OS14I-01
Seismic Imaging of a Thermohaline Staircase in the Western Tropical Atlantic.
Legacy data from a 1998 seismic reflection profiling (SRP) cruise of the R/V Ewing east of the Leeward Islands has been examined for evidence of thermohaline staircases. Strong staircase profiles in temperature and salinity are often found in the main thermocline of the western tropical Atlantic due to the salt finger instability. The fingers are active at thin, high gradient interfaces and provide a buoyancy flux that convectively stirs the adjacent 10-40 m thick mixed layers. Sound speed decreases of up to 3m/s are calculated to occur across the interfaces, with weaker sound speed increases appearing through the well-mixed layers due to the influence of pressure. When reprocessed to reveal reflections from the water column, the eastern portion of one SRP section from 1998 reveals a series of rather flat, regular reflectors in a location and depth range where staircases are often observed. Though no physical data are available from this cruise, staircase profiles from the region obtained in 1985 and 2000 provide a basis for comparison. The magnitude of estimated reflection coefficients and comparable spatial scales in the physical data provide evidence that supports the interpretation that a thermohaline staircase was imaged in the seismic profiles. Also, the amplitude of internal waves as seen in the displacement spectra of the "staircase" reflectors is notably below that of the Garrett and Munk model and in SRP data from outside the step region. This is consistent with other evidence that internal waves are suppressed by the double-diffusive convection within thermohaline staircases.
OS14I-02
Seismic images of the ocean offshore western North America: internal tides and the California undercurrent
Seismic reflection data collected over the past several decades across the continental margin of western North America in support of solid earth tectonic studies reveal reflections from within the water column that may be related to internal tides and to the California undercurrent. We have recently begun systematically reprocessing these data to define temporal and spatial variations in the reflectivity structure and to associate this structure with variations in ocean finestructure. Preliminary results suggest the presence of higher order modes of internal tides near the crest of the Gorda Escarpment and a bottom boundary layer on the southern flank of the Gorda Escarpment. Offshore Oregon, strong reflections are found at a depth and distance that corresponds to the expected location of the California undercurrent. When combined with traditional physical oceanographic datasets (e.g. GLOBEC hydrographic data), the seismic data have the potential to extend spatial and temporal constraints on these oceanographic processes.
OS14I-03
Internal Wave Energy Estimated From Seismic Reflection Data
Seismic reflection data off the Iberian Peninsula recorded in 1993 and 1997 shows pronounced reflections within the water column that are traceable over substantial distances at lateral resolutions of down to 10m. Reflections can be found in the upper 2000m of the water column partly coincident with the layer of Mediterranean water that enters of the Atlantic basin at Gibraltar. The Mediterranean water's elevated temperature and salinity, compared to other North Atlantic water masses at similar depths, is responsible for double diffusive processes in which the different diffusivities of heat and salt can lead to series of vertically well mixed layers and jumps of temperature and salinity. These jumps in temperature are, in our data, the likely causes for the seismic reflections. Their horizontal distribution can be studied by following the reflection lines and allows a highly detailed view of the boundary zone between the water masses and the mixing processes between them. Vertical excursions of the reflection lines are thought to be associated with the background internal wave field. It exhibits Garrett-Munk like spectral distributions at horizontal wave lengths of several tens of meters to a few kilometers. The seismic reflection data allows to assess the spatial distribution of internal wave energy that is important for deep ocean mixing processes. Particularly high levels of internal wave energy are found over topographic features like sea mounts but also around a Meddy, a rotating lense of Mediterranean water.
