The Changing Arctic Ocean: Focusing on Pacific Summer Water
Rangkaan topik
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The Arctic is considered to be one of the oceans most affected by climate change, and I believe that a better understanding of sea ice and temperature fluctuations in the Arctic ocean is important for the future of climate change countermeasures. Through this research, I would like to contribute to SDG 13 (CLIMATE ACTION).
The United Nations has designated the decade starting in 2021 as the "Decade of Ocean Science," with the aim of contributing to SDGs. Ocean science, as defined by the UN, includes the field of fisheries.
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Arctic sea ice has declined significantly in recent decades. For example, the area of sea ice on September 15 averaged 7.3 million square kilometers in the 1980s, but decreased by more than half to 3.6 million square kilometers by 2020. This decline in sea ice has occurred mainly in the Pacific sector of the Arctic Ocean, and one of its causes is an increase in northward heat transport by Pacific Summer Water. The relatively warm Pacific Summer Water is transported from the Bering Sea, which is the marginal sea of the Pacific Ocean, to the Chukchi Sea, which is the marginal sea of the Arctic Ocean, through the Bering Strait from spring to autumn. Then, it is further transported to the subsurface of the Canada Basin in the Arctic Ocean. It was pointed out that this affects the surface water temperature of the Canada Basin in the Arctic Ocean, contributing to the decline in sea ice in the Pacific sector of the Arctic Ocean (for example, Shimada et al., 2006).
Schematic diagram of the flow of the Pacific sector of the Arctic Ocean. Modified figure from Tsukada, Ueno et al. Polar Science (2018).
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After passing through the Bering Strait, the Pacific Summer Water is not transported directly to the Arctic Ocean basin (a sea area with a depth of several thousand meters), but is heated by sunlight in the Chukchi Sea, which is a shelf area. This heating, called solar heating, is large, not only in the south where the solar radiation is strong, but also near the coast of mainland Alaska, which is the main northward route of the Pacific Summer Water. This is likely because the albedo became small as a result of the decline in sea ice in the Chukchi Sea, making it easier for heat to enter the ocean.
Albedo is the ratio of reflected light to incident light. When the ocean is covered with ice, much of the sunlight is reflected, and the albedo is close to 1. When the ocean is not covered with ice, most of the sunlight is absorbed by the ocean, making the value of albedo 0.1 or lower.
Horizontal distribution of cumulative solar heating from May to September in the Chukchi Sea averaged over 1999–2015 (108 J m-2).
Excerpted from Tsukada, Ueno et al., Polar Science (2018).
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The volume of Pacific Water flowing northward in the Bering Strait is increasing, with an annual average of 0.7 ×106 m3 s-1 in 2001 and 1.2 ×106 m3 s-1 in 2014 (Woodgate, PiO, 2018), and the northward heat flux through the Bering Strait is also increasing (the purple line in the figure below). This increase in the northward heat flux is thought to result in reduced sea ice in the Chukchi Sea, promoting solar heating. Therefore, we calculated the cumulative solar heating throughout the Chukchi Sea from May to September between 1999 and 2015, and found its increasing trend (the red line in the figure below).
Solar heating integrated over the Chukchi Sea in summer varied interannually from 3.6 × 1020 J in 2000 to 6.7 × 1020 J in 2015, and was up to twice the northward heat flux through the Bering Strait. The total heating in the Chukchi Sea implies that the heat in the Chukchi Sea provided by northward heat flux through the Bering Strait is amplified by solar heating in the Chukchi Sea. We further compared these heat fluxes into the Chukchi Sea with the summertime northward heat flux through Barrow Canyon, an indicator of heat flux from the Chukchi Sea to the Arctic basin (no figure). The northward heat flux through Barrow Canyon was affected by the interannual variation of solar heating in the eastern Chukchi Sea. These results imply that modification of Pacific water in the Chukchi Sea by solar heating plays an important role in the interannual variation in heat transport from the Chukchi Sea to the western Arctic basin.
Interannual variation in solar heating (red), total heating (black), sea-ice melting heat (blue), and the sum of the sensible heat (SH), latent heat (LH) and longwave radiation (LW) fluxes (green) integrated in the Chukchi Sea from May to September (1020 J). Purple line indicates northward heat flux through the Bering Strait (BSHF) integrated from May to September. Modified figure from Tsukada, Ueno et al. Polar Science (2018).
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