Recent decadal warming and freshening of Antarctic-derived abyssal waters
Recent decadal warming and freshening of Antarctic-derived abyssal waters
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最近十年南极深海水域的变暖和淡水化
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发表时间：
2009
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通讯作者：
G. Johnson
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作者：
G. Johnson
起源于南极的沃茨在全球纬向翻转环流中起着重要作用（伦普金和Speer，2007年）。这些沃茨主宰着深渊，并占据了整个全球海洋的很大一部分（约翰逊，2008年）。在这里，我们审查的证据，最近十年变暖的这些舌状派生的深海沃茨周围的全球海洋，和最近的清新这些沃茨在一些盆地附近的源区。我们还试图评估这些变化对全球热量和海平面收支的潜在贡献。在1990年代的世界海洋环流实验（WOCE）期间，水文剖面大部分被占用，提供了一个高精度的近全球海洋水特性调查。这些部分的一个子集已重新占用在本十年的国际重复水文计划，以支持气候变化和二氧化碳的研究。对这些重复剖面的比较显示，在过去十年中，地球仪各地的深海沃茨在统计上显著变暖。在过去几十年里，在南极洲周围较有限的区域，利用重复剖面数据也观察到了深海淡水化。在南大洋的大西洋部分，近几十年来威德尔海的深水和底层沃茨一直在变暖（Fahrbach等人，2004年），但这一趋势在过去几年中似乎已部分逆转。重复水文剖面显示，在过去几十年里，南大西洋所有西部深水盆地中源自南极洲的深海沃茨已大幅升温（Coles等人，1996年;约翰逊和多尼，2006年），它们的特征似乎在北大西洋西部盆地也在减弱（约翰逊等人，2008年b）。在一些深海通道，如Vema海峡（Zenk和Morozov，2007年）和赤道大西洋（Andrie等人，2003年）也显示了过去几十年的变暖。自WOCE以来，印度洋尚未进行过广泛的重新调查，但澳大利亚-南极盆地和伊丽莎白公主海槽近几十年来都显示出显着的深海淡水化和变暖的迹象（Rintoul，2007年;约翰逊等人，2008年a）。然而，东南印度洋海岭以北的东印度洋盆地没有显示出这种变化（约翰逊等人，2008年a）。最近和即将进行的重复剖面应有助于探索澳大利亚-南极海盆以西和以北印度洋其余大部分地区的深海变化。在太平洋，从南极洲一直到阿留申群岛的太平洋大部分主要海盆，在垂直和横向相对均匀的深海层中，近几十年来都观察到了变暖现象（Kawasawa等人，2004年; Kawano等人，2005年）。约翰逊等人，2007年）。变暖的速度一般在南部较大，在北部较小，与两个已知的因素一致。第一，南极的深海沃茨从南部流入太平洋，在靠近源头的地方，其性质的变化可能最大。第二，横向温度梯度通常向北减小，因此作用于这些梯度的速度变化将在北部产生较小的温度异常。与印度洋类似，在最靠近南极洲的太平洋深海盆地，也有深海淡水化的迹象，这与这些沃茨在南极洲的一些来源区域的淡水化相一致（Jacobs，2004年）。水文剖面之间的距离很大，而且每隔十年才重新被占用，因此难以量化观测到的最近深海变暖对全球热量收支的贡献。由于同样的原因，难以量化观测到的变暖和淡化对全球海平面上升预算的贡献。然而，我们可以尝试量化每个部分的本地贡献。对于热收支，这些范围从沿着印度洋的澳大利亚南极海盆中底部深度超过3000米的部分施加的0.9 W m2（约翰逊等人，2008 a），沿着S.大西洋（约翰逊和多尼，2006年），到0.06 W m-2在西部的S。太平洋和0.01 W m-2的远N。Pacific（约翰逊等人，2007年）。这些数字可以与最近的全球上层海洋十年增热估计值0.6 W m-2（Willis等人，2004年）。然而，后一个数字是按地球表面积标准化的，但深海估计数是按当地增热计算的。同样，澳大利亚-南极海盆3000米以下的变暖和淡化导致该海盆最深处12年来局部海平面上升4厘米（约翰逊等人，2008年a），而自2003年以来全球平均海平面上升3.1毫米/年（Nerem等人，2006年）。其他地区的变化要小得多。尽管如此，这种定性分析表明，深海变化可能在全球热量和海平面上升预算中发挥一定作用。气候变化：全球风险、挑战和决策IOP出版IOP会议系列：地球与环境科学6（2009）032006 doi：10.1088/1755-1307/6/3/032006
Waters of Antarctic origin play a large role in the global meridional overturning circulation (Lumpkin and Speer, 2007). These waters dominate the abyss and ventilate a significant fraction of the entire global ocean (Johnson, 2008). Here we review evidence of recent decadal warming of these Antarctic-derived abyssal waters around much the global oceans, and recent freshening of these waters in some basins near their source regions. We also attempt to assess the potential contribution of these changes to global heat and sea level budgets. Hydrographic sections occupied mostly in the 1990’s during the World Ocean Circulation Experiment (WOCE) provided a high-accuracy near-global survey of ocean water properties. A subset of these sections has been reoccupied in the current decade by international repeat hydrography programs in support of CLIVAR and CO2 studies. Comparison of these repeat sections have revealed statistically significant warming in abyssal waters around various parts of the globe over the past decade. Abyssal freshening has also been observed with repeat section data in more limited regions around Antarctica over the past few decades. In the Atlantic sector of the Southern Ocean, deep and bottom waters have been warming over recent decades in the Weddell Sea (Fahrbach et al., 2004), although that trend appears to have been partially reversed in the last several years. Repeat hydrographic sections show that abyssal waters derived from Antarctica have warmed considerably over the last few decades in all the deep western basins of the South