The Alaskan Beaufort Slope: SCICEX-99

figure 1

  Figure 1


  The submarine track along the continental slope is shown above (Figure 1). Primary sampling was performed using the sail-mounted CTD and submarine-launched expendable CTDs deployed at the indicated locations. In plan view, the submarine track described a 21-leg saw-toothed pattern above the continental slope, each leg of which was approximately 30nm in length. Preliminary results are presented below. Figure 2 shows along-slope profiles of Temperature and Salinity, collected at a nominal depth of 117m. Figure 3 shows along-slope power spectra of measured depth, T, and S for the same transect. The plots in figure 4 are pseudo-contour T & S plots of along-slope T and S as resolved by XCTDs. Figures 5-8 are pseudo-contour plots of xCTD casts along sawtooth legs 1, 10 ,17 and 21. Figures 9-13 are the geostrophic velocities and dynamic height fields derived from the xCTDs for the along-slope transect and the same four cross-slope legs. For access to the original postscript files that generated these plots, click here.

Figure 2. Temperature (red) and salinity (blue) from the sail CTD at a depth of 117 +/- 2 m (within the upper halocline) from west to east along the Beaufort Sea slope. (These data represent averages over 6-minute sampling intervals). Salinities range from a minimum of 32.7 to 33.6 psu. The data show considerable variability at a variety of spatial scales. However, the broad scale, alongslope pattern suggests a transition in temperature and salinity at about 152oW, which is about 100 km east of the mouth of Barrow Canyon in the Chukchi Sea. The broad scale pattern consists of: 1) increasing salinity (~32.75 - 33.05 psu) from the west end of the transect to ~152oW and, 2) relatively constant salinities (~33.1 psu) east of this longitude. Temperatures range between -1.7 and -1.3oC and are generally higher, by about 0.1oC, west of 152oW than to the east. The two warm and salty peaks at about 150 and 148oW can be explained as vertical displacements within the halocline of about 40 m.

 

Figure 2


 Figure 2
 
  Figure 3. Spectra of bottom depth (left), temperature (middle), and salinity (right) from the sail CTD. The spectra are red although and has a significant peak at wavelengths of between 50 and 20 km, suggesting that these are the appropriate decorrelation scales for temperature and salinity along the continental slope.

 

Figure 3
 Figure 3

 

Figure 4. The alongslope temperature (left) and salinity (right) sections based on the SSXCTDs launched at the indicated stations. The view is northward with west on the left. The thermocline (between the -1.0 and 0.0oC isotherms) slopes upward from the west to the middle of the section and then downwards to the east. The change in depth (as given by the -0.5oC isotherm is about 50 m between the western end and center of the transect). Note also that layer of warm water (> 0.5oC) lying between 300 and 500 m depth thins and cools from west to east. The halocline also shoals slightly from the west to the center of the transect and then deepens again. Note that while waters between 100 and 150 m depth are fresher in the west than in the east, the surface layer (< 50 m depth) is fresher in the east than the west. Although the source of this low-salinity water cannot be determined unequivocally, it is most likely outflow from the Mackenzie shelf that is swept westward within the Beaufort Gyre.

Figure 4


 Figure 4
 
  Figures 5 - 8. The figure sequence shows the cross-slope distribution of temperature and salinity as a function of pressure based on the SSXCTD launches shown in Figure 1. The figures are for Leg 1 (stations 9-12) along ~160oW and west of Barrow Canyon, Leg 10 (Stations 16-21) east of Barrow Canyon, Leg 17 (stations 24 - 29, central Alaskan Beaufort slope), and Leg 21 (stations 31 - 35) eastern Alaskan Beaufort slope.


  1. Along each transect the halocline and thermocline slope upward and vertical gradients increase slightly (isopleths converge vertically) moving from the interior onto the inner slope. The tilt of both features is consistent with the mean anticyclonic wind stress over the Beaufort Sea, which should promote upwelling along the Beaufort slope. Maximum slopes are within the inner 20 - 40 km of each transect.


  2. There are significant alongslope differences in the surface layer structure. The upper 75 m along Leg 1 (west of Barrow Canyon) is less stratified compared to the upper 75 m along the transects east of the canyon (Legs 10, 17, and 21). As noted above (Figure 4), the upper 75 m freshens and becomes more strongly stratified moving eastward along the continental slope. Along legs 10, 17, and 21, the isohalines in the upper water column slope downward (opposite the tilt of the halocline and thermocline) on approaching the slope. This is not observed along Leg 1, where the upper ocean isohalines are either flat or tilt in the same direction as the halocline and thermocline.


