季节性海冰驱动的冰期北大西洋“电容器”效应

doi:10.13745/j.esf.sf.2021.9.57

地学前缘 ›› 2022, Vol. 29 ›› Issue (5): 334-341.DOI: 10.13745/j.esf.sf.2021.9.57

季节性海冰驱动的冰期北大西洋“电容器”效应

张旭¹^,²^,³(), 张炜晨¹(), 王振乾¹, 郑凯¹, 邓凤飞¹

1.兰州大学资源环境学院西部环境教育部重点实验室, 甘肃兰州 730000
2.中国科学院青藏高原研究所古生态与人类适应团队, 北京 100101
3.中国科学院青藏高原研究所青藏高原地球系统与资源环境国家重点实验室, 北京 100101

收稿日期:2021-08-02 修回日期:2021-09-20 出版日期:2022-09-25 发布日期:2022-08-24
作者简介:张旭(1986—),男,教授,博士生导师,从事古气候动力学与数值模拟研究工作。E-mail: xu.zhang@itpcas.ac.cn
张炜晨(1997—),女,硕士研究生,主要从事古气候模拟研究工作。E-mail: zhangwch19@lzu.edu.cn
基金资助:
国家自然科学基金项目(42075047);国家重点研发项目(2020YFA0608902)

The Glacial North Atlantic “Capacitor” effect controlled by seasonal sea ice change

ZHANG Xu¹^,²^,³(), ZHANG Weichen¹(), WANG Zhenqian¹, ZHENG Kai¹, DENG Fengfei¹

1. Key Laboratory of Western China's Environmental Systems (Ministry of Education), College of Earth and Environmental Science, Lanzhou University, Lanzhou 730000, China
2. Group of Alpine Paleoecology and Human Adaptation (ALPHA), Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China
3. State Key Laboratory of Tibetan Plateau Earth System, Resources and Environment (TPESRE), Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China

Received:2021-08-02 Revised:2021-09-20 Online:2022-09-25 Published:2022-08-24

摘要/Abstract

摘要：

第四纪冰期的千年尺度气候突变事件——Dansgaard-Oeschger Event (D-O事件),一直是古气候学领域关注的重点。近年来,数值模拟的研究发现,北大西洋副极地地区年际-年代际气候变率的振幅在D-O事件中的冰阶冷期远大于间冰阶暖期,这一现象为理解该区域海温代理指标的气候学意义提供了重要参考价值,但其动力机制尚不清晰。本文利用海气耦合气候模型(COSMOS),通过模拟氧同位素(MIS)3阶段的一个典型D-O事件过程,探讨了冰阶冷期北大西洋气候变率的放大机制。结果显示,北大西洋副极地海域的季节性海冰通过调控海气间热量交换,影响当地气候变率的幅度。冰阶期,热带暖水向北输送导致海洋次表层逐渐升温,削弱了表层-次表层海水的密度层结,有利于次表层暖水上涌,促进海冰融化及海表温度升高。这将激发出海平面气压的负异常,引起气旋式风切变,并通过Ekman抽吸作用加速表层-次表层海水的垂直混合,进一步促进次表层暖水的上涌。这一正反馈机制造成海洋次表层热量的迅速释放,海表温度快速升高。当次表层热量释放结束后,海表将无暖水补充,导致海表温度下降,海冰增多。该过程激发的海表气压正异常(即反气旋式风切变)将抑制垂直混合发生,促进次表层热量积累,为下一次放热过程提供条件。在间冰阶暖期,随着北大西洋季节性海冰消失,海气间热交换不再受海冰变化影响,海洋次表层与大气间的热交换始终处于准平衡态,气候变率的振幅显著下降。本研究结果显示,北大西洋季节性海冰的存在可以调控海洋次表层热量积累-释放的过程,产生“电容器”效应,这对理解冰期年际-年代际气候变率放大现象有重要启示意义。

关键词: 年际-年代际气候变率, 冰期气候变率发大现象, 冰海气相互作用, 千年气候事件

Abstract:

Millennium-scale climate fluctuations, known as Dansgaard-Oeschger (D-O) events during the last glacial cycle, have been a hot topic in paleoclimatology research. Previous modeling studies show that during the cold DO-stadials the amplitude of interannual-interdecadal climate variability in the subpolar North Atlantic is greater than during the warm DO-interstadials. This phenomenon provides valuable implications for interpreting temperature proxies during D-O events, yet its dynamic mechanism remains unclear. Here, via simulation of a typical D-O event during marine isotope stage 3 (MIS 3) using a fully coupled atmosphere-ocean general circulation model, we explore the mechanism behind the amplitude change in interannual-interdecadal climate variability. We find that subpolar seasonal sea-ice in the northern North Atlantic plays an important role by regulating atmosphere-ocean heat exchange. During stadials, sea subsurface gradually warms up due to the northward transport of tropical warm water masses, weakening the density stratification between sea surface and subsurface, which eventually leads to upwelling of the subsurface warm water masses, promoting sea ice melting and sea surface warming. In turn, the sea level pressure lowers and the associated cyclonic wind stress anomaly accelerates the vertical mixing by Ekman suction, which further promote local upwelling. This positive feedback mechanism results in the rapid release of sea subsurface heat and rapid sea surface warming. As the heat release gradually ceases, surface warming is deaccelerated and even paused, allowing sea ice to regrow. This weakens the low-pressure system and stimulates anti-cyclonic wind anomaly, which in turn reduces the vertical mixing. Consequently, sea temperature drops quickly, meanwhile heat starts accumulating in sea subsurface until the vertical stratification is broken again. During interstadials, with the disappearing of seasonal sea ice in the key convection sites of the North Atlantic, the atmosphere-ocean heat exchange is no longer affected by sea ice. This enables a quasi-static heat exchange between the sea subsurface layer and atmosphere. Our results suggest that the existence of seasonal sea ice effectively regulates sea subsurface heat accumulation and release (as termed sea-ice driven “capacitor” effect) in the subpolar North Atlantic, shining a light on our understanding of the glacial amplification of interannual-interdecadal climate variability.

