We used a satellite-derived global daily sea surface temperature (SST) dataset with resolution 0.25 × 0.25∘ to analyze interannual changes in the Arctic Shelf seas from 2000 to 2020 and to reveal extreme events in SST distribution. Results show that the second decade of the 21st century for the Siberian Arctic seas turned significantly warmer than the first decade, and the increase in SST in the Arctic seas could be considered in terms of marine heatwaves. Analyzing the spatial distribution of heatwaves and their characteristics, we showed that from 2018 to 2020, the surface warming extended to the northern deep-water region of the Laptev Sea 75∘ to 81∘N. To reveal the most important forcing for the northward extension of the marine heatwaves, we used three-dimensional numerical modeling of the Arctic Ocean based on a sea-ice and ocean model forced by the NCEP/NCAR Reanalysis. The simulation of the Arctic Ocean variability from 2000 to 2020 showed marine heatwaves and their increasing intensity in the northern region of the Kara and Laptev seas, closely connected to the disappearance of ice cover. A series of numerical experiments on the sensitivity of the model showed that the main factors affecting the Arctic sea-ice loss and the formation of anomalous temperature north of the Siberian Arctic seas are equally the thermal and dynamic effects of the atmosphere. Numerical modeling allows us to examine the impact of other physical mechanisms as well. Among them were the state of the ocean and winter sea ice, the formation of fast ice polynias and riverine heat influx.

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A coupled ice-ocean model forced by atmospheric reanalysis data is used to examine a change in the bottom layer of the Laptev Sea on a scale of several decades. The model shows that since the mid-1980s there has been a warming of bottom waters in the shelf region. Analyzing values of bottom temperature averaged over a decade, we show that since 2005 the intensity of warming of the bottom layer has increased. The main reason for this is disappearance of ice cover in summer accompanied by an influx of heat into the surface layer of the sea. Also, an essential factor is the dynamic state of the atmosphere. The intensification of surface currents due to wind action promotes mixing of waters and heat transfer to the bottom layer of the sea. The heat anomalies entering the near-bottom layer of the sea during the autumn cooling could exist during the winter period.

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