Holocene Upper Atlantic Temperature Trend: AMOC Variability or Anomalous Heat Uptake

大西洋子午线海洋环流(AMOC)在过去11,000年中是否以及如何发生了变化尚不确定。然而，AMOC影响着热量、盐度和营养物质的分布，其强度的变化可能会影响全球气候，并对生态造成影响。初步数据表明，热带大西洋的水深约为1000米，从大约11,000年前冷却到5000年前。其他证据也暗示，在这段时间里，该地区也出现了类似的降温。AMOC强度的变化影响了热量在深海中的分布，并可能导致了观测到的冷却趋势。深海的温度也受到地表水沉入深海区域的温度变化的影响。这些表面温度可能会由于全球温度变化或高纬度区域气候变化(例如风、海冰分布)而变化。这项研究的目的是产生数据，以证实观察到的冷却趋势，并评估这种趋势是由于流向这些地点的高纬度水的冷却，还是由于AMOC的变化，或者两者兼而有之。确定这种冷却的原因对于理解全新世气候和变异性具有重要意义。一名博士后科学家将参与这一项目。初步结果表明，热带北大西洋西部发生了早-中期全新世降温，深度约为950米，其中南极中间水(AAIW)是主要水团。这种冷却大约开始于11kyBP，并在接近现代的底层水温(BWT)达到约7-5kyBP。解释这种冷却的假设包括：(1)随着北大西洋脱冰期淡水输入的减少而加强的大西洋子午线翻转环流(AMOC)，或(2)一个或多个源水向该地点的冷却。研究人员将通过重建现代AAIW深度内几个浅热带地点的BWT来证实这种冷却，并用AAIW和北部水源水域内其他亚热带地点的测量来测试替代假设。一套来自北部、南部和热带大西洋的海洋沉积物岩芯，水深从300米到1100米不等，将用于重建温度变化的空间模式，并帮助区分温度趋势的两个可能原因，即流向热带大西洋的水域的高纬度冷却，或这段时间内AMOC的加强。由于模型模拟不能预测这些地点在这段时间内的降温，确认和了解这种降温的原因可能有助于确定模型或模型设置可以改进的地方。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Whether and how the Atlantic Meridional Ocean Circulation (AMOC) has changed during the last 11,000 years is uncertain. Yet, the AMOC influences the distribution of heat, salinity, and nutrients, and variations in its strength can affect global climate, with ecological impacts. Initial data suggest the tropical Atlantic, at about 1000 meters water depth, cooled from approximately 11,000 to 5,000 years ago. Other evidence also hints similar cooling in this region during this time. Changes in the strength of the AMOC influence how heat is distributed within the deep ocean and may have caused the observed cooling trend. Temperature of the deep ocean is also affected by temperature changes in the regions where surface waters sink into the deep ocean. These surface temperatures may change due to global temperature change or due to regional climate changes at high latitudes (e. g. winds, sea ice distribution). The goal of this study is to generate data to confirm the observed cooling trend and to assess whether the trend is due to cooling of high latitude waters flowing to these sites or to changes in AMOC, or to both of these mechanisms. Identifying the cause of this cooling has important implications for understanding Holocene climate and variability. A post-doctoral scientist will participate in this project.Initial results suggest an early – middle Holocene cooling in the western tropical north Atlantic, at a depth (about 950 m) where Antarctic Intermediate Water (AAIW) is the dominant water mass. This cooling began at approximately 11 kyBP and culminated in near-modern bottom water temperatures (BWT) by about 7-5 kyBP. Hypotheses to explain this cooling include (1) a strengthening Atlantic Meridional Overturning Circulation (AMOC) in response to a decrease in the input of deglacial freshwater to the North Atlantic, or (2) cooling in one or more of the source waters to the site. Researchers will confirm this cooling by reconstructing BWT at several shallow tropical sites within modern AAIW depths, and test the alternative hypotheses with measurements from other subtropical sites within AAIW and northern source waters. A suite of marine sediment cores from the north, south, and tropical Atlantic, and ranging from 300 to 1100 meters water depth will be used to reconstruct the spatial pattern of temperature change and help distinguish between the two likely causes of the temperature trend, a high-latitude cooling in waters flowing to the tropical Atlantic, or a strengthening of the AMOC during this time period. As model simulations do not predict a cooling at these sites over this time period, confirming and understanding the cause of this cooling may help identify where models or model set-up can be improved.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.