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Free Content Cross-shelf mixing and mid-shelf front dynamics in the Mid-Atlantic Bight evaluated using the radium quartet

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Mid-shelf fronts (MSFs) are thought to be ubiquitous in shelf areas. However, their dynamical role in cross-shelf mixing has yet to be fully characterized. In January, February, and April of 2007, radium isotopes (223Ra, t1/2 = 11 d; 224Ra, t1/2 = 3.7 d; 226Ra, t1/2 = 1600 yr; 228Ra, t1/2 = 5.7 yr) were measured along a transect in the Mid-Atlantic Bight to constrain mixing rates at and around a MSF. Cross-shelf eddy diffusivities (Kx) were determined from 223Ra and 224Ra distributions using a variable-depth model. Two key assumptions – minimal advection and negligible benthic radium input – involving the use of 223Ra and 224Ra as tracers of mixing were quantitatively evaluated in order to assess the accuracy of the Kx estimates. Eddy diffusivities over the three-month sampling period range from 0.1 ± 0.05 – 1.6 ± 0.5 × 102 m2 s–1 for 223Ra and from 1.7 ± 0.4 – 2.2 ± 0.6 × 102 m2 s–1 for 224Ra. The temporal variability in Kx is low in comparison to the uncertainty of the derived values, indicating that eddy diffusivity in this area is relatively constant throughout the sampling period. Observations in the Mid-Atlantic Bight differ from theoretical data corresponding to the tidal dispersion frontogenesis model, suggesting that a different mechanism is responsible for MSF formation. Variability in supported 223Ra and 228Ra observed near the front indicates that cross-shelf mixing may be inhibited by MSFs. Conversely, along-shelf transport is enhanced by the front's presence. These results indicate that the equatorward jet associated with the front is capable of effectively transporting dissolved chemicals over hundreds of kilometers.

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Document Type: Research Article

Publication date: 2012-01-01

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  • The Journal of Marine Research publishes peer-reviewed research articles covering a broad array of topics in physical, biological and chemical oceanography. Articles that deal with processes, as well as those that report significant observations, are welcome. In the area of biology, studies involving coupling between ecological and physical processes are preferred over those that report systematics. Authors benefit from thorough reviews of their manuscripts, where an attempt is made to maximize clarity. The time between submission and publication is kept to a minimum; there is no page charge.
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