ASYNC - Resolving asynchronous responses of North Atlantic climate to deglacial changes in ocean circulation

这个拟议的项目将检验这样一种假设，即大西洋子午线翻转环流(AMOC)--一个对地球气候施加主要控制的表层和深海洋流系统--的逐渐变化，导致北大西洋气候在从最后一个冰期过渡到现在的温暖间冰期(约20,000-10,000年前)期间发生突变。格陵兰冰芯记录清楚地表明，这一时期的特点是在不到十年的时间里发生了重大的气候突变，这归因于与高纬度北大西洋向北的地表热量输送减少及其向南的深层回流有关的AMOC制度的变化。最重要的是，由于北极冰川融化和冰出口通量的增加，近几十年来北极冰川融化和冰出口的异常减弱引发了这样一个问题：目前北极冰川融化和冰出口的减少是否预示着一个类似于冰芯和海洋沉积物记录的新的突变阶段，以及北大西洋气候对高纬度地表和深海环流变化的响应时间有多长？因此，解决和量化海-气耦合系统中的非同步变化对于提高我们对气候过程的理论理解和气候模式的预测能力，以及识别在哪些条件下发生气候突变至关重要。Async是由剑桥大学领导的一个国际合作项目，将解决这一根本问题。该项目将利用北大西洋独特的沉积物记录，生成一套准确的海洋环流和气候变化的年代和数十年分辨率的替代记录。Async是第一个通过以连续方式精确集成基本时间尺度来比较高分辨率北大西洋代理记录的有针对性的努力。海洋记录将与格陵兰冰芯年表同步，方法是利用来自海底沉积物和已公布冰芯的数十年分辨率的宇宙成因10Be记录，独立和连续地重建传入宇宙射线通量的全球同步变化。拟议的项目将导致从三个北大西洋海洋沉积物岩芯重建新的宇宙成因10Be、海冰、融水排放以及底层和地表水通风。古海洋学重建，特别是反映大洋中游向南深层组成部分的底水通风记录，将直接与格陵兰岛冰芯气候重建中记录的事件进行比较。ASYNC将共同产生第一个连续同步记录大气、海洋和海冰变化的网络，这将解决北大西洋扰动在气候系统上跨越上次冰川消融(约20,000-10,000年前)的主要气候转变的时空传播问题。ASYNC的结果将促进对以下问题的当前理解：i)气候档案中气候突变的性质和时间；ii)AMOC和气候对逐渐的格陵兰冰盖和北冰洋冰川融水强迫的非线性响应；以及iii)北大西洋地区快速气候变化的海洋前兆。

The proposed project will test the hypothesis that gradual changes in Atlantic Meridional Overturning Circulation (AMOC) -a system of surface and deep ocean currents that exerts a primary control on Earth's climate, led to abrupt shifts in North Atlantic climate during the transition out of the last ice age and into the present warm interglacial (~20,000-10,000 years ago). Greenlandic ice-core records show clear evidence that this period was characterised by major abrupt climate shifts in less than a decade, which have been attributed to changes in the AMOC regime associated with reduced northward surface heat transport in the high-latitude North Atlantic and its deep southward return flow. Critically, the anomalous weakening of the AMOC in the last decades caused by enhanced fluxes of meltwater and ice export from the Arctic in response to Arctic change prompts the question: Is the current decline in AMOC heralding a new phase of abrupt change similar to those recorded in ice cores and ocean sediments, and what is the response time of North Atlantic climate to changes in high-latitude surface and deep ocean circulation?Resolving and quantifying asynchronous changes within the coupled ocean-atmosphere system is hence essential to improve our theoretical understanding of climate processes and predictive capacity of climate models, as well as identifying under which conditions abrupt climate change occurs. ASYNC is an international collaborative project led by the University of Cambridge that will tackle this fundamental problem. The project will avail of unique North Atlantic Ocean sediment records to generate a suite of precisely dated and multidecadally-resolved proxy records of ocean circulation and climate change. ASYNC represents the first targeted effort to compare high resolution North Atlantic proxy records by precisely integrating the underlying timescales in a continuous fashion. The marine records will be synchronised to the Greenland ice-core chronology via independent and continuous reconstructions of globally synchronous variations in the incoming cosmic ray flux using multidecadally-resolved cosmogenic 10Be records from seafloor sediments and published ice cores.The proposed project will result in new cosmogenic 10Be, sea ice, meltwater discharge, and bottom- and surface-water ventilation reconstructions from three North Atlantic marine sediment cores. The palaeoceanographic reconstructions, and in particular the bottom-water ventilation records, which reflect the southward deep component of AMOC, will be directly compared to events recorded in ice-core climate reconstructions from Greenland. Together, ASYNC will result in the first network of continuously synchronised records of atmospheric, oceanic and sea ice change that will resolve the temporal and spatial propagation of North Atlantic ocean perturbations on the climate system across the major climatic transitions that punctuated the last deglaciation (~20,000-10,000 years ago). Results from ASYNC will advance the current understanding of i) the nature and timing of abrupt climate shifts across climate archives, ii) nonlinear responses of AMOC and climate to gradual Greenland Ice Sheet and Arctic sea ice meltwater forcing, and iii) ocean precursors of rapid climate change in the North Atlantic region.