  3. Finally, we note that there are alongslope differences in the cross-shore extent of the deep (300 - 500 m) layer of warm water (T ~ 0.5oC). This water mass extends across the entire 60 km width of Leg 1, is confined to the innermost 40 km along Leg 10, and is virtually absent at Legs 17 and 21.

 

Figure 5

 Figure 5
 
 
Figure 6

 Figure 6
 
 
Figure 7

 Figure 7
 
 
Figure 8

 Figure 8

 

Figures 9 - 13.
  This sequence of figures shows the along-slope distribution of dynamic height (Figure 9) and the cross-slope distribution of dynamic height for Legs 1, 10, 17, and 21 (Figures 10 - 13). In each figure we show the dynamic height as a function of depth relative to two different reference depths. Superimposed on the figures are geostrophic velocities (in cm/s) based upon thermal wind and assuming that the reference depth is a level of no motion.
 
  Figure 9 shows the dynamic height field along the slope and as a function of depth. Dynamic heights slope downward from the Chukchi slope to 150 - 145W and then upward toward the eastern Beaufort. The dynamic topography is relatively flat below 200 m depth, indicating that most of the gradient is explained by changes in the halocline slope. The geostrophic tendencies include weak onshore flow in the eastern Beaufort and offshore flows along the western Beaufort and Chukchi slopes.

 

figure 9


 Figure 9
 
  Figure 10 shows the dynamic height and baroclinic flow field along Leg 1 (west of Barrow Canyon). Calculations relative to 1000 m depth indicate maximum eastward flow at about 300 m depth or just below the depth of the thermocline. The geostrophic calculations can be extended further inshore by choosing a shallower reference level (e.g., 400 db) as shown in the left panel. The relative flow is westward and decreases with depth, with weak eastward flow indicated at about 300 m. The results suggest a swift westward flow of 15 - 20 cm/s within the upper 150 m along the inner part of the transect. The flow is narrow O(10 km) and has a strong horizontal shear (10-5 s-1) within the upper 200 m. The vertical shears inshore are a factor of 5 greater than those further offshore.

 

figure 10


 Figure 10
 
  The dynamic height and baroclinic flow along Leg 10 (east of Barrow Canyon) is shown in Figure 11. The section shows considerable horizontal and vertical complexity between the surface and 600 m, although most of the dynamic relief is above 200 m depth. Relatively strong eastward flow occurred between 150 and 600 m depth on the inshore portion of this transect and also in the upper 150 m on the outermost end of the transect. The regions of eastward flow were separated by a swift, westward flow with near surface speeds of ~30 cm/s. The velocity structure suggests an eddy and indeed the isohalines in Figure 6 show a downward bowing of the isohalines at station 20. However, the vertical extent of this feature is greater than usually associated with the Arctic Ocean eddies.

 

Figure 11


 Figure 11
 
  The relative flow is westward over virtually the entire water column along Legs 17 (Figure 12) and 20 (Figure 13). This transition in the alongshore flow structure from eastward along the western end of the study area to westward along the eastern end, is consistent with the change in the along-slope dynamic topography or baroclinic pressure gradient (Figure 9). Along both sections calculations relative to 400 db suggest a strong westward flow over the inner slope with significant vertical and horizontal shear. In aggregate the along-slope flows suggest convergence along the Beaufort continental slope, particularly within the upper 250 m of the water column. Mass transport calculations (relative to 1000 db) support this insofar as the net alongslope transport across Leg 1 is 0.8 Sv (eastward), diminishing to 0.2 Sv (eastward) along Leg 10, and then reversing to -0.2 Sv (westward) along Leg 17 and -0.9 Sv along Leg 21.
 
  The cross-shore extent of these sections was limited so that the mass transport calculations and suggestions of alongshore convergence must be viewed with some caution. Nevertheless, the geostrophic calculations and the hydrographic structure show considerable along- and cross-shore variability in terms of the vertical stratification and horizontal and vertical current shears. The submarine did not sample in water depths much less than 300 m. Our data suggest that current shears are substantial along the inner slope. Insofar as the shelfbreak vorticity structure controls exchange between the shelf and slope, future sampling efforts should include this region.

 

Figure 12

 Figure 12
 
Figure 13

 Figure 13