Ocean parameters

Salinity and temperature profiles, collected during the summer mooring surveys show that the coldest water (-1.7°C) in summer is found at mid-depth. It has a salinity of 32.8-33.0 and represents water mass remnants of the winter surface mixed layer (Prinsenberg and Hamilton 2005). Above this water mass lies a very stable surface layer formed by dilution by ice melt and local runoff and warming by the atmospheric heat flux. The warmest and freshest water is organized into buoyancy boundary currents that flow in opposite directions along the northern and southern shores. Below the cold mid-depth water mass, the water temperature and salinity increases with depth. This warmer deep water enters the area from northern Baffin Bay. Geostrophic current fields derived from density distributions shows an eastward flowing current that decreases with depth and extends from the southern shore of Lancaster Sound to 2/3 of the way across the Sound. A depth-varying current along the northern shore appears to be restricted to 1/3 of the northern part of the Sound and does not appear to contribute to the total flux through the cross-section.

Figure 3 shows a 2-month example of the current variability seen in Lancaster Sound. The 2-month data sections are bi-hourly along-shore currents at 10 m depth from the southern and northern sites. These small sections of the total 8 year record show the temporal variability that exists throughout the 8 year time series. Ocean currents vary hourly due to tidal components, vary daily due to atmospheric forcing and vary seasonally due to long-term variability in sea level pressure gradients. In addition to these temporal variabilities at each location, there exist large horizontal and vertical spatial variabilities. Tidal currents vary on a 12 h cycle by up to 35 cm/s when the sun and moon tidal constituents generated in the

Lancaster Sound Barrow
Fig. 3. Along-shore ocean surface velocities (10 m depth) from the southern and northern mooring sites in Lancaster Sound. Bi-hourly data are from August 10 to October 9, 2005.

Atlantic Ocean reinforce or oppose each other. The contribution from the Arctic Ocean tide is weaker as the Arctic tides are smaller and reflected back to the Arctic from the sill and island arch located in western Barrow Strait at 96° W longitude.

The largest long-term mean velocity components, driven by atmospheric and sea level pressure gradients along the NW Passage, are found along the southern shore where daily mean values of the exiting Arctic surface waters reach 50 cm/s and set towards Baffin Bay. Along the northern shore, the long-term currents are smaller and do not have a persistent preferred direction (Fig. 4). In the summer they generally are directed towards the west (Arctic Ocean) and in the winter towards the east (Baffin Bay). Currents normally decrease with depth in response to surface atmospheric forcing and bottom friction. The exception being that during land-fast ice conditions, the ice isolates the ocean surface from wind cm/s cm/s cm/s

total year 1999-2000 winter 1999-2000 summer 1999-2000

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-5 0 5 10 15 20 25 30 -5 0 5 10 15 20 25 30 -5 0 5 10 15 20 25 30

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total year 1999-2000 winter 1999-2000 summer 1999-2000

total year 1999-2000 winter 1999-2000 summer 1999-2000

Fig. 4. Along-shore current profiles tor the northern (top) and southern (bottom) sites in Lancaster Sound. Profiles are for year-long deployment of August 1999 to August 2000, the winter 2000 (January to April) and the summer 2000 (June to mid August).

forcing and acts as a friction boundary thereby reducing the surface currents in winter relative to mid-depth values (Fig. 4). The seasonal and yearly mean currents vary interannually in response to large scale atmospheric forcing and as seen later to the sea surface level set-up due to surface winds at the western entrance of the NW Passage section in the CAA (Peterson 2008).

Fig. 5. Eight years of bi-monthly ocean velocity data observed at 10 m depth at the southern site in Lancaster Sound. Shown are bi-monthly vector velocity means (solid circles), standard deviations (bars), and the maximum east (squares) and maximum west (triangles) bi-hourly velocities for each bi-monthly section.

Fig. 5. Eight years of bi-monthly ocean velocity data observed at 10 m depth at the southern site in Lancaster Sound. Shown are bi-monthly vector velocity means (solid circles), standard deviations (bars), and the maximum east (squares) and maximum west (triangles) bi-hourly velocities for each bi-monthly section.

1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 Fig. 6. Same as Fig. 5 but for the northern site in Lancaster Sound.

Figures 5 and 6 show the bi-monthly ocean along-shore current data for the 8 year period. The bi-monthly mean velocities (solid dots) and magnitude of the standard deviation about the mean show that at both sites, the mean currents are lower and are less variable during the winter months than the summer months, when far-field and local wind forcing on the ocean surface can occur. In the summer months the mean currents and variability about the means are larger for the southern site and are setting eastwards to Baffin Bay. For the northern site, the summer means are small but generally setting to the west. The maximum velocities are similarly largest for the southern site, occur during the open water period and are in the direction of Baffin Bay. The maximum velocities towards the west are less as they oppose the mean currents flowing to Baffin Bay. In contrast, the largest summer maximum velocities along the northern shore are part of the buoyancy coastal currents directed towards the west.

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