Background climate dependence of Atlantic Meridional Overturning Circulation responding to precessional change
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摘要: 大西洋经向翻转环流(Atlantic Meridional Overturning Circulation,AMOC)是气候系统重要的组成部分,其强度变化可直接影响南北半球的热量分配,厘清其变化机理对全球变暖背景下的未来预估至关重要。海洋沉积物记录发现,在晚更新世,AMOC的变化与地球岁差周期有紧密联系,但其物理机理尚不清楚。本文利用COSMOS模型,通过敏感试验,分析在冰盛期冷期和间冰期暖期气候背景下,AMOC对地球岁差变化的响应机理。结果表明:岁差降低引起的北半球夏季太阳辐射增强,会导致间冰期暖期背景下的AMOC显著减弱,但对冰盛期AMOC的影响并不明显。通过进一步分析发现,在间冰期暖期,夏季太阳辐射增强,造成高低纬大西洋海表的升温,同时促进北大西洋高纬度地区的局地降水,两者导致北大西洋表层海水密度降低,共同削弱大西洋深层水生成。而在冰盛期冷期,大西洋高低纬度地区的响应对AMOC的影响反向——副热带升温触发的海盆尺度低压异常,通过其南侧的西风异常削弱大西洋向太平洋的水汽输送,导致净降水增多,海表盐度下降;同时,高纬度升温造成的海冰减少,促进了海洋热丧失,海表失热变重,有利于大西洋深层水的生成,最终两者的共同作用导致AMOC对岁差变化的响应偏弱。本文系统揭示了不同气候背景下,岁差尺度AMOC变化的控制机理,对理解晚更新世AMOC重建记录中持续存在的岁差周期具有重要启示意义。Abstract: The Atlantic Meridional Overturning Circulation (AMOC) is an important component of the climate system, of which change in the strength can affect meridional heat distribution between the northern and southern hemisphere. Proxy records show that changes in Atlantic ocean circulation during the Late Pleistocene is associated with precessional cycle (~20 kyr), but its physical mechanism remains unclear. Here we use a fully coupled climate model to investigate dynamics associated with AMOC changes in precessional band under glacial-interglacial climate conditions. Our results show that increase in boreal summer insolation can effectively weaken the AMOC during warm interglacial periods, while this weakening effect is reduced under glacial maximum. We further demonstrate that during the warm interglacial period increase in boreal summer insolation leads to sea surface warming and subpolar rainfall increase in North Atlantic, which jointly reduces sea surface density and hence the strength of deep water formation. During the glacial maximum period, climate responses to precessional change is of anti-phase impacts on the AMOC. At the low latitudes, a low pressure anomaly triggered by subtropical warming weakens atmospheric moisture export from the subtropical Atlantic to Pacific, increasing in net precipitation and hence freshening tropical sea surface in the North Atlantic. At the high latitudes, the warming-induced sea ice retreat promotes ocean heat loss via the enlarged ice-free area, and hence tends to strengthen the vertical mixing. The combined effects of low- and high-latitude responses finally leads to a trivial weakening of the AMOC. Overall, our results provide a systematic understanding of governing mechanism for precessionally-induced AMOC change under glacial-interglacial climatic backgrounds, shedding light on our interpretation of precessional periodicity in reconstructed ocean circulation changes during the Pleistocene.
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Key words:
- AMOC /
- Precessional cycle /
- Tropical hydroclimate /
- Climate background dependence
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图 3 PI和LGM背景下不同气候要素的Pmin-Pmax差异场 ,a) 填色图是PI时期夏季海表温度的差异场,叠加PI时期夏季海表面气压的差异场;b)同a,是LGM时期;c)填色图是PI时期夏季海表净降水的差异场,矢量图是PI时期整层水汽输送的差异场;d)同c,是LGM时期
Fig. 3 Climate response to changes in precession under PI (a, c) and LGM (b, d) backgrounds. a) shaded is summer sea surface temperature anomaly, and contour represents summer sea surface pressure; b) same as a, but for LGM period; c) shaded is the summer sea surface salinity anomaly, and vector is vertical integrated water vapor transport; d) The same as C, is LGM period
图 6 PI和LGM时期,Pmin与Pmax的夏季海冰密集度和冬季垂直混合层深度的差异场。 a)PI时期夏季海冰的差异;b)LGM时期夏季海冰的差异场;c)PI时期冬季垂直混合的差异场;d)LGM时期冬季垂直混合的差异场。绿线和红线分别是对应Pmax和Pmin时期15%海冰密集度分界线。
Fig. 6 Anomalous fields of summer sea ice concentration and winter vertical mixing layer depth between Pmin and Pmax under PI and LGM conditions. a) the difference of sea ice concentration in summer in PI; b) The difference field of summer sea ice in LGM period; c) The difference field of vertical mixing in winter in PI period; d) Difference field of winter vertical mixing in LGM period. , Green and red lines represent 15% sea ice concentration in Pmax and Pmin, respectively.
表 1 具体实验设置
Tab. 1 specific experimental settings
实验
名称CO2
(ppm)CH4
(ppb)N2O
(ppb)偏心率 倾角
(°)岁差
(°)全球冰量
(e.s.l.)ORB001 280 760 270 0.04 23.446 90 0 ORB002 280 760 270 0.04 23.446 270 0 ORB01lgm 185 350 200 0.04 24.5 90 116 ORB02lgm 185 350 200 0.04 24.5 270 116 S1 经过90W,75W,6N,14N这一区域的水汽通量,对水汽输送情况定量化,可以看到,PI和LGM时期,水汽输送减少相当
S1 Quantifying the water vapor flux in 90W, 75W, 6N and 14N, water vapor transport decrease considerably in PI and LGM periods
年均 春季 夏季 秋季 冬季 夏季(5−9月) ORB001 uq −126.76 −182.286 −144.391 8.16864 −188.532 −113.281 vq −9.96022 −26.442 30.3474 12.409 −56.1552 30.7718 Qspd 127.151 184.194 147.545 14.8563 196.717 117.386 ORB002 uq −175.303 −195.216 −233.976 −73.5462 −198.475 −200.19 vq −13.9999 −32.7454 31.0561 16.0139 −70.3243 29.0856 Qspd 175.861 197.943 236.028 75.2694 210.565 202.292 ORB01lgm uq −92.5443 −159.29 −49.1301 −15.8694 −145.887 −59.1867 vq −11.8552 −17.5885 27.6077 −12.4783 −44.9616 22.9494 Qspd 93.3006 160.258 56.3556 20.1877 152.659 63.4803 ORB02lgm uq −142.635 −176.768 −165.993 −58.9051 −168.875 −146.019 vq −17.2861 −27.6586 22.6438 −1.23924 −62.8904 20.189 Qspd 143.679 178.919 167.53 58.9181 180.205 147.408 -
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