OS54A-01 INVITED
Diagnostic Studies of Mesoscale Variability in the Florida Current at the Downstream Boundary of the Intra-Americas Sea (IAS)
Most of the throughflow of the Intra-Americas Sea (IAS) passes through the Straits of Florida as the Florida
Current. The Florida Current forms an intense jet and frontal system along the shelfbreak of the East Florida
Shelf. It exhibits extrinsic variability associated with tides, synoptic weather systems, and seasonal and longer-
term variations of atmospheric forcing and oceanic general circulation. The Florida Current also exhibits intrinsic
variability associated with dynamical instabilities resulting in the formation of the well-known cyclonic Florida
Current frontal eddies (FCFEs), their less well-known anticyclonic counterparts, and meanders on time scales
similar to those of the synoptic atmospheric forcing. Here, the results of a limited-area, two-year numerical
simulation using the Princeton Ocean Model (POM) [with mesoscale-admitting resolution; realistic bottom
topography for the Straits of Florida; and realistic tidal, atmospheric, and open boundary forcing] are used for
diagnostic studies of the aforementioned Florida Current variability. The simulations are validated against various
types of observations and then used to examine processes beyond the scope and grasp of available
observations. For example, powerful downwelling events occur along the East Florida Shelf (and upwelling
events along the Bahamas) as the consequence of the passage of wintertime cold fronts which available
observations can only partially characterize, while the simulations provide a much more comprehensive view of
the associated countercurrent flows and related phenomena. Overall, a scientific strategy of diagnostic analyses
based on numerical simulations is illustrated that should prove useful in improving the understanding of how
other elements of the Gulf Stream System interact with the continental shelf and slope regions of the IAS.
http:efsis.rsmas.miami.edu
OS54A-02
Nitrogen and Phosphorus Fluxes in the Gulf of California: Geostrophic Approach
Continental margins exert a strong influence on global biogeochemical cycles. There have been relatively few studies to quantify either the magnitude or nature of temporal variability in material fluxes. Nutrient fluxes studies at the mouth of the Gulf of California (GC) are needed to estimate values from direct measurements. From 1995- 1999 during five cruises covering all seasons, seawater samples were collected and measured the nutrient content from the surface to the bottom (some deeper than 2500 m) from a repeated hydrographic sections at the mouth of the GC. This chemical and physical database is unique because it covers an area with important biogeochemical signs, which has been detected as one of the highest in primary productivity of the world oceans. These sections are perpendicular to the coastlines of the Mexican states of Baja California Sur (BCS) and Sinaloa. We choose only one section, with very dynamic surface water. Strong geostrophic currents appear in February 1999 with temperatures of 20±1.5°C; salinity 35.091°0.156; pH 8.16°0.13; phosphate 0.85°0.42μM, nitrate+nitrite 2.35°2.94μM, and ammonia 2.00°1.25μM (average ° standard deviation). The geostrophic velocities were computed from high-resolution CTD sections across the entrance to the GC. During winter and spring, the outflow occurred near BCS and the inflow occurred either through the center of the section and/or along the Sinaloa coast. Both inflow and outflow cores were 45 km wide and extended deeper than 700 m. Summer and fall showed a complex pattern, alternating cores of inflow and outflow but with inflow along Sinaloa on all cruises. The maximum flow into the Gulf occurs during May in the center of the section while outflow was concentrated along BCS. The section mean geostrophic velocity was composed of two alternating cores of inflow and outflow. The two cores that were adjacent to either coast were broader and contained the highest inflow (0.40 m s-1) and outflow (-0.25 m s-1) velocities, supporting the general idea of inflow along the Sinaloa and an outflow along BCS. The highest nutrient fluxes occur during El Niño conditions in November 1997 with outflows as high as 54.5 Tg yr-1 for phosphate, 43.0 Tg yr-1 for nitrate+nitrite and 31.7 Tg yr-1 for ammonia, this values were at least three times higher than in February 1999.
OS54A-03
Autigenic and Anthropogenic Uranium in the Marine Sediments of the Gulf of California in Front of Santa Rosalia Mining District
To verify the possibilities of U enrichments in the marine sedimentary environment of the eastern sector of the central Gulf of Califoria (GC), eleven sediment cores were collected in front of the Santa Rosalia mining region, peninsula of Baja California. Uranium and some other trace element contents in sliced core layers, dried and homogenized, were determined using instrumental neutron activation analysis. Average total U contents in sediments of five cores collected in the open GC in front of Santa Rosalía at sites with water depths from 265 m to 1030 m and in the Guaymas Basin with 2019 m, ranged from 1.36±0.26 mg kg-1 (Guaymas Basin) to 9.31±3.03 mg kg-1 (SR63 core, depth 630 m). To distinguish non-lithogenic U from the lithogenic one, the normalization of total U contents to the concentrations of Sc in the samples was used. That because this element is a reliable indicator of crustal materials, mainly aluminosilicates in the marine sediments. The relative contribution of non-lithogenic (authigenic) U varied from 49.8±3 % (Guaymas Basin) to 84.2±8.2 % (SR62 core) of the total U content in the sediments of the open central GC. Surprisingly, in three sediment cores from the coastal zone adjacent to the town of Santa Rosalía in water depth range 3-6 m very high concentrations of total U were found, ranging from 54.2±7.3 mg kg-1 (SR4 core) to 110±13 mg kg-1 (SR2 core) and exceeding not only U average abundance in the earth´s crust (2.7 mg kg-1), but also its levels found for SR62 core, as well as those reported for natural enrichments of U in suboxic-anoxic environments, e.g. at Mexico and Peru margin sites (3.04 mg kg-1 - 24.54 mg kg-1, McManus et al., 2006). The relative contribution of non-lithogenic U in the sediments of these three anomalous cores varied from 97.2±0.4 % (SR4 core) to 98.80.2 % (SR1 and SR2 cores) of their total U content. The sediments were also depleted in organic C (0.05 % - 0.18 %), which is not typical for marine solid phases enriched in authigenic U. Additional surface sampling around the cores with high levels of U, helped to define the spatial distribution of this element, as well as Co, Cu, Zn, light lanthanides and europium, which also showed "anomalies". The association with anthropogenic impact is discussed because the geochemical fingerprints of these sediments are the same as for solid wastes of copper smelting, which has occurred in Santa Rosalía in the past century till 1984.
