**4. Linear trends**

When analyzing long-term data series, a question arises – are there unidirectional tendencies of one or another parameter in the specified period of time? This is especially important in the context of global warming, the most noticeable results of which are an increase in winter air temperature in the Arctic latitudes and a decrease in ice cover (area of the water area covered with ice) both in the Arctic Ocean and in the Sea of Okhotsk.

In order to answer this question, in each spatial cell of the 1000 × 1022 matrix, covering the investigated water area (42–60°N and 135–163°E), the linear trend coefficients were calculated using the least squares method, meaning the temperature increase over the year … The calculation was carried out on a 20-year series of data for each month separately, for annual averages, as well as for average temperatures for the season. The seasons for the calculations are shifted by a month relative to the calendar ones due to the specifics of the region: January–March (winter), April–June (spring), July–September (summer) and October–December (autumn).

The linear trend coefficients calculated from the full annual series in the study region are mostly negative, and there are no regions with positive values in the Sea of Okhotsk (**Figure 6** [9]). Significant cooling of the surface layer (about 1.5°C over 10 years) was observed in the northern and western parts of the sea; most clearly this process is manifested in the northern part of the sea at some distance from the coast. A less pronounced decrease (at a rate of about 0.5°C over 10 years) is observed in the Kuril Islands and in the band along the western coast of Kamchatka beyond the shelf zone, as well as in the influence zone of the Amur River outflow (Amur Estuary, Sakhalin Bay, northern shelf of Sakhalin Island).

A significant decrease (more than 0.5°C over 10 years) was found in the northern part of the Strait of Tatary; further southward, the cooling rate of surface waters decreases and, south of 45° S, a slight increase in the temperature of the surface waters in the Sea of Japan was recorded. The warming process is more pronounced in the northwestern part of the Pacific Ocean, i.e., in the southeastern part of the study region.

Let us now consider the results of calculating the parameters of the linear trend in different seasons. In winter, there is not enough data for a reliable calculation in the northwestern part of the sea (the "Sea of Okhotsk fridge"), on the northern shelf of the sea, and on the northeastern shelf of Sakhalin. In many regions of the basin, where the ice cover is significantly influenced, the calculation is not reliable enough; nevertheless, some ideas of the trends in the thermal regime during the cold period can be put forward.

In the entire Sea of Okhotsk, especially in the northern and western parts of the sea, as well as in the Strait of Tatary of the Sea of Japan and in the regions of the Pacific Ocean adjacent to the Kuril Islands, there is a tendency for the temperature decrease in the surface layer. Signs of the opposite trend are recorded only in the southeastern part of the study region, in the Pacific Ocean, in the northern part of the Sea of Japan, and in the very southern part of the Strait of Tatary (south of 47° N).

The most pronounced decrease in the surface layer temperature is in the springtime. It covers the entire study region, including the northwestern part of the Pacific Ocean, and it is particularly significant in the northern and western parts of the Sea of Okhotsk (the rates of temperature decrease range from 1.0 to 1.5°C over 10 years and even higher in some regions).

The cooling of the surface layer in spring is the most evident consequence of a decrease in ice cover, since in the absence of the ice cover the cooling processes

**Figure 6.**

*Distribution of linear trend coefficients by number of average annual (above) and average seasonal values. The scale corresponds to degrees celsius over 10 years [9].*

cover a layer of water thicker than in the presence of the ice cover. Undoubtedly, this has a significant impact on the climate of the region and, above all, on the weather conditions of Sakhalin, which are the most affected by this process.

While calculating the trends in sea surface temperature in individual months, it was found that the most intense cooling is observed in the month of May, especially in the northwestern part of the Sea of Okhotsk and along the entire eastern coast of Sakhalin Island from Cape Elizabeth in the north to Cape Aniva in the south. Exceptions are the western part of the Amur Estuary, which accounts for the bulk of the Amur flood, and the basin adjacent to Tauiskaya Bay. It is obvious that the hydrological regime of coastal areas experiencing the influence of the river outflow differs from that typical for the Sea of Okhotsk as a whole.