OS14I-04
Seismic Imaging of Fronts Associated with Antarctic Circumpolar Current Near Drake Passage
The evolution of the Antarctic Circumpolar Current and its components is undoubtedly of considerable importance in moderating climate change. Despite this global significance, temporal and spatial interactions between circumpolar water masses and the general thermohaline circulation of the oceans are poorly understood. Here, we present observations from a controlled-source seismic reflection experiment which we believe demonstrate the potential of four-dimensional (i.e. time-lapse) monitoring of acoustically distinctive water masses. The approximately north-south seismic reflection profile is 1200 km long and crosses both the Sub-Antarctic and Polar Fronts where they loop around the Falkland Plateau, east of Drake Passage. These data were acquired with a 5,600 cubic inch airgun array and a 6 km streamer in October 1998. Vertical and horizontal resolution is 5-10 meters. In the Argentine Abyssal Plain, the interaction between North Atlantic Deep Water and different components of the Antarctic Circumpolar Current is spectacularly imaged: thermohaline fronts, salt fingering and mesoscale eddies are observed in detail. Our seismic observations can be independently corroborated by legacy hydrographic measurements for which we have calculated reflectivity response. The lack of detailed correlation implies temporal variation. Across the Falkland Plateau, the Sub-Antarctic Front is imaged in several places as expected. In the Scotia Sea, stratification of the water column occurs south of the Polar Front, which is clearly imaged dipping northwards. These preliminary results suggest that a co-ordinated series of experiments in the southern oceans could yield important information about diapycnal mixing.
OS14I-05
A Seismic-Oceanographic Joint Observation in the Kuroshio Extension Front Offshore East of Japan -- Preliminary Report From KT05-21 Cruise
We conducted a seismic-oceanographic survey to observe the finestructure of the sea water near the Kuroshio extension front off east of Japan from the end of August through early September 2005. Multi-channel seismic reflection survey (MCS) and oceanographic observation, including XCTD, XBT, XCP and ADCP, were carried out using R/V Tansei-maru of JAMSTEC. A basic 150 miles-long survey line ran across the main stream of Kuroshio current in N-S direction. The MCS survey was operated in north and south of Kuroshio stream axis separately; the length of the MCS line was 60 miles in the northern part and 45 miles in the southern part. An additional MCS survey was also conducted south of Kuroshio current on E-W 30 miles survey line. The streamer cable used in this survey equipped 48 channels with 25m group interval. During the MCS survey, we simultaneously operated both the vertical oceanographic profiling and air gun shooting. We carried out the MCS shooting three times in the northern MCS line to evaluate the effect to the quality of seismic profile brought by different air gun setting; Bolt 1500C with 20 litters chamber, Bolt 1500C with 9 litters chamber, Sodera/SSI GI gun with 3.5 litters chamber. It turned out that Bolt 1500C with 9 litters chamber was the best in our survey. This setting was then used in the MCS profile in southern lines. Five days survey yielded 450 miles ADCP profile, 36 measurements of XCTD, 5 measurements of XBT, 5 measurements of XCP and 255 miles of MCS profile. The brute stack profiles of MCS data show clear reflection event at 200-700m depth in the sea water dipping southward. The depth of reflector matches with that of temperature jump in XCTD, XBT profile, which suggests that the reflector is formed by the subduction of the Oyashio (cold current) beneath the Kuroshio (warm current).
OS14I-06
Thermohaline Interleaving in the Kuroshio Extension Front: Seismic Reflection Imagery and in situ Measurements
Oceanic fine structures, especially in a layered form, is a common feature in frontal regions. So far our observational knowledge on these structures has principally relied upon series of CTD profiles with sparse horizontal resolution, which often requires risky interpolation between adjacent profiles. Recent developments in the seismic imagery and the expendable instrumentation enabled thevisualization of the layered structures with surprisingly high resolution ($\sim$10m) both vertically and horizontally, which is high enough to track a single layer for horizontal distance over 50km. We conducted a survey from 29 August to 2 September 2005 in the region of the Kuroshio extension front, along which cold and fresh Oyashio water subducts into warm and salty Kuroshio current and thus exhibits strong thermohaline contrast. In the survey we acquired seismic reflection data simultaneously with XCTD (expendable CTD), XCP (expendable current profiler), and onboard ADCP (acoustic Doppler current profiler) measurements. Three seismic sections along 143$\deg$E (across the front) all show distinct reflection surfaces at the depth of 400$\sim$600 m, with horizontal span of $\sim$50 km and vertical separation of $\sim$50m, which are well consistent with the observed salinity and temperature profiles and also with velocity fields. Position of reflection surfaces varied considerably in association with short-term variations of the Kuroshio current during 3 days of the survey. Possible mechanisms which generate and maintain the layered structure are discussed and some estimation of mixing inside the layered region is made by analyzing the XCP data.