Timescales of AMOC decline in response to fresh water forcing
Timescales of AMOC decline in response to fresh water forcing
复制标题
AMOC 因淡水强迫而下降的时间尺度
DOI：
10.1007/s00382-017-3957-6
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发表时间：
2018
期刊：
影响因子：
4.6
通讯作者：
R. Wood
中科院分区：
文献类型：
--
作者：
L. Jackson;R. Wood
由于温室气体变暖和北大西洋淡水输入的增加，预计大西洋纬向翻转环流（AMOC）在未来世纪将减弱，但AMOC减弱的数量和速度还存在很大的不确定性。了解是什么控制着AMOC减弱的速度和时间尺度，可能有助于减少这种不确定性，从而减少相关影响的不确定性。作为实现这一目标的第一步，我们考虑与响应于理想化的清新场景的弱化相关的时间尺度。在这里，我们探讨的AMOC减弱的时间尺度，在一套实验中与涡流允许的全球气候模式（GCM）的北大西洋的清新。当注入北大西洋的淡水速率较小时（0.1 Sv; 1 Sv = 1 ×106 m3/s），AMOC减弱的时间尺度主要取决于淡水输入速率本身，可能长于世纪。然而，当淡水添加率较大时（$$\ge$$≥ 0.3 Sv），时间尺度为几十年，并且对淡水输入的实际速率不敏感。这种不敏感性是因为随着淡水输入速率的增加，平流反馈在输出淡水异常方面变得更加重要，因此淡水化速率是相似的。我们发现平流反馈：出口的新鲜异常的平均流量;通过白令海峡进口量少;一个减弱的AMOC运输较少的副热带水北向和异常的副热带环流放大出口的新鲜异常。后一种环流变化是由副极地环流通过地转输出的新异常的存在所驱动的。这种反馈尚未确定在以前的模式研究，当新鲜的速度是强大的，它被发现占主导地位的总出口的新鲜异常，因此AMOC下降的时间尺度。虽然结果可能是依赖于模型，定性相似的机制也发现在一个单一的实验与不同的GCM。
The Atlantic meridional overturning circulation (AMOC) is predicted to weaken over the coming century due to warming from greenhouse gases and increased input of fresh water into the North Atlantic, however there is considerable uncertainty as to the amount and rate of AMOC weakening. Understanding what controls the rate and timescale of AMOC weakening may help to reduce this uncertainty and hence reduce the uncertainty surrounding associated impacts. As a first step towards this we consider the timescales associated with weakening in response to idealized freshening scenarios. Here we explore timescales of AMOC weakening in response to a freshening of the North Atlantic in a suite of experiments with an eddy-permitting global climate model (GCM). When the rate of fresh water added to the North Atlantic is small (0.1 Sv; 1 Sv $$=1\times 10^6$$=1×106 m$$^3$$3/s), the timescale of AMOC weakening depends mainly on the rate of fresh water input itself and can be longer than a century. When the rate of fresh water added is large ($$\ge$$≥ 0.3 Sv) however, the timescale is a few decades and is insensitive to the actual rate of fresh water input. This insensitivity is because with a greater rate of fresh water input the advective feedbacks become more important at exporting fresh anomalies, so the rate of freshening is similar. We find advective feedbacks from: an export of fresh anomalies by the mean flow; less volume import through the Bering Strait; a weakening AMOC transporting less subtropical water northwards; and anomalous subtropical circulations which amplify export of the fresh anomalies. This latter circulation change is driven itself by the presence of fresh anomalies exported from the subpolar gyre through geostrophy. This feedback has not been identified in previous model studies and when the rate of freshening is strong it is found to dominate the total export of fresh anomalies, and hence the timescale of AMOC decline. Although results may be model dependent, qualitatively similar mechanisms are also found in a single experiment with a different GCM.
影响因子：
18.3
作者：
Boening, Claus W.;Behrens, Erik;Bamber, Jonathan L.
通讯作者：
Bamber, Jonathan L.