Viktor Ivshin, Alexander Trofimov, Oleg Titov
Knipovich Polar Research Institute of Marine Fisheries and Oceanography, Murmansk, Russia Abstract
The paper presents the research on interannual variability of the Barents Sea thermal frontal zones.
The length index of the frontal zones and their mean temperature gradients at 50 m depth in August–September 1960–2017 were calculated for an area between 73–78°N, 15–30°E where the frontal zones are more evident. Thermal frontal zones were determined in areas where temperature gradients exceeded 0.04°C/km. Since the beginning of this century, the length index of the frontal zones has been decreasing and temperature gradients have been weakening. From the 1960s to the 2010s, decadal mean centroids of thermal frontal zones shifted northeast by 150 km.
Keywords: Barents Sea, Polar Front, thermal frontal zone, length index, temperature gradient, interannual variability, centroids
Introduction
The oceanographic conditions of the Barents Sea are largely determined by the interaction of Atlantic and Arctic waters that results in the occurrence of extended frontal zones (Agenorov, 1946;
Izhevsky, 1958; Johannessen, Foster, 1978; Ozhigin, 1989; Loeng, 1991; Ozhigin et al., 2016). The Polar Frontal Zone is the most evident of them and it separates warm and saline Atlantic waters from cold and fresh Arctic waters. The position of frontal zones in the Barents Sea is closely related to the bottom topography (Ozhigin, 1989; Loeng, 1991; Parsons et al., 1996; Lien, 2010). In the areaofthe Spitsbergen and Great Banks,the thermal front position is trapped to an isobathof250m (Harris et al., 1998; Morozov et al. 2017; Harris, 1996). The areas with the sharpest temperature gradients are about 3 km wide (Morozov et al. 2017), and the total length of the Polar Front in the Barents Sea is about 1 500 km (Vage, 2010).
Given the fact that the frontal zones exist in areas where waters of different origins with widely varying properties interact, the main characteristic that defines frontal zones among other phenomena in the ocean is a sharp horizontal gradient of one or several hydrophysical parameters in a particular sea or ocean area (Fedorov, 1983; Gruzinov, 1986; Ozhigin et al., 2016). Despite the fact that a lot of research papers on the study of the Barents Sea frontal zones have been published, unfortunately, there is no clear understanding of their spatial pattern and interannual variability.
Using instrumental observations collected over a long period, this paper attempts to analyze and make a quantitative assessment of the spatial-temporal variability of the Barents Sea thermal frontal zones, namely the frontal zones identified in the water temperature field, over the period from 1960 to 2017.
Material and methods
Oceanographic data from the PINRO database for August–September 1960–2017 were used in the paper. These months were chosen because almost the entire Barents Sea is ice-free during this season, and the international ecosystem survey carried out in these months covers the sea sufficiently with oceanographic stations. To analyze the variability of thermal frontal zones in the Barents Sea, we chose an area between coordinates of 73–78°N and 15–43°E where the zones can be obviously observed (Ozhigin, 1989; Ozhigin et al., 2016).
The extent of the thermal frontal zones was estimated using the length index (Titov et al., 2007a;
Titov et al., 2007b). To calculate the index for the selected area, water temperature fields were obtained at standard depths in grid nodes with spatial steps of 10' in latitude and 30' in longitude.
The temperature fields obtained were used to calculate horizontal temperature gradients in every grid node. The length index of thermal frontal zones was defined as the number of grid nodes where temperature gradients (GradT) exceeded a critical value of 0.04°C/km that is used to identify thermal frontal zones (Titov et al., 2007a). The mean temperature gradient characterizing the
“sharpness” of the frontal zones was calculated for those grid nodes as well.
To estimate the interannual variability in the position of thermal frontal zones, their geographical centroids were calculated taking into account weighting coefficients (horizontal temperature gradients). The centroids were calculated in ArcGis 10.2.2 for Desktop using the tool set “Spatial Statistics – Measuring Geographic Distribution – Mean Center” where the temperature gradient was used as a weight function.