In summertime, the region is dominated by an increase in the sea surface temperature most pronounced in the northwestern part of the Pacific Ocean. In the Sea of Okhotsk, it is noted in its northwestern part, in the region of the Shantar Islands,

#### *Analysis of Spatiotemporal Variability of Surface Temperature of Okhotsk Sea and Adjacent… DOI: http://dx.doi.org/10.5772/intechopen.94918*

in Sakhalin Bay, and in the Amur Estuary, to the east of the northern tip of Sakhalin Island, at the northwestern coast of the Kamchatka Peninsula, and near the Urup and Simushir islands.

In the northern part of the sea, near the Kuril Islands, on the southeastern coast of Sakhalin Island and along the Primorye coast in the Sea of Japan, there is a trend towards a decrease in temperature, although it is more moderate than in springtime. Moreover, in July, the tendency towards a decrease in the sea surface temperature still prevails in the study region; the trend changes only in the northwestern part of the Pacific Ocean. In August, in these regions, warming reaches the highest rates (about 1.5°C over 10 years); the temperature increase was noted in the central part of the Sea of Okhotsk, along the western coast of Kamchatka and off the southeastern coast of Sakhalin, in the Strait of Tatary and the Amur Estuary. In the northern part of the sea, on the northeastern shelf of Sakhalin, and in the Kuril region, the trend towards a decrease in sea water temperature continues.

In September, on the contrary, the intensity of warming in the Pacific Ocean decreases; the greatest warming rate was noted in the northern and northwestern parts of the sea (especially in the shelf zone between Ayan and Okhotsk settlements).

A diffuse pattern is observed in the fall. Relatively small negative trends prevail in the greater part of the Sea of Okhotsk; positive trends (also insignificant) are noted in the eastern part of the sea on the western shelf of Kamchatka and in a relatively narrow band along the northern coast. Smaller areas were also found in the Shantar Islands and Kashevarov Banks and in the southwestern part of the analyzed basin of the Sea of Japan.

The highest rates of the temperature decrease in the surface layer are observed in the Strait of Tatary, in its northern part, and along the western coast of Sakhalin, as well as in the Sea of Okhotsk, in the northern part of it, beyond the shelf, off the southeastern coast of Sakhalin, and also in the band between 145° and 150° E from the Urup and Iturup islands to the Kashevarov Banks.

It was found in the calculations for individual months that a positive trend prevailed in October, and the trend reversed in November and December.

In the work [10], it was found that, in the Sea of Okhotsk and in the region of the Kuril Islands, negative trends were also noted in the spring months; the largest positive trends were recorded in October. Similarity results become very interesting if we take into account the differences in the studied time periods and spatial characteristics of the regions in which the trends were calculated. In the Sea of Japan, no negative trends were identified in this work.

As a result of the analysis of data on the surface temperature of the Sea of Okhotsk over a 20-year period (1998–2017), it was found that the global climate change in this basin caused a decrease in the ice cover and a decrease in the temperature of the upper water layer during the winter–spring period. The negative trends in temperature in the spring in the northern and western parts of the study region, as well as in the Strait of Tatary of the Sea of Japan (from 0.5° to 1.5°C over 10 years), are especially large. This effect in the reduction of the ice cover, both in time and in space, is the most evident and can be explained by an increase in the depth of winter convection. The predominance of a decrease in sea surface temperature, although less pronounced, was also noted in winter and autumn, and generally throughout the year.

In summer, the region is dominated by an increase in sea surface temperature, most pronounced in the northwestern part of the Pacific Ocean. In the Sea of Okhotsk, it was noted in its northwestern part, east of the northern tip of Sakhalin Island, off the northwestern coast of the Kamchatka Peninsula, as well as in some other regions. Moreover, in July, the main role belongs to the processes of cooling

of the surface layer; the change in the trend occurs in August and continues in September–October.

These processes play a significant role in the climate variations in the Sea of Okhotsk; in particular, a decrease in the temperature of sea water is noted in the coastal waters of Sakhalin Island. In addition to the weather conditions, the results are important for studying the habitat conditions of commercial fish species and invertebrates in the basin, which is of great fishery importance.
