Hydraulic criticality of the exchange flow through the Strait of Gibraltar

Author Posting. © American Meteorological Society, 2009. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 39 (2009): 2779-2799, doi:10.1175/2009JPO4075.1.The hydraulic state of the exchange circulation through the Strait of Gibraltar is defined using a recently developed critical condition that accounts for cross-channel variations in layer thickness and velocity, applied to the output of a high-resolution three-dimensional numerical model simulating the tidal exchange. The numerical model uses a coastal-following curvilinear orthogonal grid, which includes, in addition to the Strait of Gibraltar, the Gulf of Cadiz and the Alboran Sea. The model is forced at the open boundaries through the specification of the surface tidal elevation that is characterized by the two principal semidiurnal and two diurnal harmonics: M2, S2, O1, and K1. The simulation covers an entire tropical month. The hydraulic analysis is carried out approximating the continuous vertical stratification first as a two-layer system and then as a three-layer system. In the latter, the transition zone, generated by entrainment and mixing between the Atlantic and Mediterranean flows, is considered as an active layer in the hydraulic model. As result of these vertical approximations, two different hydraulic states have been found; however, the simulated behavior of the flow only supports the hydraulic state predicted by the three-layer case. Thus, analyzing the results obtained by means of the three-layer hydraulic model, the authors have found that the flow in the strait reaches maximal exchange about 76% of the tropical monthlong period

Ancestral proportions and admixture dynamics in geographically defined African Americans living in South Carolina.

We analyzed admixture in samples of six different African-American populations from South Carolina: Gullah-speaking Sea Islanders in coastal South Carolina, residents of four counties in the "Low Country" (Berkeley, Charleston, Colleton, and Dorchester), and persons living in the city of Columbia, located in central South Carolina. We used a battery of highly informative autosomal, mtDNA, and Y-chromosome markers. Two of the autosomal markers (FY and AT3) are linked and lie 22 cM apart on chromosome 1. The results of this study indicate, in accordance with previous historical, cultural, and anthropological evidence, a very low level of European admixture in the Gullah Sea Islanders (m = 3.5 +/- 0.9%). The proportion of European admixture is higher in the Low Country (m ranging between 9. 9 +/- 1.8% and 14.0 +/- 1.9%), and is highest in Columbia (m = 17.7 +/- 3.1%). A sex-biased European gene flow and a small Native American contribution to the African-American gene pool are also evident in these data. We studied the pattern of pairwise allelic associations between the FY locus and the nine other autosomal markers in our samples. In the combined sample from the Low Country (N = 548), a high level of linkage disequilibrium was observed between the linked markers, FY and AT3. Additionally, significant associations were also detected between FY and 4 of the 8 unlinked markers, suggesting the existence of significant genetic structure in this population. A continuous gene flow model of admixture could explain the observed pattern of genetic structure. A test conditioning on the overall admixture of each individual showed association of ancestry between the two linked markers (FY and AT3), but not between any of the unlinked markers, as theory predicts. Thus, even in the presence of genetic structure due to continuous gene flow or some other factor, it is possible to differentiate associations due to linkage from spurious associations due to genetic structure