南極底層水と全球海洋子午面循環

• Main Author: 勝又, 勝郎
• Format: Article in Journal/Newspaper
• Language: Japanese
• Published: 低温科学第76巻編集委員会
• Subjects: 南極底層水; 海洋循環; 452; Antarctic; Indian; Pacific; Antarc*; Antarctica
• Online Access: http://hdl.handle.net/2115/70320; https://doi.org/10.14943/lowtemsci.76.1

南極沿岸のポリニヤで冬季に冷却された海水は高密度水として周囲の海水と混合しながら大陸斜面を下り南極底層水となる．低温・高溶存酸素で特徴づけられるこの水塊はインド洋・太平洋の底層を中心に世界の海水の大きな割合を占める．この南極底層水を含む海洋の循環は乱流が卓越し，数年程度の観測では熱や物質の輸送量を正確に見積もれていないことが分かってきた．数十年から百年程度平均化した循環はしばしばその南北の輸送を強調して子午面循環とよばれる．この子午面循環は複雑な三次元構造を持つ．この循環場は大気から風・熱・淡水による強制を受けるだけでなく潮汐による混合や地熱の影響を受けて流れている．これらの外力と海洋循環との関係はよく分かっていないが，南極底層水と周囲の海水との混合が全球の海洋循環に影響を与えている可能性が指摘されている． Antarctic Bottom Water (AABW) originates from dense shelf water mixing with ambient water masses as it flows down the shelf slope. Dens shelf water is mainly produced by winter brine rejection in polynyas along the Antarctica coast. Cold and highly oxygenated AABW occupies a significant part of world oceans, mainly in the bottom layers in the Indian and Pacific oceans. It is recently recognised that ocean currents including the flow of AABW are turbulent and measurement with a duration of a few years is not appropriate for accurate transport estimate. Even after averaging for decadal or centennial time scales, the global system of ocean currents has a complicated threedimensional structure. This circulation, often referred to as the meridional overturning circulation, is forced not only by wind, heat, and freshwater fluxes from the atmosphere but also gravitational tidal forces and geothermal heating. We do not fully understand quantitative relationships between these forcing and ocean circulation, but recent studies point out possible mechanisms where AABW mixing with ambient water masses can affect the global meridional overturning circulation.