Recent field studies have revealed a frontal boundary layer present in the near shore waters off the southeast coast of Florida, which is related to the Gulf Stream Front. This boundary layer was observed using sonar instruments, and was characterized by a distinct layer of high backscatter. The layer was visible on both 83 kHz and 200 kHz frequency, with each frequency displaying a front at a somewhat different depth. Figure 1 shows an example of the fronts measured along the southeast coast of Florida. The sonar record was taken by making a transect heading straight inshore from approximately 5 miles offshore until the start of the reef was reached. The two images represent a continuous record, where the right edge of the first continues at the left edge of the second image. Additional sonar records have been collected that exhibit the same frontal boundary structure, displaying similar shape and appearing in both day and night cruises. 

Figure  SEQ Figure \* ARABIC 1: Sonar record of fronts observed along the southeast coast of Florida on Sept. 25, 2009.

The sonar record shown above in Figure 1 was recorded within two hours of a TerraSAR-X stripmap image capture of the South Florida coast (see Soloviev et al., 2010). This image is shown below in Figure 2, and displays signatures of varying origins. The dotted trajectory line indicates the path the research vessel took while recording sonar measurements. Numbered yellow circles correspond to the numbers shown on the sonar plot above in Figure 1. A faint front can be seen at the beginning of the sonar plot and is represented in the SAR image as a very faint line that is more predominant to the south of the first yellow point. Between the second and third yellow circles, frontal features can be seen on the SAR image and seem to correspond to the approximate location where the fronts observed with sonar reach the surface.

Figure  SEQ Figure \* ARABIC 2: A TerraSAR-X stripmap image of the coast of South Florida, from Boca Raton to Pompano Beach, taken on Sept 25, 2009 during a surfactant release experiment (Soloviev et al., 2010).  The boat's trajectory while collecting sonar data is displayed and the three fronts encountered are denoted by the numbered, yellow circles.  Other features such as ship wakes, natural slicks, and surfactant releases are labeled on the full image, with enlarged views to the right.

Figure  SEQ Figure \* ARABIC 3: Sonar record from Mar. 25, 2010 displaying the front penetrated.

The frontal structure observed in the sonar and SAR records acts to separate the coastal and Gulf Stream water masses. Generally these fronts are continuous as displayed in the sonar record above in Figure 1, however occasionally it has been observed that the front is penetrated as shown in the sonar record below in Figure 3. This observation lead to the hypothesis of a semi-stable frontal system that is occasionally breached by sub-mesoscale eddies. These eddies allow for cross-shelf circulation and mixing of the coastal and Gulf Stream water masses, thus controlling the cross-shelf propagation of land-based sources pollution and possible oil spills.

References:

A. Soloviev, C. Maingot, A. Fujimura, J. Fenton, M. Gilman, S. Matt, S. Lehner, D. Velotto, S. Brusch. Fine Structure of the Upper Ocean from High-Resolution SAR imagery and In-situ Measurements.  IEEE International Geosciences and Remote Sensing Symposium 2010.  Extended abstract.