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back to main page... AWACS Recon: R/V Tioga Cruise 194 July 17-19, 2006 Chief Scientist: Glen Gawarkiewicz, WHOI Captain: Ken Houtler, Deck: Ian Hanley Watchstanders: Frank Bahr and Chris Linder, WHOI Objective This cruise was aimed at establishing initial ocean conditions for the SW06/NLIWI/AWACS Experiment off New Jersey. The cruise plan was to sample along three long cross-shelf sections, two coinciding with the dimensions of the mooring array and a third located upstream, east of Hudson Canyon. An along-slope line connecting the offshore ends of the cross-shelf transects was intended to resolve slope eddies adjacent to the outer shelf. We were able to complete the two cross-shelf sections at the southwest and northeast ends of the mooring array domain as well as the slope section, but were unable to do the section east of Hudson Canyon due to an impending Tropical Storm (Beryl). In addition to hydrographic sampling, we also were noting position of fishing gear as well as marine life. Preliminary plots - station map 
A map of the stations sampled. Preliminary plots - Transmission Line (TL)  An 80 km cross-shelf section along the mooring line/cross-shelf transmission line. Note the relatively cool temperatures in the "cold pool" (8 Degrees C) and the warm surface temperatures (25 Degrees C). Offshore, there are relatively warm temperatures at depth (18 Degrees C at 50 m) associated with a slope eddy. Scott Glenn of Rutgers University identified this eddy as being centered at 72.5 W 38.5 N. There are strong cross-shelf temperature gradients between x=40 and 50 km, which would be the position of the front at 40 m depth. Maximum vertical temperature gradients are at approximately 15 m depth, and the thermocline weakens considerably further offshore in the slope eddy. 
Salinity along the same transect. Two key features; the slope eddy with salinities of up to 36.0 PSU offshore, and the very fresh surface layer (31 PSU) which extends to the offshore edge of the section. The low salinities may be associated with large outflows of fresh water from the Hudson River (suggested by Scott Glenn). The fresh surface layer,combined with the warm surface layer, resulted in fairly high stratification in the seasonal pycnocline. The foot of the front, defined by the 34.0 PSU Isohaline, is at the 70 m isobath, but note that there are strong along bottom gradients at the 90 m isobath, essentially at the shoreward edge of the slope eddy. The front is quite steep, suggesting that the section is on the western side of the slope eddy, where there are onshore flows which tend to steepen the front.  Density shows a strong seasonal pycnocline with steeply sloping isopycnals within the shelfbreak front. Note that the density gradients reverse at 60 km. This suggests the slope eddy circulation is opposing the flow in the shelfbreak front, and there are likely strong lateral shears in this cross-shelf position.  Profiles are upward refracting until x=20 km. There is a distinct mid-depth duct at x=40 km at the offshore edge of the shelfbreak front. 
The buoyancy frequency along the cross-shelf mooring line (TL). This is a plot of N2 and units are 1/seconds*seconds. Preliminary plots - AS  A coarse section along the 80 m isobath. There are some alongshelf gradients below 20 m depth, but this is difficult to interpret due to the temporal gaps between the four stations.  Some suggestion of slope water intruding at 10 km, but again temporal gaps make interpretation difficult.  The gradients are likely to be strongly aliased by internal tides but show a fairly uniform pycnocline.  Some interesting gradients between 30 and 50 m depth. Preliminary plots - EB  This is from the eastern line of the mooring box. The slope eddy is not evident. The foot of the front is at 70 m, and again there is a temperature minimum at mid-depth, this time at x=60 km. Note that the stations are spaced at 20 km instead of 10 km intervals.  The salinity structure again indicates a fairly steeply sloping front. The near surface layer is very fresh, below 30 PSU at the most onshore station. Note that the maximum salinities in this section are 35 PSU, not 36 PSU as in the TL transect.  Major cross-shelf density gradients are near the foot of the shelfbreak front.  Mid-depth duct is at x=60 km, Preliminary plots - SL  This section shows a significant band of cool shelf water presumably moving offshore in a filament or streamer to the northeast of the slope eddy to the south.  This section shows the eastern extent of the near-surface fresh layer as well as the low salinity shelf water between 20-50 m depth. A second slope eddy is at the eastern edge of this section.  This section shows a nice reversal of density gradients near station 20. West of this, the shelf streamer has a thermal wind shear consistent with offshore motion, to the east of station 20 the shear is consistent with onshore motion, relative to 100 m. Preliminary plots - ADCP 
Zonal velocity from the shipboard ADCP (section TL). Westward velocities were a maximum of 30 cm/s in the shelfbreak frontal jet. This section has not been de-tided. 
Meridional velocity from the shipboard ADCP.  A planview of the velocity vectors in the depth-range of 26 to 34 m. The shelfbreak jet is apparent in the southwesternmost cross-shelf section, but in the northeast section the flow is more confused. Note the offshore transport in the northeast corner of the cross-shelf sections- possibly indicative of the offshore flow of Hudson River plume water to the shelfbreak. Photos by Glen  A view of the WHOI dock as the Tioga departs Woods Hole.  Ian Hanley anchors for the night.  A surface slick passes by the Tioga.  Typical fishing gear.  CTD operations: Chris Linder (left) and Frank Bahr (right).
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