eddy kineYc energy is high along the northeastern periphery of the mean Loop Current posiYon.
4. Loop Current Eddies Ekman and Franklin
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Array placed where historical analysis indicated eddy separaYon was most likely to occur and designed to encompass the Loop Current from east to west.
Eddy Ekman
Aber the iniYal detachment meanders develop (~300 km wavelength) along the Loop Current.
IniYal detachment (June-‐July) occurs when a trough forms in the southeast edge of the Loop Current. The trough expands and ulYmately causes detachment.
SeparaYon occurs when a frontal cyclone along the northern Loop Current strengthens and moves southward.
A northward shib (meander crest) of the Loop Current, lower-‐layer thickness decreases, and deep anYcyclonic vorYcity develops to conserve potenYal vorYcity. With favorable verYcal offset, a lower-‐layer anYcyclone leads a meander crest, upper-‐layer and lower-‐layer eddies grow.
Joint development also occurs when a southward shib (meander trough) increases lower-‐layer thickness and generates a deep cyclone that leads the upper trough.
Eddy Franklin
Similar to Eddy Ekman, a growing and southward propagaYng meander along the west Florida slope iniYates the first detachment.
Also similar to Eddy Ekman, a second detachment occurs when a frontal cyclone along the northern Loop Current strengthens and moves northward.
A deep anYcyclone is offset from an upper crest, a deep cyclone offset from upper trough and deep anYcyclone offset from upper crest.
A strong deep cyclone with swirl speed in excess of 20 cm/s develops leading an upper trough, the pair jointly intensify, move south and Eddy Franklin detaches from the Loop Current.
We acknowledge support from BOEMRE contract M08PC20043. See related posters: Rosburg, K., Performance evaluaYon of HYCOM in GOM, B1333 Hamilton, P., Loop Current Eddies Ekman and Franklin, A historical perspecYve, B1322
• Preliminary invesYgaYons show strong interacYon of upper-‐layer flows with deep currents. • Upper and lower-‐layer responses during Ekman and Franklin detachments are organized and suggesYve of baroclinic instability. • Loop Current meanders are an important mechanism to transfer energy to the lower layer. • Four Loop Current Eddies formed in the Gulf of Mexico during the 30-‐month observaYonal Yme-‐frame, analysis is ongoing.
18 MAY 2010
20 cm s!1
88°W 87°W 86°W
25°N
26°N
27°N
Pressure (dbar)!0.2 0 0.2
25 MAY 2010
20 cm s!1
88°W 87°W 86°W
Upper-‐layer and lower-‐layer coupling revealed from mapped bo=om pressure (shaded color contours) and currents (gray vectors) as well as measured deep currents (black vectors).
Mapped round-‐trip travel acousYc travel Yme (thick lines) with a contour interval of .002 s comparable to ~9 cm in sea surface height.
The 17 cm contour (black) denotes the locaYon of the Loop Current and Loop Current eddy fronts in the alYmeter-‐derived sea surface height.
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90oW 88
oW 86
oW 84
oW 82
oW 80
oW
20oN
22oN
24oN
26oN
28oN
30oN
Short MooringTall MooringPIES
0
200
400
600
depth
[m
]
Ekman
Forms
Ekman
SeparationFranklin Forms
tem
pera
ture
[oC
]
4
12
20
28
!40!20
02040
2000 m
velo
city [cm
s!1]
07/01/09 10/01/09 01/01/10 04/01/10 07/01/10!40!20
02040
2935 m
!1000
100
152 m
!1000
100
352 m
velo
city [cm
s!1]
A2 Tall Mooring
24oN
26oN
28oN
89oW 87
oW 85
oW
10 cms!1
mean Loop Current
eddy kinetic energy [cm 2 s!2
]
0 0.004 0.008 0.012 0.016
Deep Upper
24oN
26oN
28oN
89oW 87
oW 85
oW
10 cms!1
mean Loop Current
tau [s]1.12 1.122 1.124 1.126 1.128