CBL Atlantic Zigzag paper JGR 2005

Hydrographic and tracer data from 2002 illustrate Atlantic water pathways and variability in the Mendeleev Ridge and Chukchi Borderland (CBLMR) region of the Arctic Ocean.
Thermohaline double diffusive intrusions (zigzags) dominate both the Fram Strait (FSBW) and Barents Sea Branch Waters (BSBW) in the region. We show that details of the zigzags temperature-salinity structure partially describe the water masses forming the intrusions. Furthermore, as confirmed by chemical tracers, the zigzags peaks contain the least altered water, allowing assessment of the temporal history of the Atlantic waters. Whilst the FSBW shows the 1990s warming and then a slight cooling, the BSBW has continuously cooled and freshened over a similar time period. The newest boundary current waters are found west of the Mendeleev Ridge in 2002.
Additionally, we show the zigzag structures can fingerprint various water masses, including the boundary current. Using this, tracer data and the advection of the 1990s warming, we conclude the strongly topographically steered boundary current, order 50 km wide and found between the 1500 m and 2500 m isobaths, crosses the Mendeleev Ridge north of 80ºN, loops south around the Chukchi Abyssal Plain and north around the Chukchi Rise, with the 1990s warming having reached the northern (but not the southern) Northwind Ridge by 2002. Pacific waters influence the Atlantic layers near the shelf and over the Chukchi Rise.
The Northwind Abyssal Plain is comparatively stagnant, being ventilated only slowly from the north. There is no evidence of significant boundary current flow through the Chukchi Gap.

This work is funded by NSF and made possible by the dedicated, hardworking, professional support of the USCGC Polar Star crew, the science party of cruise CBL2002, and the USCG Science Liaison. We also thank the Barrow Arctic Science Consortium, and the North Slope Borough for their assistance.

Figure Caption

Figures
For details, see paper

Figure 1. Maps of the study region, showing (left) the position of the Chukchi Borderland and Mendeleev Ridge in relation to the rest of the Arctic, and (right) details of the bathymetry in the Mendeleev Ridge and Chukchi Borderland (CBLMR) area. Black dots mark the CTD casts of the CBL2002 research cruise on the USCGC Polar Star. Depth contours are (left) schematic and (right) from IBCAO, at depth interval of 500 m. Pl stands for Plain. CGap stands for Chukchi Gap. Herald Valley and the Chukchi Slope extend off the bottom of the map.

Figure 2. Plots of CTD, XBT or XCTD data from a variety of Arctic missions, taking place between 1993 (top left) and 2002 (bottom right). Dot color (as per color bar) indicates maximum temperature (in °C) deeper than 150 db, i.e., the temperature maximum in the Atlantic water layers. Depth contours are from IBCAO, at depth interval of 500 m. See Section 3 for discussion.

Figure 3. Schematic of effects of mixing processes in temperature-salinity space, as described in the text. Thin dashed lines represent schematic isopycnals. (a) Starting from two distinct water masses (black dots) in temperature-salinity space, mechanical mixing yields a resultant with water properties lying on a straight line between the parent water masses (dashed line with arrows labeled MIX ). In contrast, double diffusive processes act to equalize temperature faster than salinity, with a resultant change in temperature-salinity space as indicated by wiggly arrows labeled DD . A combination of these processes (double diffusion and mixing) allows the resultant water to lie within the dark grey zone (assuming both processes are roughly equally present). Whatever combination of processes act, the resultant cannot lie outside the light grey area. (b) Schematic of the mixing of two distinct water columns, as described in the text. (c) Thick solid line shows the resultant of mixing the two water columns of Figure 3b (shown here as dashed lines) isopycnally in equal quantities. (d) Thick solid line shows a possible outcome of double diffusive (DD) processes acting on the interface between the two water columns (dashed lines). Note in the region where the difference between the two water columns is larger (here at lower salinities, labeled Big ), the zigzags are of larger amplitude and the peaks are more spaced in density than in the region where the two water columns are similar (here at higher salinities, labeled Small ). (e) Thick solid line shows the hypothetical decay of the structure of the solid line zigzags of Figure 3d by small scale vertical mixing. The amplitude of the peaks erodes, but the spacing of the peaks in density space remains much the same. Thin solid lines mark the new envelope of the maximums and minimums of the zigzag structures, and this envelope will be used in Figure 3f. (f) Taking the thin solid lines of Figure 3e as the parent water columns, thick solid line shows a possible outcome of double diffusive processes acting between these columns. Note that although the zigzag amplitude is the same that of the thick solid line in Figure 3e, the spacing of the peaks in density space is much smaller. This difference in structures between Figures 3e and 3f allows us to distinguish the initial separation of the parent water masses.

Figure 4. Potential temperature (theta) versus salinity plots for CBL2002 CTD data from the northwest slope of the Chukchi Rise. For Theta-S plots (right), grey dots show the entire CBL2002 data set, with locations given by grey dots in the left-hand maps. Oblique dotted lines are sigma-0 isopycnals in kg/m3. Within each row, individual profiles are marked in color, both on the map and on the Theta-S plot. For the maps, depth contours are from IBCAO, at depth interval of 500 m.

