In August–September 2018, the area covered by warm water (above 4, 3 and 1°С at 50, 100 m and near the bottom respectively) was close to that in 2017 at 50 and 100 m, and 12% smaller at the bottom (Fig. 3.1.16). The area covered by cold water (below 0°С) was also close to that in 2017 at 50 m but 7 and 8% larger than in 2017 at 100 m and near the bottom respectively (Fig. 3.1.16). Since 2000, the area covered by cold bottom water was the largest in 2003 and rather small in 2007, 2008, 2012, 2016 and 2017; in 2016, it reached a record low value since 1965.
In the past decades, the area of Atlantic waters has increased in the Barents Sea, whereas the area of Arctic waters has decreased (Karsakov et al., 2018). In August–September 2018, the Atlantic waters (>3°C) covered a relatively large area that decreased compared to 2017; the Arctic waters (<0°C) still covered rather small area like they did in the previous year; and the area of Mixed waters (0°C<T<3°C) reached a record high value since 1965 (Fig. 3.1.17).
Figure 3.1.16. Areas covered by water with different temperatures at 50 m (upper panel), 100 m (middle panel) and near the bottom (lower panel) in the Barents Sea (71–79°N, 25–55°E) in August–September 2000–2018.
The transition zone between relatively warm and saline Atlantic Water and colder and less saline Arctic Water is commonly termed the Barents Sea Polar Front. Due to the steeper topography around the Spitsbergen Bank and Hopen Deep, the front is most pronounced in this area of the western Barents Sea. Using a grid of 30’ by 10’ in longitude and latitude, respectively, and defining the thermal frontal zone as the gradient in temperature exceeding 0.04 °C/km at 50 m depth during the August-September period, the extent index and center of gravity of the frontal zone around the Hopen Deep area are calculated. The extent index (defined as the number of grid nodes in which temperature gradients exceeded a threshold value of 0.04°C/km) ranged from 105 to 644 in 1960–2018, while the long-term (1960–2010) mean value is 393 (Figure 3.1.18). The index showed significant interannual variations in the 1960s and early 1970s. Then it was gradually increasing from the late 1980s and the early 2000s; thereafter it began to decrease to its minimum value in 2010. At the same time, the area of thermal frontal zones diminished five-fold between 2001 and 2010. After 2010, the index remained low, and only in 2011 and 2017 was it close to the average. Most notably, the center of gravity of the front has moved northeast, especially from the 1960s to the 1970s (by about 75 km), and again from the 2000s to the 2010s (Fig. 3.1.18). As a result, from the 1960s to the 2010s, the centroids of the Barents Sea thermal frontal zones shifted northeast by approximately 150 km. Moreover, the average temperature gradient in the frontal area has decreased since 2000 (not shown), from an average of 0.065 °C/km in the early 2000s to 0.055 °C/km in the 2010s.
Figure 3.1.18. The extent index (left panel) and decadal mean centroids (right panel) of the thermal frontal zones in the Barents Sea (73–78°N, 15–43°E) at 50 m depth in August–September (From Ivshin et al., 2018). The dashed line shows the long-term (1960–2010) mean value.