Ocean circulation drifts in multi-millennial climate simulations: the role of salinity corrections and climate feedbacks
Ocean circulation drifts in multi-millennial climate simulations: the role of salinity corrections and climate feedbacks
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DOI：
10.1007/s00382-018-4243-y
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发表时间：
2019-02
期刊：
影响因子：
4.6
通讯作者：
J. Dentith;R. Ivanović;L. Gregoire;J. Tindall;Robin S. Smith
中科院分区：
文献类型：
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作者：
J. Dentith;R. Ivanović;L. Gregoire;J. Tindall;Robin S. Smith
低分辨率、复杂的大气环流模式（GCM）是研究千年时间尺度上的地球系统的宝贵工具。然而，缓慢演变的盐度漂移可能导致数千年来气候和海洋状况的巨大变化。我们测试了两种不同的方案，用于中和非强迫盐度漂移的FAMOUS GCM：表面通量校正和体积通量校正。虽然这两种方法都成功地维持了稳定的全球平均盐度，但局部漂移和随后的反馈促进了北大西洋的冷却（6000年内为1.4 °C）和淡化（6000年内为1.2 psu），以及北太平洋的逐渐变暖（每千年1.02 °C）和盐化（每千年1.015 psu）。这些区域表面密度的变化影响了纬向翻转环流（MOC），因此，几千年后，大西洋MOC（AMOC）处于崩溃状态，并且存在强大的深太平洋MOC（PMOC）。此外，AMOC表现出一段亚稳态，这是唯一的运行长度超过1500年的识别。我们还比较了模拟与两种不同的陆面方案，表明在地表气候的小偏差可能会导致区域盐度漂移和显着的转变，在MOC（削弱的AMOC和启动，然后振兴的PMOC），即使全球水文循环已被强制关闭。虽然没有模拟AMOC崩溃的具体前兆，但北太平洋西北部和北大西洋东北部是应密切监测此类偏差所产生趋势的重要区域。
Low-resolution, complex general circulation models (GCMs) are valuable tools for studying the Earth system on multi-millennial timescales. However, slowly evolving salinity drifts can cause large shifts in climatic and oceanic regimes over thousands of years. We test two different schemes for neutralising unforced salinity drifts in the FAMOUS GCM: surface flux correction and volumetric flux correction. Although both methods successfully maintain a steady global mean salinity, local drifts and subsequent feedbacks promote cooling (≈ 4 °C over 6000 years) and freshening (≈ 2 psu over 6000 years) in the North Atlantic Ocean, and gradual warming (≈ 0.2 °C per millennium) and salinification (≈ 0.15 psu per millennium) in the North Pacific Ocean. Changes in the surface density in these regions affect the meridional overturning circulation (MOC), such that, after several millennia, the Atlantic MOC (AMOC) is in a collapsed state, and there is a strong, deep Pacific MOC (PMOC). Furthermore, the AMOC exhibits a period of metastability, which is only identifiable with run lengths in excess of 1500 years. We also compare simulations with two different land surface schemes, demonstrating that small biases in the surface climate may cause regional salinity drifts and significant shifts in the MOC (weakening of the AMOC and the initiation then invigoration of PMOC), even when the global hydrological cycle has been forcibly closed. Although there is no specific precursor to the simulated AMOC collapse, the northwest North Pacific and northeast North Atlantic are important areas that should be closely monitored for trends arising from such biases.