Results and discussion
According to the above proposed method, the horizontal temperature gradients in the grid nodes and the length indices of thermal frontal zones within the study area in the Barents Sea (73–78°N and 15–43°E) were calculated. Table 1 shows that the largest area with high thermal gradients is observed at depths of 30–50 m in August–September, that corresponds to the previous studies (Ozhigin, 1989; Oziel et al. 2016). The seasonal cycle with a minimum in the winter months (February–April) is clearly visible at these depths. However, when studying seasonal variations, it is necessary to take into account the fact that different ice conditions occur in this area during the year: it is completely ice free in the summer months, partially ice free in winter, and as a result, the calculated fields and characteristics of the frontal zones will be different.
Table 1. Long-term (1960–2010) mean length indices of the thermal frontal zones in the Barents Sea at standard depths.
Month
Further analysis of the variability in the thermal frontal zones was carried out for a depth of 50 m and the period from August to September. Figure 1 shows the distribution of probability of significant temperature gradients (more than 0.04°C/km) at 50 m depth for 1960–2017. Apparently, in the period under review significant gradients were observed more frequently in the western and central parts of the Barents Sea. The areas with the highest occurrence of frontal zones (more than 65%) drove round Bear Island from the west and south, spreading further to the northeast. Figure 1 confirms that the area between 73–78°N and 15–43°E was chosen correctly to assess the variability in thermal frontal zones: the sites where significant temperature gradients are observed the most frequently are entirely located within the marked area.
The long-term (1981–2010) mean distribution of significant (more than 0.04°С/km) temperature gradients at 50 m in August–September (Figure 2) is almost identical to the distribution of
probability of such gradients (see Figure 1), while the areas with the highest values of both parameters coincide. The similarity in the distribution of these parameters indicates the quasi-stationarity of thermal frontal zones in the Barents Sea.
Figure 1. Probability (%) of relevant thermal frontal zones (GradT ≥ 0.04°С/km) in the Barents Sea at 50 m in August–
September 1960–2017.
The highest temperature gradients (more than 0.08°C/km) within the marked frontal zones occur west of Bear Island (see Figure 2). Compared to other areas, the increased sharpness of gradients in this area is most likely the result of their more accurate calculation due to the occurrence of standard oceanographic section “Bear Island – West” (along 74°30'N, 9°50'–18°30'E), where the density of standard stations is high. The sharpness of the frontal zone is also observed east of Bear and Hope Islands, and it is probably determined by the close interaction of warm Atlantic waters and cold Arctic waters.
The interannual variations in the length index of the thermal frontal zonesintheBarentsSeaat50m depth in August–September have a high amplitude (from 100 to 600) for the period under consideration, while the long-term (1960–2010) mean value is 393 (Figure 3). There were significant interannual variations in this index before the mid-1970s that was probably caused by the different density of oceanographic observations. The period since the early 1980s shall be considered as the most “stable” period in the nature of variations in the index. By that time, a rather clear pattern of making observations during the ecosystem survey had been developed which included almost a regular network of oceanographic stations that may have resulted in decreased interannual variations in the index. It should be mentioned that the index gradually increased between the late 1980s and the early 2000s, thereafter it began to decrease to the absolute minimum
2010. After 2010, the index remained low, and only in 2011 and 2017, it was close to the long-term average.
Figure 2. Long-term (1981–2010) mean temperature gradients in the Barents Sea at 50 m in August–September. Points indicate grid nodes.
Figure 3. Interannual variability of the length index of the thermal frontal zones in the Barents Sea at 50 m in August–
September. Dotted line shows a long-term (1960–2010) mean value.
The mean horizontal temperature gradient varied from 0.05 to 0.07°C/km in the marked frontal zones while the long-term (1960–2010) average was 0.061°C/km (Figure 4). Since the mid-1990s, its gradual decrease has been observed. In 2010, the index reached the absolute minimum, and after 2010, it remained at a low level (Table 2).
Figure 4. Interannual variability of the mean temperature gradient in the thermal frontal zones of the Barents Sea at 50 m in August–September. Dotted line shows a long-term (1960–2010) mean value.
Table 2. The length index of the thermal frontal zones in the Barents Sea and the mean horizontal temperature gradient (°C/km) in them at 50 m in August–September.
