DTIC ADA555822: Eddy Processes of the Western Adriatic Current Near Cape Gargano

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Conference Proceedings
15-02-2012
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Eddy Processes of the Western Adriatic Current Near Cape Gargano
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0601153N
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6. AUTHOR(S)
Jeffrey Book, D. Burrage, S. Carniel, R. Hobbs, P. Martin, E.A. Midgely, M.
Rixen, K. Schroeder, I. Vilibic, W. Wood
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73-9858-A9-5
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Naval Research Laboratory
NRL/PP/7330-09-0097
Oceanography Division
Stennis Space Center, MS 39529-5004
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14. ABSTRACT
Eddy processes of the Western Adriatic Current near Cape Gargano are highly modulated by the wind, growing during calm periods following strong
wind events. Both single anticyclones and trains of multiple eddies with a regular spacing are observed. Suppression of a single anticyclone in the lee
of the Cape was observed by profiling SEPTR moorings to occur when the horizontal gradient of the thermocline depth was increased by the wind.
Eddies also form cyclonic filaments extending offshore. Such a filament was observed through the new technique of siesmic oceanography to have
downslope tilting isotherms and a long, thin, offshore extension in the bottom boundary layer.
15. SUBJECT TERMS
fronts, mesoscale phenomena, Adriatic Sea, instruments and techniques, turbulence
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Jeffrey Book
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228-688-5251
Standard Form 298 (Rev. 8/98)
Prescribed by ANSI Std. Z39.18
EDDY PROCESSES OF THE WESTERN ADRIATIC CURRENT NEAR CAPE GARGANO
2 3 3 4 5 6
J. W. Book '*, D. M. Burrage ', S. Camiei , R. Hobbs , P. J. Martin ', E. A. Midgely , M. Rixen , K. Schroeder , I. Vilibic and
W. Wood '
1
U.S. Naval Research Laboratory, Stennis Space Center, MS, U.S.A. - [email protected]
2
C.N.R.-Institute of Marine Science, Venezia, Italy
3
University of Durham, Durham, U.K.
4
NATO Undersea Research Centre, La Spezia, Italy
5
C.N.R.-Institute of Marine Science, La Spezia, Italy
6
Institute of Oceanography and Fisheries, Split, Croatia
Abstract
Eddy processes of the Western Adriatic Current near Cape Gargano are highly modulated by the wind, growing during calm periods
following strong wind events. Both single anticyclones and trains of multiple eddies with a regular spacing arc observed. Suppression
of a single anticyclone in the lee of the Cape was observed by profiling SEPTR moorings to occur when the horizontal gradient of the
ihcrmocline depth was increased by the wind. Eddies also form cyclonic filaments extending offshore. Such a filament was observed
through the new technique of seismic oceanography to have downslope tilting isotherms and a long, thin, offshore extension in the
bottom boundary layer.
Keywords: Fronts. Mesoscale Phenomena, Adriatic Sea, Instruments And Techniques, Turbulence
Recent niea.su re me ins from two international collaborative research programs
[3]). Both microstruclure profiler and seismic sections crossed the filament
reveal new details of eddy activity and instability of the Western Adriatic multiple times. The filament frontal structure was baroclinic with down-
Current (WAC) as it rounds Cape Gargano in the central Adriatic Sea. The sloping isotherms. Near the bottom, the tilt extended into a long, thin, offshore
"Dynamics of the Adriatic in Real-Time" (DART) program was focused on
extension of coastal waters in the bottom boundary layer, followed by a subtle
understanding the predictability of this system with observation and
up-tilting structure offshore. These structures were all successfully imaged
modelling from October 2005 through September 2006. These included, among
(Figure 2) by seismic measurements, revealing new fine-scale details of the
other things, measurements from long-term current moorings, profiling SEPTR filament structure that couldn't have been measured otherwise.
moorings, tow-yo CTD profiles, remote sensing, and high-resolution
modelling using the U.S. Navy Coastal Ocean Model. Remote sensing and
Although Adriatic wind regimes and Cape topography provide the background
modelling, supported by in situ observations, revealed two distinct cases of
setting for eddy formation, frontal instabilities and mixing processes determine
WAC eddy activity [1], In the first case, a single anti-cyclonic circulation cell
many details of eddy structure and evolution.
(typically 20-30 km horizontal and 25 m vertical scale) forms on the
downstream side of the cape in the Gulf of Manfrcdonia (Figure la). In the
Distance (km)
second ease, a multiple eddy train (30-40 km crcst-to-crest wavelength) forms
along the coast upstream of the cape and proceeds around it (Figure lb). Both
cases are linked to wind patterns; eddies grow and propagate as southeastward
winds relax, and arc suppressed when these winds are strong. Profiling SEPTR
mooring measurements observed anticyelonic eddies in the lee of the Cape
when the ihcrmocline slope was weak and the absence of eddies when strong
southeastward winds strengthened this thermocline slope.
Fig. 2. Seismic reflectivity image showing the structure of the vertically tilting
WAC filament edge (left), thin offshore extension of cold bottom water
(center), and offshore up-tilting structure (right). Reflectivity is measured at 5-
10 m horizontal resolution and is associated with band-limited vertical
temperature gradient. E.g.. the colder water at the bottom causes a decrease in
sound speed and a negative reflection (black over white). The largest
reflections correspond to temperature changes of about one degree C over ten
meters of depth.
Acknowledgments: We thank the Italian CNR for providing R/V Urania ship
time and CNR-ISMAR for coordinating the AdriaScismic09 cruise. This work
was supported by the Office of Naval Research under Program Elements
Isolated Eddy East ol Cape Multi-Eddy field spanning Cape
0602435N and 0601153N.
Fig. 1. Remote sensing Chlorophyll-a images from MOD1S Aqua showing
References
representative cases of (a) single anticyelonic WAC eddy downstream of Cape
1 - Burrage D.M., Book J.W. and Martin P.J., 2009. Eddies and Filaments of
Gargano and (b) multiple eddy train around the Cape. This is a reproduction
the Western Adriatic Current near Cape Gargano: Analysis and Prediction../.
of Figure 2 from [1].
Marine Syst. 78: S205-S226, doi: 10.1016/j jmarsys.2009.01.024.
t
2 - Ruddick B., Song H., Dong C. and Pinheiro L., 2009. Water Column
Seismic Images as Maps of Temperature Gradient. Oceanogr., 22 (1): 192-
205.
The AdriaSeismic09 research campaign used the new technique of seismic
3 - Vilibic I., Book J.W., Beg Paklar G„ Orlic M., Dadie V., Tudor M., Martin
oceanography [2] to image the temperature gradient structure of the WAC
P.J., Pasaric M., Grbec B., Matic F,, Mihanovic H. and Morovic M., 2009.
frontal system during March 2009 at very fine horizontal scales (order 10 m).
West Adriatic coastal water excursions into the East Adriatic. J. Marine Syst.,
Sampling captured the details of a third WAC eddy case, a cyclonic filament
78:S132-SI56,doi:I0.l016/j.jmarsys.2009.01.015.
extending offshore following a strong wind event (also observed during DART
%
Ropp. Co
. Mer Medit. 39. 2010