Fratantoni David M.

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Fratantoni
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David M.
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  • Article
    Equilibration and circulation of Red Sea Outflow water in the western Gulf of Aden
    (American Meteorological Society, 2005-11) Bower, Amy S. ; Johns, William E. ; Fratantoni, David M. ; Peters, Hartmut
    Hydrographic, direct velocity, and subsurface float observations from the 2001 Red Sea Outflow Experiment (REDSOX) are analyzed to investigate the gravitational and dynamical adjustment of the Red Sea Outflow Water (RSOW) where it is injected into the open ocean in the western Gulf of Aden. During the winter REDSOX cruise, when outflow transport was large, several intermediate-depth salinity maxima (product waters) were formed from various bathymetrically confined branches of the outflow plume, ranging in depth from 400 to 800 m and in potential density from 27.0 to 27.5 σθ, a result of different mixing intensity along each branch. The outflow product waters were not dense enough to sink to the seafloor during either the summer or winter REDSOX cruises, but analysis of previous hydrographic and mooring data and results from a one-dimensional plume model suggest that they may be so during wintertime surges of strong outflow currents, or about 20% of the time during winter. Once vertically equilibrated in the Gulf of Aden, the shallowest RSOW was strongly influenced by mesoscale eddies that swept it farther into the gulf. The deeper RSOW was initially more confined by the walls of the Tadjura Rift, but eventually it escaped from the rift and was advected mainly southward along the continental slope. There was no evidence of a continuous boundary undercurrent of RSOW similar to the Mediterranean Undercurrent in the Gulf of Cadiz. This is explained by considering 1) the variability in outflow transport and 2) several different criteria for separation of a jet at a sharp corner, which indicate that the outflow currents should separate from the boundary where they are injected into the gulf.
  • Article
    Mixing and entrainment in the Red Sea outflow plume. Part I : plume structure
    (American Meteorological Society, 2005-05) Peters, Hartmut ; Johns, William E. ; Bower, Amy S. ; Fratantoni, David M.
    When the salty and heavy water of the Red Sea exits from the Strait of Bab el Mandeb, it continues downslope into the Gulf of Aden mainly along two channels. The 130-km-long “Northern Channel” (NC) is topographically confined and is typically only 5 km wide. In it, the Red Sea plume shows unanticipated patterns of vertical structure, turbulent mixing, and entrainment. Above the seafloor a 25–120-m-thick weakly stratified layer shows little dilution along the channel. Hence this bottom layer undergoes only weak entrainment. In contrast, a 35–285-m-thick interfacial layer shows stronger entrainment and is shown in a companion paper to undergo vigorous turbulent mixing. It is thus the interface that exhibits the bulk of entrainment of the Red Sea plume in the NC. The interfacial layer also carries most of the overall plume transport, increasingly so with downstream distance. The “Southern Channel” (SC) is wider than the NC and is accessed from the latter by a sill about 33 m above the floor of the NC. Entrainment into the bottom layer of the SC is diagnosed to be strong near the entry into the SC such that the near-bottom density and salinity are smaller in the SC than in the NC at the same distance from Bab el Mandeb. In comparison with winter conditions, the authors encountered weaker outflow with shallower equilibration depths during the summer cruise. Bulk Froude numbers computed for the whole plume varied within the range 0.2–1. Local maxima occurred in relatively steep channel sections and coincided with locations of significant entrainment.
  • Technical Report
    Red Sea Outflow Experiment (REDSOX) : DLD2 RAFOS float data report February 2001 - March 2003
    (Woods Hole Oceanographic Institution, 2005-01) Furey, Heather H. ; Bower, Amy S. ; Fratantoni, David M.
    This is the final data report of all acoustically tracked second-generation Deep Lagrangian Drifter (DLD2) RAFOS float data collected by the Woods Hole Oceanographic Institution in 2001-2003 during the Red Sea Outflow Experiment (REDSOX). The float component of REDSOX was comprised of two deployments on the R/V Knorr and R/V Ewing: the first in February-March 2001, with 26 floats, and the second in August-September 2001, with 27 floats. The isobaric floats were ballasted for 650 decibars to target the intermediate-depth, high-salinity outflow waters from the Red Sea. The objectives of the Lagrangian float study were (1) to identify the spreading pathways of the equilibrated Red Sea outflow, and to quantify the velocities and eddy variability typical of this outflow and of the background oceanic environment in the Gulf of Aden, and (2) to identify and describe the mesoscale processes which contribute to the seaward transport of Red Sea Overflow Water properties through the Gulf of Aden and into the western Indian Ocean. In addition to floats activated and launched during the two cruises, four time-series sites were chosen for dual-release float moorings. The dual-release floats were released every two months between cruises and every two months after the second cruise, with the final release in March 2002. A pirate attack on the R/V Ewing forced some modification of the float deployment plan during the second cruise.
  • Article
    Somali Current rings in the eastern Gulf of Aden
    (American Geophysical Union, 2006-09-30) Fratantoni, David M. ; Bower, Amy S. ; Johns, William E. ; Peters, Hartmut
    New satellite-based observations reveal that westward translating anticyclonic rings are generated as a portion of the Somali Current accelerates northward through the Socotra Passage near the mouth of the Gulf of Aden. Rings thus formed exhibit azimuthal geostrophic velocities exceeding 50 cm/s, are comparable in overall diameter to the width of the Gulf of Aden (250 km), and translate westward into the gulf at 5–8 cm/s. Ring generation is most notable in satellite ocean color imagery in November immediately following the transition between southwest (boreal summer) and northeast (winter) monsoon regimes. The observed rings contain anomalous fluid within their core which reflects their origin in the equator-crossing Somali Current system. Estimates of Socotra Passage flow variability derived from satellite altimetry provide evidence for a similar ring generation process in May following the winter-to-summer monsoon transition. Cyclonic recirculation eddies are observed to spin up on the eastern flank of newly formed rings with the resulting vortex pair translating westward together. Recent shipboard and Lagrangian observations indicate that vortices of both sign have substantial vertical extent and may dominate the lateral circulation at all depths in the eastern Gulf of Aden.