Intra-interglacial variability: are warmer periods climatically more unstable?

随着当前高纬度变暖和北大西洋淡水输入的加速，预计大西洋子午线翻转环流(AMOC)将减弱，因此迫切需要考虑AMC在超温条件下的稳定性。虽然目前的监测不能跨越足够长的时间，但可以询问过去的间冰期，以获得对AMOC变化和随之而来的气候影响的洞察。一个突出的例子是末次间冰期(大致相当于海洋同位素5E亚期)，其特征是全球平均温度峰值比工业化前高出约0.8℃，北极剧烈变暖，全球海平面比现在高出约6-9米，其中0.6-3.5米来自格陵兰冰盖(GRIS)的融化。最近的一项研究(Tzedakis等人，2018年)表明，最后一次间冰期被北大西洋百年规模的冷水团扩张和南欧的干旱事件打断，并表明格陵兰冰川融化可能是导致AOC间歇性减弱的原因之一。相比之下，我们目前间冰期(全新世，MIS1)的气候变化相对较小。这提出了两个重要的问题：气候比全新世更温暖的时期是否更不稳定，这种变化是否与AMOC的变化有关？为了解决这些问题，我们建议检验一组“自然实验”：冷间冰期MIS7a-c和MIS7e，暖间冰期MIS9e，并将它们与MIS5e和MIS1进行比较。研究表明，格陵兰南部的气候变暖，MIS5e和MIS9e期间大范围的GRIS后退，MIS1和MIS7期间南部GRI的最小退缩。拟议的项目(VARING)将记录这些间冰期百年尺度气候变化的程度，并探索其起源。我们将根据下列站点网络构建一个广泛的时空框架：(1)北大西洋五个海平面和深水条件变化的记录；(2)来自意大利中部的长花粉序列，其具有独立的年代学，以火山灰为依据，证明北大西洋的多变性对植被和降水制度的影响；(3)葡萄牙边缘的‘Rosetta stone’深海序列，将同一核心的海洋和花粉记录连接起来；(4)北大西洋-南欧地层格网，具有间冰期变化的共同年代学框架。我们的研究有几个可能的结果：(I)没有间冰期内的变化，这对气候保持基本稳定的条件构成了限制；(Ii)只有表层-海洋的变化，导致向北的海洋热量输送减少，而不会削弱AMOC的深层。(3)表层和深海的变化，这意味着海洋和海洋生态系统的参与。通过限制AMOC分量，我们将更深入地了解海洋环流变化是如何发生的，以及它们对南欧下游的影响。Ving将超越Tzedakis等人的研究。(2018)，通过从具有不同气候背景的多个时间间隔的多个地点获得记录，以记录变化并探索机制。最终，它将有助于改变我们对一系列国家气候变异性的理解，并将为间冰期条件下AMOC的稳定性及其与南欧气候的联系提供亟需的约束。

With current high-latitude warming and accelerated freshwater input into the North Atlantic expected to weaken the Atlantic Meridional Overturning Circulation (AMOC), there is a pressing need to consider the stability of AMOC under conditions of 'excess warmth'. While present monitoring does not span a sufficiently long period, past interglacials can be interrogated to gain insights into AMOC changes and attendant climate impacts. A prominent example is the Last Interglacial (broadly equivalent to Marine Isotope sub-Stage [MIS] 5e), characterized by peak global mean temperature ~0.8degC above pre-industrial values, intense Arctic warming and global sea-level ~6-9 m above present, with 0.6-3.5 m derived from melting of the Greenland Ice Sheet (GrIS). A recent study (Tzedakis et al., 2018), has shown that the Last Interglacial was punctuated by centennial-scale cold water-mass expansions in the North Atlantic and arid events in S. Europe and suggested that Greenland ice-melt may have contributed to episodic AMOC weakening. By comparison, climate variability during our current interglacial (Holocene, MIS 1) has been relatively subdued. This raises two important questions: are periods warmer than the Holocene climatically more unstable and is this variability related to AMOC changes? To address these questions, we propose to examine a set of 'natural experiments': cool interglacials MIS 7a-c and 7e, and warm interglacial MIS 9e and compare them with MIS 5e and 1. Studies have indicated warmer conditions off southern Greenland and extensive GrIS retreat during MIS 5e and 9e, and minimal retreat of the southern GrIS during MIS 1 and 7. The proposed project (VARING) will document the extent of centennial-scale climate variability during these interglacials and explore its origin. We will construct a wide spatio-temporal framework from a network of sites: (1) five North Atlantic records of changes in ocean surface and deep-water conditions; (2) a long pollen sequence from central Italy with an independent chronology, based on dating of volcanic tephras, to provide evidence of the impact of North Atlantic variability on vegetation and precipitation regimes; (3) a 'rosetta stone' deep-sea sequence from the Portuguese Margin, linking marine and pollen records in the same core; (4) a North Atlantic - S. Europe stratigraphic lattice with a common chronological framework for the timing and duration of interglacial changes. There are several possible outcomes to our study: (i) no intra-interglacial variability, which places constraints on conditions under which climate remains largely stable; (ii) surface-ocean only variability, leading to less northward oceanic heat transport without a weakening of the deep limb of the AMOC. (iii) surface- and deep-ocean changes, which implies AMOC involvement. By constraining the AMOC components, we will gain greater insight into how ocean circulation changes were occurring and their downstream effects over S. Europe. VARING will go beyond the study of Tzedakis et al. (2018), by deriving records from multiple sites across multiple time intervals with different climate contexts to document changes and explore mechanisms. Ultimately, it will contribute to a step change in our understanding of climate variability across a range of states and will provide much needed constraints on the stability of AMOC under interglacial conditions and its linkage with S. European climate.