Observations of shelf-basin exchange processes west of Spitsbergen and dense water outflow from the Storfjorden completed during IO PAS summer Arctic experiments.

 

As in previous years during the IO PAS Arctic experiment in summer 2008, a large number of CTD measurements were performed on the Spitsbergen shelf and slope area and in the deep basins connected with them. Among other CTD stations three sections located in the Storfjordrenna region and three towed CTD sections on the west Spitsbergen shelf-slope area (figure 1) were obtained as well.

 

 

 

 

 

 

 

 

Figure 1. Locations of towed CTD sections performed on the west Spitsbergen shelf area in summer 2008: three sections across the shelf near Hornsund and Bellsund fjords and three sections across Isfjord and Hornsund mouths.

 

 

 

 

 

 

 

 

 

 

 

Towed CTD profile across the west Spitsbergen shelf near entrance to the Bellsund Fjord shows interactions between the West Spitsbergen Current, Sørkapp Current and fjord outflow and intensive mixing processes (figure 2).

 

 

 

 

 

 

 

 

 

 

Figure 2. Temperature (θ), salinity and density (σθ) distribution on the section (marked as TSB section on the figure 1) across west Spitsbergen shelf near the Bellsund fjord in summer 2008. Warm and saline Atlantic water is mixing with Arctic type water, a product of ice melting and river runoff is visible as a light, fresh, surface layer.

 

 

 

 

 

 

 

 

 

 

 

This year dense water plume cascaded from the Storfjorden along the Strofjordrenna and shelf break was observed (figure 3) and, compared to the last years’ measurements, extreme values of main physical parameters were noted (figure 4). In core of the plume near bottom density reached 28.4 kgm-3, salinity 35.3 and potential temperature -1.77 ºC. Cross-section performed on the north bank of Storfjordrenna confirms that the pathway of this gravity current initially proceeds across the north bank of Storfjordrenna in accordance with the shape of isobaths and location of underwater canyons. Further, after overflow the shelf-break dense water plume cascades in a similar way as assumed by Killworth (2001) and comparably to ROMS simulation of Fer and Ådlandsvik (2008) – downward with small angle to isobaths.

 

 

 

 

 

 

 

 

 

Figure 3. Temperature (θ), salinity and density (σθ) distribution on the section along the Storfjordrenna in summer 2008. Dense water plume is visible as near bottom cascade moving down to the shelf-break as gravity current. Maximal values of physical paramets are equal to: -1.77 ºC (θ), 35.3 (S) and 28.4 kgm-3 (σθ). Above the most dense layer intensive mixing and entrainment processes occur, which forms plume shape.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 4. TS (θ-S) diagrams for CTD stations perfomed by IO PAS during last 8 summer experiments in Storfjordrenna area. (Water mass classification according to Skogseth at al., (2005): MW - Melt Water, NAW - North Atlantic Water, MAW - Modified Atlantic Water, SSW - Storfjorden Surface Water, PW - Polar Water, AW - Arctic Water, ESW - East Spitsbergen Water, BSW - Brine-enriched Shelf Water). Density (σθ) isolines drawed with step 0.25 kgm-3. In 2004 only outer part of Storfjordrenna was examined. In 2008 all parameters reched maximal values in the BSW bottom layer.

 

 

 

 

 

 

 

 

 

 

We recognized occurrence of dense water originating from the Storfjorden outflow on a few stations located on the slope west of Spitsbergen (figure 5). Increasing of near bottom density due to increased salinity and higher temperature as an effect of entrainment and mixing processes can be clearly visible on the TS diagrams. However, observations of dense water close to the bottom on the slope were made a few times during the last 8 years, similar specific increase of salinity was observed only in summer 2002. (figure 6). Appearance of this water mass indicates intensive dense water production presumably resulting from huge transport of warm and saline Atlantic water into the Barents Sea during last years. Furthermore, minimum ice extent in 2007 allowed for intensive new ice formation and brine rejection in the whole Barents Sea shelf area, especially in polynya regions, as for instance in the Storfjorden polynya.

 

 

 

 

 

 

 

 

 

Figure 5. West Spitsbergen shelf, slope and adjacent deep basins. Dense water cascades from Storfjorden along Storfjordrenna (green dots) and can be observed on the slope area (red dots) as denser bottom water. Observation of plume in summer 2008.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 6. TS (θ-S) diagrams for bottom layer of CTD stations perfomed in summer 2002 and 2008 west of Spitsbergen. Data avaraged once per 5 dbar. Red line indicates level of σ0.5 = 30.444, green line – level of σ1.5 = 35.142 and magenta line – level of σ.2 5 = 39.738 kgm-3. Increase of near bottom salinity (>34.915) and density (>σ1.5 = 35.142 kgm-3) was observed in those two years. Temperature (θ) is slightly higher as an effect of entrainment and mixing processes.

 

 

 

 

 

 

References:

I. Fer and B. Ådlandsvik: Descent and mixing of Storfjorden overflow., Ocean Science, 4, 115–132, 2008.

P. D. Killworth: On the rate of descent of outflows.,2001, Journal of Geophysical Research, 106, 22267-22275, 2001.

R. Skogseth, P. M. Haugan, M. Jakobsson: Watermass transformations in Storfjorden., 2005, Continental Shelf Research, 25, 667–695, 2005b.