4.3.1 Interpretations from model transports
Different roles of the straits in the Northern Barents Sea Opening
Table 3.3 of 12-year mean transports shows the strait between Nordaustlandet and Kvitøya determine the net mass transport through the NBSO even though mass exchange is much larger in the Franz Victoria Trough. The reason is the highly balanced mass transport in and out of Franz Victoria Trough, also seen in the interannual mass transport variation in Figure 3.12. Since the outflow in Franz Victoria Trough is cold and the inflow is warm, this water mass exchange dominate the heat transport through the NBSO.
Correspondence of Svalbard branch variation and inflow to the northern Bar-ents Sea
Figure 4.9 shows deviations from the 12-year mean of model mass and heat transport eastward north of Nordaustlandet (Section A1) and southward between Svalbard and Franz Josef Land (Section B; NBSO). The transports are 365 days running means (Hanning window), and the deviations were normalized in order to compare their variations. Section B transports were multiplied by−1 for easier comparisons so that positive deviations in Figure 4.9 resemble larger than average transport.
The eastward mass transport north of Nordaustlandet (black filled line in Figure 4.9) was larger than average from autumn 1992 to the summer of 1995 and from the end of 1998 to spring 2000. This intensified mass flow was accompanied by increased heat transport, especially in the period 1992-1995. A correlation coefficient of 0.8 significant on a 1%
level was found for the interannual variation of eastward mass and heat transport north of Nordaustlandet, meaning 64% of the interannual variation can be explained by this correlation.
It is reasonable to assume intensified mass flow in the Svalbard branch lead to increased inflow of AW through the NBSO. Figure 4.9 shows inflow increased the same periods as the Svalbard branch intensified, however with a time lag, reflected in the increased correlation coefficient from 0.2 to 0.7 when the mass inflow through the NBSO was shifted one year.
Hence, the one year time lagged significant correlation of 0.7 can explain nearly 50% of the interannual variation of mass inflow to the northern Barents Sea.
Comparisons of yearly mean model hydrography fields at each depth (not shown) indicated changes in the Svalbard branch first occurred in upper layers, then propagated to deeper layers the next two to three years. Hence, these analyses may indicate the intermediate or deeper layer mass transport interannual variation in the Svalbard branch is important for interannual variation of mass inflow through the NBSO, and induces a time lag of approximately one year in the correlation.
Figure 4.9 also shows the interannual variation of heat transport southwards through the NBSO follows the interannual variation of heat transport in the Svalbard branch to some extent. Here a significant correlation coefficient of 0.6 was found, meaning 37% of the
1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002
Pos mass transport Section A1 Pos heat transport Section A1 Neg mass transport Section B Neg heat transport Section B
Figure 4.9: Deviations from 12-year mean of model mass and heat transport eastward (positive) in Section A1 north of Nordaustlandet and southward (negative) in Section B Svalbard-Franz Josef Land (the NBSO). Black lines are for Section A1; gray lines for Section B. Filled lines are mass transport; dashed lines are heat transport. 365 days running mean 1990-2001. Deviating transports are scaled in order to compare interannual variation. Section B transports are multiplied with−1 for easier comparisons; positive deviations are transports larger than the 12-year mean.
interannual variation of heat transport southward through the NBSO could be explained by this correlation.
This comparison indicate the Svalbard branch variation (Section A1) largely determine the interannual variation of mass and heat transport into the northern Barents Sea. In addition, local winds are likely to affect the inflow. Winters with intensified cold water formation in the northern Barents Sea could lead to increased outflow through the Franz Victoria Trough. The balanced mass flow in this trough might indicate this situation could be favourable for increased inflow.
Influence on northern Barents Sea hydrography: Case study 1995/1996.
The winter 1995/1996 was unusual in many respects. Kwok et al. (2005) showed the Barents Sea had especially high freshwater content and exceptionally high ice transport
Mean temperature 1996 200m depth
−1.8−1.7
N Mean salinity 1996 200m depth
34.7
Figure 4.10: 1996 mean model temperature (left figure) and salinity (right figure) at 200 m depth.
from the Arctic Ocean to the Barents Sea this winter. Additionally, Figure 4.9 shows ROMS gave a significant drop in mass and heat transport of the Svalbard branch in 1995.
The mentioned abnormalities of this winter could be related to a shift in the NAO index from a highly positive phase to a highly negative phase this winter. This change in NAO index means less westerly winds and reduced Norwegian Atlantic Current, hence colder air and less AW inflow to the Barents Sea.
Figure 4.10 shows the model temperature and salinity at 200 m depth in 1996. In comparison to the 12-year mean temperature and salinity at 200 m depth in Figure 3.7, it is clear ROMS give much higher salinities and slightly lower temperatures inside the northern Barents Sea at in 1996. Accordingly, this might indicate an increased dense water production in the northern Barents Sea this year. Figure 3.10 supports this conclusion as the model gives the bottom-intensified northward cold current on the eastern side of the Franz Victoria Trough increased velocity and extent in 1996. It is also seen the model gave an upper-level intensified southward current on the western side of the Franz Victoria Trough the same year.
Similar comparisons showed the model surface layer was much fresher and somewhat colder in 1996 than the 12-year mean (not shown), possibly caused by lower air temperature and less AW inflow in the upper layer (as discussed in Chapter 4.1.2 the model generally has AW inflow too high up in the water column, however not this year).
Sea ice transport through the Northern Barents Sea Opening
Kwok et al. (2005) investigated 10 years of satellite records 1994-2003 and found the direction of transport between the Arctic Ocean and the Barents Sea is controlled by the location of the mean atmospheric low in the Barents Sea. If the mean atmospheric low pressure is situated in the eastern Barents Sea and the winter Perennial Ice Zone is shifted southwards towards the northern Barents Sea, conditions are favourable for large inflows of multiyear ice into the northern Barents Sea. They find as many years with ice export from the Arctic to the Barents as opposite directed ice transport, adding up to 40 km3 mean export from the Arctic through the NBSO with hugh variability ranging from -280 km3 to 340 km3. Kwok et al. (2005) also report a mean ice thickness of 2.5 m during a winter of net ice import to the Arctic, possibly an indication of thickness of first year ice in the northern Barents Sea.
Comparable water mass and flow structure in all openings of the Barents Sea The typical water mass distribution and flow field of straits in ROMS have similarities to that seen in the Franz Victoria Trough in Figure 3.9. Warm water enters the Barents Sea on one side and cold water leaves the Barents Sea on the other side. The outflow typically has an intensified bottom boundary current with CBW, as observed in the BSX in 1991-1992 (Gammelsrød et al., 2008) and modelled by ROMS in the Franz Victoria Trough. The geographical orientation of this simplified model of the Barents Sea straits is so that the cold flow is to the right of warm inflow, seen from inside the Barents Sea. This simplification of the water mass exchange between the Barents Sea and its surrounding oceans highlights the Barents Sea as a heat sink and modifier of AW.