Spatio-Temporal Analysis of Summer Salinity Fronts in the Greenland Sea

Using daily average data from the global ocean eddy resolution reanalysis product 25 (GLORYS12V1) from 1993 to 2018, sea surface salinity horizontal gradient is 26 calculated to obtain the spatio-temporal distribution and the intensity characteristics of 27 the salinity front in the Greenland Sea. Combined with the sea ice concentration data, 28 the salinity front and sea ice relationship is also studied. There is a significant spatial 29 relationship between the main position of the salinity front and the ice edge before the 30 sea ice shrinks toward the continental shelf. In the climatological seasonal variation, 31 the intensity of the salinity front beyond the continental shelf reaches its strongest 32 (~0.22 psu/km) in July. The front area beyond the continental shelf reaches its peak 33 value (~7.80 × 10 4 km 2 ) in August. The interannual variation of sea ice extent 34 averaged from July to August has a downward trend of 5.83 × 10 3 km 2 /year. Under the 35 background of rapid change in sea ice, the intensity and area of the salinity front 36 beyond the continental shelf are reducing at about 1.7 × 10 -3 psu/km and 142 km 2 37 /year, respectively. 38


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The Greenland Sea, one of the main marginal seas in the Arctic Ocean, is located to 42 the east of Greenland. Its shelf slope area is covered by sea ice all-year-round, and its 43 dynamic thermal environment is affected by sea ice formation and melting. The upper ocean front also moved fast and kept pace with the retreating ice edge, but the 63 deeper front was nearly stationary 7 . The ~3-day evolution of a density front located at 64 the ice edge in the Beaufort Sea was captured by an underway 65 conductivity-temperature-depth system. In the same period, the synthetic aperture 66 radar image also showed that the ice edge underwent concurrent evolution 11 .

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The formation and melting of sea ice play an important role in global climate 68 change 12,13,14 . The sea ice in the Greenland Sea has been changing rapidly in recent 69 years, and this change also affects various ocean processes, including ocean fronts 15,16 .

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The ocean front is considered to be an important factor in the energy transfer chain 71 from global-scale circulation to small-scale phenomena 17 . It can affect the structure of 72 the ocean flow field, ocean heat exchange, material transport and air-sea interaction 18 .

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In addition, various oceanic or atmospheric phenomena and processes are related to 74 the ocean front, such as high biological productivity, abnormal wind and waves, 75 dramatic changes in ocean color, strong vertical motion and local weather conditions.

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Large-scale fronts can influence the weather and even the climate 19 . Therefore, the 77 variation of fronts under the background of the rapid change in sea ice is an important 78 point for research.

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The horizontal gradient of the climatological sea surface salinity in the Nordic 80 Sea is not greater than 0.001 psu/km. Some researchers considered the spatial and 81 temporal variations of the oceanic front. They defined a salinity front threshold as 82 0.002 psu/km in the Nordic Sea to research the distribution characteristic of the 83 salinity front and its seasonal variation 2,20 . Since an oceanic front is a mesoscale 84 phenomenon, the position and intensity of a front cannot be accurately defined using 85 climatology and low-resolution data.

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In order to obtain the salinity front variation as accurately as possible, we use a 87 multi-year daily average reanalysis data to study the spatial distribution and intensity 88 characteristics of the salinity front beyond the continental shelf in the Greenland Sea 89 from July to August (JA). We also investigate the interannual variations of the salinity 90 front under the background of sea ice shrinking.

Results
As the sea ice melts in the Greenland Sea from June to August, the sea ice extent 94 shrinks toward the continental shelf, and the salinity front moves and keeps pace with   Fig. 4c), and reaches its minimum (7.58×10 4 km 2 ) in September.

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To explore the sea ice melting-induced changes in the Greenland Sea salinity 156 front, we use two indices, the front area and the averaged front intensity. The front 157 area is defined as the area occupied by a high salinity gradient greater than a threshold.

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The averaged front intensity is defined as the average value of the high salinity 159 gradient greater than a threshold. In the analysis, this threshold is set to 0.1 psu/km.

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In the climatological seasonal variation, the front area beyond the continental 161 shelf increases rapidly in May, reaches its peak value of 7.80×10 4 km 2 (solid blue 162 line in Fig. 4c) and then decreases. The July-August surface front area is larger than in 163 June and September (figure not shown). This is due to the rapid rate of sea ice melting 164 in July and August, which results in a large influx of fresh water, thus inducing strong 165 salinity gradients and increasing the frontal area.

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The seasonal intensity of the salinity front beyond the continental shelf (Fig. 4c) 173 increases rapidly in May. It reaches its strongest (0.22 psu/km) in July and then decreases until April of next year. The lower intensity in September is due to the sea ice formation, which weakens the salinity gradient. 176 The interannual variation of the average intensity of the salinity fronts beyond 177 the continental shelf in June and September increases year by year, and decreases in 178 July and August (not shown). However, the amplitude of the front intensity has an 179 overall decreasing trend of about 1.7×10 -3 psu/km (dotted orange line in Fig. 4d).   Methods. Some studies use the isobath of temperature, salinity or density to 241 determine the existence of a front 24,25,26 . This method is suitable for the frontal 242 analysis of profile data. In this paper, we use a front detection algorithm proposed by  First, we calculate the distance between every two grid points (equation (1)  where is the distance in direction between two grid points and is the 256 distance in direction. is the original salinity data. So, the gradient value G is 257 obtained by the square root of the gradient in the X and Y direction (equation (3)).

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The value G is used to determine the position and intensity of the salinity front.

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The intensity of the front varies in different seas. In this paper, we choose 0.1 psu/km 261 as the threshold value to define the salinity front in the Greenland Sea. In addition, we 262 use two indices, the front area and the mean intensity, to describe the salinity front.

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The front area of the front is calculated as follows (equation (4)): in which the part corresponding to greater than 0.1 psu/km is set to 1, and the rest 269 is set to 0. The average intensity of the front is defined as equation (5): The above front area calculation method is affected by the resolution of the data 272 grid, but the average intensity is not. In this paper, we use the average intensity and 273 front area to analyze the variation of the salinity front in the Greenland Sea.