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

当前高纬度地区的变暖和北大西洋淡水输入的加速预计将削弱大西洋经向翻转环流（AMOC），因此迫切需要考虑在“过热”条件下AMOC的稳定性。虽然目前的监测没有涵盖足够长的时间，但可以对过去的间冰期进行调查，以深入了解AMOC的变化和随之而来的气候影响。一个突出的例子是末次间冰期（大致相当于海洋同位素亚阶段[MIS] 5e），其特征是全球平均温度峰值比工业化前高0.8℃，北极强烈变暖，全球海平面比现在高6-9 m，其中0.6-3.5 m来自格陵兰冰盖（GrIS）的融化。最近的一项研究（Tzedakis et al., 2018）表明，北大西洋百年尺度的冷水团扩张和欧洲南部的干旱事件打断了末次间冰期，并表明格陵兰岛的冰融化可能导致了间歇性的AMOC减弱。相比之下，当前间冰期（全新世，MIS 1）的气候变率相对较低。这就提出了两个重要的问题：比全新世更温暖的时期气候是否更不稳定？这种变化是否与AMOC变化有关？为了解决这些问题，我们建议研究一组“自然实验”：冷间冰期MIS 7a-c和7e，暖间冰期MIS 9e，并将它们与MIS 5e和1进行比较。研究表明，格陵兰岛南部变暖，MIS 5e和9e期间GrIS大面积退缩，MIS 1和7期间GrIS南部退缩最小。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。我们将从一个站点网络构建一个广泛的时空框架：(1)北大西洋海洋表面和深水条件变化的五个记录；(2)意大利中部的长花粉序列，具有独立的年代学，基于火山泥的定年，为北大西洋变率对植被和降水制度的影响提供证据；(3)来自葡萄牙边缘的“罗塞塔石”深海序列，将同一岩心的海洋和花粉记录联系起来；(4)在间冰期变化的时间和持续时间方面具有共同的年代框架的北大西洋-欧洲南部地层格。我们的研究有几个可能的结果：(i)间冰期内没有变率，这限制了气候保持基本稳定的条件；（ii）表层-海洋的变率，导致向北的海洋热输送减少，而AMOC的深层分支没有减弱。（iii）表层和深海的变化，这意味着AMOC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，以及它们对欧洲南部的下游影响。VARING将超越Tzedakis等人（2018）的研究，从不同气候背景的多个时间间隔的多个地点获取记录，以记录变化并探索机制。最终，它将有助于我们对一系列州的气候变化的理解，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急需的约束。

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.