Figure 6. Scatter-plots of potential temperature (theta) against salinity for the CBL2002 data set for (top) CTD data colored with CTD-oxygen; (middle) CTD data colored with oxygen saturation; and (bottom) bottle data colored with CFC-11. Note that for clarity only 1/10th of the 2 db CTD data are used for the top two plots

Figure 5. From left to right, profiles with pressure of potential temperature (theta), salinity, dissolved oxygen (oxyg) and CFC-11 for station 33 (80º 14 N, 172º 50 W, black dot and profiles in top two panels of Figure 4) in the Canada Basin. Asterisks denote bottle data, with horizontal lines marking estimated error bars as per Section 2. For CFC11, these error bars are too small to be visible on this scale. Errors for profile data are as discussed in Section 2, i.e., ~ 0.002 °C; ~ 0.002 psu; ~ 1 dbar; < 2 umol/kg (CTDoxygen, calibrated against bottle samples).

Figure 7. Potential temperature (theta) versus salinity plots for CBL2002 data set. Oblique dotted lines are sigma-0 isopycnals in kg/m3. In each panel, grey indicates the entire data set, with colored lines representing various profiles from locations shown schematically in the inset map. Color indicates approximate water depth, with black being deep and red being shallow. The range of stations numbers used for each panel is labeled in the top right of each panel. Panel layout represents geography of the region. For example, sections from north of the study zone are on the top row of the figure, and stations from the west are on the left-hand side of the figure. See Section 4.3 for discussion

Figure 8. Composite plot of location (top row); potential temperature (theta) versus salinity (second row); CTD oxygen versus salinity (third row); and CFC-11 versus salinity (fourth row) for the CBL2002 data in the property regimes corresponding to the approximate depth range of the Fram Strait Branch Water (FSBW). See Section 4.3 for discussion.

Figure 9.
Composite plot of location (top row); potential temperature (theta) versus salinity (second row); CTD oxygen versus salinity (third row); and CFC-11 versus salinity (fourth row) for the CBL2002 data in the property regimes corresponding to the approximate depth range of the Barents Sea Branch Water (BSBW) and the deeper waters of the Arctic Ocean. See Section 4.4 for discussion.

Figure 10. CBL2002 data showing for each station (top left) CFC-11 value linearly interpolated onto the pressure surface of 375 db; (bottom left) CFC-11 value linearly interpolated onto the pressure surface of 700 db; (top right) mean potential temperature (theta) averaged between 200 and 700 db; and (bottom right) mean CTD oxygen averaged between 600 and 1000 db. (Note that the eastmost station of our data is outside the boundary current and thus cooler and lower in CFC-11 and oxygen than its neighbors, and that due to the order of plotting, this data point partially obscures the higher values in adjacent stations.)

Figure 11. Schematic of Atlantic water circulation in the CBLMR (Chukchi Borderland and Mendeleev Ridge) region of the Arctic Ocean. The newest boundary current waters (red, entering in the west) exhibit small zigzag ( zz ) structures in temperature-salinity space (red inset schematic). Here the FSBW (Fram Strait Branch Water) is slightly cooler than the maximum observed in the CBL2002 data, but the BSBW (Barents Sea Branch Water) is the coldest found in the CBL2002 data. Black arrows by the Mendeleev Ridge indicate that the boundary current, in some unspecified way, traverses along and somewhere crosses the Mendeleev Ridge. East of the Mendeleev Ridge, the core of the boundary current is marked by a point and bump (PtandBump) structure in temperature-salinity space (magenta inset schematic). The pathway of the boundary current, firstly south of the Chukchi Abyssal Plain and then north and east along the edge of the Chukchi Rise, is indicated with magenta arrows. In these waters the FSBW has its maximum temperatures, while the BSBW is slightly warmer than the newer BSBW to the west. Beyond the point of the Chukchi Rise, two pathways are evident in our data - one follows topography to the north of the Northwind Ridge and then moves south; the second follows the eastern flank of the Chukchi Rise southwards. The isolated relic waters of the Northwind Abyssal Plain are marked here in blue, with the northern part of the plain (light blue shaded region) being better ventilated than the southern part of the plain (dark blue shaded region). The large zigzags (blue inset schematic) are found to the north of the Chukchi Rise, where the boundary current interleaves with the older, colder basin waters. Green wiggly arrows indicate regions where shelf processes affect the FSBW Atlantic water layer. Black arrows north and west of the Chukchi Rise show hypothesized transport from the boundary current into the deep basin and a possible shortcut from the eastern Mendeleev Ridge to the Chukchi Rise. These routes are not confirmed or denied by the present paper. Note that the proposed shelf route through the Chukchi Gap is not marked here, since the CBL2002 data do not show the warm, point and bump boundary current taking this route.

We gratefully acknowledge financial support for this work from the National Science Foundation (NSF), under grant numbers NSF-OPP-0117480, NSF-OPP-0117040, and NSF-OPP-0117367.

Atlantic Circulation over the Mendeleev Ridge and Chukchi Borderland from Thermohaline Intrusions and Water Mass Properties

Rebecca Woodgate, Knut Aagaard, Jim Swift, Bill Smethie and Kelly Falkner