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

大西洋经向大洋环流 (AMOC) 在过去 11,000 年间是否以及如何发生变化尚不确定。然而，AMOC 会影响热量、盐度和营养物质的分布，其强度的变化会影响全球气候，从而产生生态影响。初步数据表明，水深约 1000 米的热带大西洋在大约 11,000 至 5,000 年前变冷。其他证据也暗示该地区在此期间出现类似的降温。 AMOC 强度的变化会影响热量在深海内的分布，并可能导致观察到的冷却趋势。深海温度还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）而发生变化。本研究的目的是生成数据来确认观察到的冷却趋势，并评估该趋势是否是由于流向这些地点的高纬度水域冷却或 AMOC 的变化，或这两种机制造成的。确定这种变冷的原因对于了解全新世的气候和变化具有重要意义。一名博士后科学家将参与该项目。初步结果表明，在热带北大西洋西部，在南极中层水（AAIW）是主要水体的深度（约 950 m），全新世早期至中期出现了冷却。这种冷却从约 11 kyBP 开始，最终达到接近现代底部水温 (BWT) 约 7-5 kyBP。 解释这种冷却的假设包括（1）加强大西洋经向翻转环流（AMOC），以响应北大西洋冰消淡水输入的减少，或（2）该地点的一种或多种源水冷却。研究人员将通过在现代 AAIW 深度内的几个浅热带地点重建 BWT 来确认这种冷却，并通过 AAIW 内其他亚热带地点和北部水源水域的测量来测试替代假设。一组来自北大西洋、南大西洋和热带大西洋、水深从 300 米到 1100 米的海洋沉积物岩心将被用来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：流向热带大西洋的高纬度水域变冷，或该时期 AMOC 的加强。由于模型模拟无法预测这段时间内这些地点的降温情况，因此确认和了解降温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点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.