Key words: interannual-decadal climate variability, glacial amplification, air-sea-sea ice interaction, Dansgaard-Oeschger Event

中图分类号:

张旭, 张炜晨, 王振乾, 郑凯, 邓凤飞. 季节性海冰驱动的冰期北大西洋“电容器”效应[J]. 地学前缘, 2022, 29(5): 334-341.

ZHANG Xu, ZHANG Weichen, WANG Zhenqian, ZHENG Kai, DENG Fengfei. The Glacial North Atlantic “Capacitor” effect controlled by seasonal sea ice change[J]. Earth Science Frontiers, 2022, 29(5): 334-341.

图/表 7

图1 北大西洋经向翻转环流(AMOC)指数的时间序列图,灰色线为原始数据,黑色线为30年滑动平均

Fig.1 Time series of Atlantic Meridional Overturning Circulation (AMOC) Index. Gray lines represent the original outputs, and black line is its 30-year running mean

图2 北大西洋北部不同要素标准差的空间分布 a—冰阶期间海表气温(SAT,℃) 标准差空间分布;b—冰阶期间海冰密集度(SIC,%)标准差空间分布;c—冰阶期间垂直混合层深度(ZMLD,m)标准差空间分布;d-f与a-c相同,但为间冰阶时期标准差空间分布图。

Fig.2 Spatial distribution of standard deviations of different variables in the northern North Atlantic

图3 北大西洋不同要素场在冰阶(模式年份1600—1800年)和间冰阶(2200—2400年)气候平均态的空间分布 a—冰阶期间SST(℃,填色)与海平面气压(hPa,等值线)场;b—冰阶期间海冰密集度,其中实线为90%浓度海冰线,虚线为15%浓度海冰线,两线之间范围代表季节性海冰区;c—冰阶期间垂直混合层深度;d-f与a-c相同,但为间冰阶。

Fig.3 Climatology annual mean of different climate variables in the northern North Atlantic during cold stadial (model year 1600-1800) and warm interstadial (model year 2200-2400) periods

图4 冰岛南部和北欧海南部各要素的时间序列 a—冰岛西南部海域表层海温 (SST,℃) ;b—冰岛西南部海域SAT(℃) ;c—冰岛西南部海域SIC (℃) ;d—冰岛西南部海域ZMLD (m) ;e—冰岛西南部海域次表层海水温度(250~450 m,℃)。其中灰色细线为模拟原始场,彩色粗线为30年滑动平均。f-j与a-e相同,但为北欧海域时间序列图,所选研究区域 (冰岛西南部:32°~25°W,55°~62°N;北欧海:8°W~8°E,65°~72°N) 。

Fig.4 Time series of climate variables in south of Iceland and southern Nordic Sea

图5 冰岛南部气温变化引起的年均合成差异场 a—f依次为SST (℃) 、SIC (%) 、ZMLD (m) 、次表层温度 (℃)、海表盐度 (SSS,psu)、SLP (Pa)的空间差异场,其中打点区域通过了95%的显著性检验。

Fig.5 Annual mean composite anomaly of different variables based on SAT variation in the south of Iceland during the cold stadial

图6 冰岛南部海域各个要素场在AMOC不同状态下的超前滞后相关图 a—AMOC突变前不同要素场的超前、滞后相关图,其中绿色为SST与SIC的超前、滞后相关,x轴正方向代表SST超前;黄色为SLP与SIC的超前、滞后相关,x轴正方向代表SLP超前;黑色为SubT与SIC的超前、滞后相关,x轴正方向代表SubT超前;蓝色为SubT与SST的超前、滞后相关,x轴正方向代表SubT超前;红色为SLP与SST之间的超前、滞后相关,x轴正方向为SLP超前。 b与a相同,但时间为AMOC突变后的300年。

Fig.6 Cross-correlations of climate variables in different AMOC states and histogram of SST and SIC variability of the cold stadial in south of Iceland

图7 冰阶期冰岛南部SST (a)和SIC (b) 变率的概率分布直方图

Fig.7 Histogram of (a) SST and (b) SIC variability during stadial in south of Iceland

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季节性海冰驱动的冰期北大西洋“电容器”效应

The Glacial North Atlantic “Capacitor” effect controlled by seasonal sea ice change

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