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The Beaufort Gyre is a wind-driven ocean current located in the Arctic Ocean polar region. The gyre contains both ice and water. It accumulates fresh water by the process of melting the ice floating on the surface of the water.〔. Expanded Academic ASAP. Web. 13 Oct. 2009.〕 ==Dynamical Mechanisms== Conditions in the Arctic have favored sea ice loss in recent years during the Northern Hemisphere summers. At the end of the 20th century, analyses of increasing Pacific Surface Water temperatures led to the discovery of a connection between these rising temperatures and the onset of severe loss of Arctic sea ice in the Beaufort Sea. A reason for the existence of this link was proposed: “...delayed winter ice formation allows for more efficient coupling between the ocean and wind forcing.” These dynamical mechanisms are observed in the spin-up and circulation of the Beaufort Gyre. Housed in the western part of the Arctic Ocean is the Beaufort Gyre, whose growing reservoir of freshwater is shrouded in mystery. In recent years, this increasing freshwater content (FWC) has been the focal point of many studies, particularly those concerning coupled ocean-atmosphere dynamics. The majority of the Arctic’s freshwater content resides in the Beaufort Gyre. Although biased toward the Northern Hemisphere summer months, observations from submarines, ships, and stations on drifting ice suggest that the gyre has been expanding over the past two decades. Researchers have employed coupled sea-ice-ocean general circulation models in order to thoroughly analyze these observations. Model results show that Ekman transport plays an integral role in the variability of freshwater in the gyre, and thus in the Arctic Ocean. The prevailing rotational direction of the Beaufort Gyre is clockwise, following the prevailing wind circulation of the Polar High. Coriolis veers moving objects to the right in the northern hemisphere and 'to the right" is inwards in a clockwise rotating system. This is why anything floating, including fresher water tends to move toward the centre of the system. Indeed there is a slight bulge in the centre of the Beaufort gyre when it is rotating in its predominant clockwise direction. If as is speclated, as the Arctic ocean becomes a heat collector resulting in a low pressure, counter clockwise rotating system, the Beaufort Gyre can be expected to follow suit and send the fresher water outward to be captured by the transpolar current. This could well bring up the saltier, slightly warmer Atlantic water which lies under the floating, fresher Arctic water. Variations in the Ekman transport change the sea surface height and depth of the halocline, resulting in Ekman pumping. During anticyclonic regimes—where the wind stress curl is negative—freshwater is pumped into the Beaufort Gyre; during cyclonic regimes—where wind stress curl is positive—freshwater is released into the Arctic Ocean, where it can then flow into the North Atlantic. Giles et al. (2012) conclude that the variability in freshwater content varies with wind stress curl. The wind stress curl used by Giles et al. (2012) is from the NCEP/NCAR Reanalysis data at the National Oceanic & Atmospheric Administration, Earth System Research Laboratory, Physical Sciences Division (NOAA/OAR/ESRL PSD) in Boulder, Colorado, USA. See http://www.esrl.noaa.gov/psd/ for more details. The seasonal cycle of freshwater content does not only concern mechanical (Ekman pumping) processes, but thermal (ice formation) processes as well. The Beaufort Gyre contains a mean volume of 800 km^3 of frozen freshwater, or sea ice, based on a mean ice thickness of 2 meters. During the June–July months, the mean seasonal cycle of freshwater content peaks; in this season, sea ice thickness reaches a minimum, implying that the amount of melted sea ice has reached a maximum. The maximum in freshwater content released into the ocean waters coincides with a maximum in wind stress curl (i.e., a minimum in Ekman pumping), allowing for a high volume of freshwater to seep into the Arctic Ocean circulation. This rapid influx of freshwater into the Arctic circulation forces a large volume of freshwater to outflow into the North Atlantic basin, affecting the Atlantic Meridional Overturning Circulation. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Beaufort Gyre」の詳細全文を読む スポンサード リンク
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