Year Index Gradient Year Index Gradient Year Index Gradient
1960 417 0.063 1980 288 0.061 2000 474 0.065
1961 1981 398 0.059 2001 565 0.064
1962 291 0.063 1982 469 0.066 2002 537 0.066
1963 418 0.066 1983 477 0.065 2003 481 0.064
1964 204 0.061 1984 460 0.060 2004 515 0.059
1965 277 0.063 1985 378 0.063 2005 442 0.061
1966 201 0.059 1986 434 0.057 2006 366 0.059
1967 490 0.065 1987 278 0.059 2007 311 0.057
1968 241 0.056 1988 468 0.060 2008 285 0.054
1969 424 0.064 1989 644 0.066 2009 118 0.052
1970 385 0.062 1990 575 0.066 2010 105 0.051
1971 385 0.061 1991 461 0.061 2011 437 0.056
1972 601 0.068 1992 375 0.059 2012 308 0.058
1973 223 0.058 1993 451 0.065 2013 256 0.055
1974 193 0.062 1994 497 0.069 2014 256 0.051
1975 263 0.055 1995 509 0.065 2015 267 0.053
1976 241 0.061 1996 529 0.067 2016 265 0.054
1977 229 0.056 1997 459 0.062 2017 385 0.057
1978 359 0.060 1998 494 0.060 2018
1979 393 0.057 1999 562 0.065 2019
Significant positive correlation was found between the length index and the mean temperature gradient of the Barents Sea frontal zones. The coefficient of determination was 0.52 (with series length n=58) (Figure 5). The plot shows the best relationship of these parameters (with less deviations from regression line) for the highest values. In years when frontal zones are widespread, the mean temperature gradient is usually rather large as well. The worse relationship is observed for
weak frontogenesis (the length index varies in the range 200–300), when the mean temperature gradient can almost equiprobable be either large or small.
Figure 5. Correlation between the length index and mean temperature gradient (°С/km) in the thermal frontal zones of the Barents Sea at 50 m in August–September 1960–2017.
In order to estimate spatial shifts in thermal frontal zones from year to year, their centroids were calculated using the ArcGIS software. Weight functions of centroids are employed with the temperature gradient in frontal zones set as “weight”. The centroids calculated for each year from 1960 to 2017 are spread from southwest to northeast and located between the Spitsbergen Bank and Hopen Trench (Figure 6). The distance between the two outermost centroids (1962 and 1974) is about 290 km. Most centroids are concentrated within the small area (75–76°N and 26–28°E), that indicates relative stationary character of thermal frontal zones in the Barents Sea. Figure 6 shows that centroids had tended to be located southwest in the 1960s and shifted northeast in the 2010s.
Patterns in centroid shifting are more precise if we consider decadal mean centroids (Figure 7). For instance, in the 1960s, the centroids of the Barents Sea thermal frontal zones were located in the extreme southwest. The fact that these years' observations were mainly conducted along standard sections may have resulted in such a location of centroids. In the 1970s, the centroids shifted northeast by approximately 75 km, and during the next 40 years (1970s, 1980s, 1990s and 2000s), they were quasistationary and shifted within a tiny area (75.5–75.7°N and 26.8–27.4°E). Since the early 2010s, the centroids continued to shift northeast. As a result, from the 1960s to the 2010s, the centroids of the Barents Sea thermal frontal zones shifted northeast by approximately 150 km.
L. Oziel et al. (2016) indicated that the front generally shifted northwards (from the 1970s to the 2010s) and they explained this shift by “atlantification” of the Barents Sea.
Conclusions
It was proved that thermal frontal zones of the Barents Sea are more evident in the 30–50 m layer in August–September. The length index of thermal frontal zones was used for quantitative assessment of their extent; the length index and mean temperature gradients in the zones were calculated for 1960–2017. It was noted that the length index of the thermal frontal zones had been decreasing since the early 2000s and their temperature gradients had been weakening; in 2010, the length index and the mean temperature gradient were record low since 1960. It was indicated that despite the quasistationary nature of the thermal frontal zones in the Barents Sea their decadal mean centroids shifted northeast by 150 km from the 1960s to the 2010s.
Figure 6. Centroids of the Barents Sea thermal frontal zones at 50 m in August–September 1960–2017.
Figure 7. Decadal mean centroids of the Barents Sea thermal frontal zones at 50 m in August–September.
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