OS54A-04
Circulation in the SAV, Shelf of Veracruz, Mexico
Data from current meter, hydrographic measurements, and numerical modeling are analyzed to determine the circulation patterns in the Sistema Arrecifal Veracruzano (SAV), Veracruz, Mexico, region. Results show that the main forcing in the region is the along-coast wind stress component. Tides contribute with one order of magnitude less energy, during the fall-winter period. Two main regimes are identified: one between the coast and the 20 m isobath, with waters of large turbidity and small influence from rivers, and the other between the 20 m isobath and the external shelf, with clearer waters. Stratified water column with low temperature upwelled waters near the bottom are observed when southeasterly winds are present. Downwelling and southeastward currents are associated with northerly winds linked with northers.
OS54A-05
Effect of the wave-induced currents on the contaminants dispersion in Veracruz, Mexico.
he influence of the waves on the circulation of the Sistema Arrecifal Veracruzano and the beaches of Veracruz and Boca del Rio Counties is examined by using the NearCoM model. The surf zone wave-averaged circulation is forced primarily by the gradients of the radiation stress due to the breaking waves. Results show that a "valve effect" associated with mass convergence due to wave-induced currents is an important mechanism for the dispersion of contaminants in the beaches with coral barriers.
OS54A-06
The diel zooplankton motion at 1000 and 300 m depth
A well known feature of a large fraction of the zooplankton in the 300 m near-surface layer is the daily vertical migration cycle, which is phase locked with the light cycle. Less documented is the similar cycle in deeper layers away from significant light. Here we show, based on direct measurements, the mean vertical velocity cycle for layers 170 m thick, one centered at 1000 m and another, for reference, at 300 m depth, in the central Gulf of Mexico. Averages over 473 days are computed timing each cycle with the sunrise and sunset. Both cycles are highly symmetrical odd functions where, in the deeper layer, the downward/upward migration is delayed/advanced relative to the upper layer by about 1 h 40 min. In the upper layer, the peak speeds are close to 40 mm/s, with corresponding vertical displacements about 250 m, and in the deeper layer the peak speeds are 10 mm/s with vertical displacements of approximately 120 m. In the upper layer, the peak downward motion occurs minutes before sunrise, wheras in the lower layer, the peak downward velocity occurs 1.5 hours after the sunrise. The peak upward velocities are found 1.5 hours before sunset in the deeper layer and just minutes after sunset in the upper layer. Therefore, the nightly permanence towards shallower water lasts longer, for about 3.5 hours, in the deeper layers than closer to surface.
OS54A-07
Three-Layer Regime of the Water-Column in the Central Gulf of Mexico
Recent observational results have shown that the water column in the Gulf of Mexico is characterized by different hydrographic and diffusive regimes into three layers. The variability within the upper layer (above the sill level at the Florida Straits, ~ 700 m depth), is driven mostly by the mesoscale eddies shed from the Loop Current, which dominate the 20 to 150 day spectral band. An intermediate layer, extending from about 700 m to about 1300 m depth, is a transitional zone that connects the near-surface dynamics with that of the lower layer, which extends from 1300 m depth to the bottom (~ 4000 m). Although it is forced by the layers above, this lower layer is dominated by bottom-trapped motions, possibly Topographic Rossby Waves, centered at the 20-60 day spectral band and whose decaying scale exceeds 1000 m.
OS54A-08
The Bottom Boundary Layer in the Deep Gulf of Mexico
A year-long time series of current profiles, upward from ten meters off the bottom in 3750 m of water in the central Gulf of Mexico, shows that the measuring range of the instrument, and the velocity of the current in the boundary layer vary as a function of time, the former because the concentration of backscattering suspended matter fluctuates with the strength of the currents above the boundary layer. Therefore, currents near the bottom, typically of O(0.1 m·sec-1) are often strong enough to cause sediment resuspensions. Estimates of the thickness of the boundary layer based on the friction velocity support the notion of an active boundary layer that varies with the bottom-intensified motions of the lower, very homogeneous, layers of the Gulf of Mexico. The deeper layers in the gulf have been shown to be actively ventilated and therefore highly oxygenated. Hence, organic matter near the bottom must be efficiently oxidized. The dynamical and environmental implications of these findings are discussed.