Atlantic (Coles et al., 1996; Johnson and Doney, 2006) and their signatures appear to be weakening in the western basins of the North Atlantic as well (Johnson et al., 2008b). More frequent bottom temperature data in a few deep passages such as the Vema Channel (Zenk and Morozov, 2007) and the equatorial Atlantic (Andrie et al., 2003) also show warming over the past few decades. The Indian Ocean has not yet been extensively resurveyed since WOCE, but the Australian-Antarctic Basin and the Princess Elizabeth Trough both shows signs of significant abyssal freshening and warming in recent decades (Rintoul, 2007; Johnson et al., 2008a). However, the eastern Indian Ocean Basins to the north of the Southeast Indian Ridge show no such changes (Johnson et al., 2008a). Recent and upcoming repeat sections should be useful in exploring for abyssal changes in much of the rest of the Indian Ocean west and north of the Australian-Antarctic Basin. In the Pacific Ocean, warming has been observed over recent decades in the relatively vertically and laterally homogenous abyssal layers throughout much of the main basins of the Pacific, from Antarctica all the way to the Aleutian Islands (Fukasawa et al, 2004; Kawano et al., Johnson et al., 2007). The rate of warming is generally larger in the south and smaller in the north, consistent with two known factors. First, abyssal waters of Antarctic feed into the Pacific from the south, and changes in properties might be expected to be largest near the source. Second, lateral temperature gradients generally decrease to the north, so velocity changes working on these gradients would produce smaller temperature anomalies in the north. Similarly to the Indian Ocean, in the deep Pacific basins closest to Antarctica, there are also indications of abyssal freshening, consistent with freshening in some of the Antarctic source regions for these waters (Jacobs 2004). The large distances between hydrographic sections, and the fact that they are reoccupied only from decade to decade makes quantification of the contribution of the observed recent abyssal warming to the global heat budget difficult. Quantification of the contribution of the warming and freshening observed to the global sea level rise budget is difficult for the same reasons. However, we can attempt to quantify the local contributions for each section. For the heat budget, these range from 0.9 W m2 applied along the portion of a section with bottom depths exceeding 3000 m in the Australian Antarctic Basin of the Indian Ocean (Johnson et al., 2008a), to 0.5 W m-2 along the western S. Atlantic (Johnson and Doney, 2006), to 0.06 W m-2 in the western S. Pacific and 0.01 W m-2 in the far N. Pacific (Johnson et al., 2007). These numbers can be compared with a recent global upper ocean decadal heat gain estimate of 0.6 W m-2 (Willis et al., 2004). However, the latter number is normalized to the surface area of the Earth, but the deep ocean estimates are made in terms of local heat gains. Similarly, the warming and freshening below 3000 m in the Australian-Antarctic Basin contributes to a local sea level rise of 4 cm over 12 years in the deepest portions of that basin (Johnson et al., 2008a), compared with a global average sea level rise of 3.1 mm yr-1 since 2003 (Nerem et al., 2006). Changes in other regions are much smaller. Nevertheless, this qualitative analysis suggests that abyssal changes may play some role in global heat and sea level rise budgets. Climate Change: Global Risks, Challenges and Decisions IOP Publishing IOP Conf. Series: Earth and Environmental Science 6 (2009) 032006 doi:10.1088/1755-1307/6/3/032006