Collaborative Proposal: Exploring Hypotheses of Southern Hemisphere Westerly Wind Changes on Southern Ocean Circulation and Biogeochemistry

合作提案：探索南半球西风变化对南大洋环流和生物地球化学的假设

• 批准号: 1537496
• 负责人: John Chiang
• 金额: $ 3.3万
• 依托单位: University of California-Berkeley
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2015
• 资助国家: 美国
• 起止时间: 2015-10-01 至 2018-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1537496&HistoricalAwards=false
• 关键词: Collaborative; Proposal; Exploring; Hypotheses; Southern

The circulation of the Southern Ocean plays a prominent role in regulating Earth's climate, in particular connecting the deep ocean to the surface. This connection holds particular importance for the Earth's carbon cycle as upwelling of waters from the deep ocean brings stored carbon of the deep to the surface, eventually entering into the atmosphere as carbon dioxide. Increased upwelling is thought to have brought about the initial rise in atmospheric CO2 concentrations during the last deglaciation, caused by changes to the surface westerlies and resulting changes to the wind-driven upwelling. This project investigates a recent hypothesis for how the westerlies changed during this time period affects Southern ocean circulation and carbon cycle, through the use of a state-of-the-art climate model. The predictions of this hypothesis will be contrasted against those made by the "annular-mode" view for westerly changes that dominates current thinking, and both compared against available paleoproxy observations. The research provides concrete tests
to each hypothesis, and the process will help advance our understanding of how southern hemisphere westerlies affect the ocean climate and carbon cycle. The project will also enhance collaboration between two US-based scientists with an overseas collaborator based in Taiwan.Previous studies have largely assumed that paleo-westerly changes were zonally uniform, analogous to the so-called "annular mode" behavior that dominates year-to-year variations in the southern hemisphere westerlies today. However, in reality there are insufficient paleoproxy data to confirm this interpretation, and in a recent study one of the PIs proposed an alternative hypothesis for westerly changes through the modulation of the South Pacific Split Jet. The Split Jet is a regional feature of the South Pacific whereby the core of the westerlies split into two around Australia, and this split extends across the Pacific. Based on dynamical reasoning and comparison to paleoproxy records, the PI argued that the Split Jet feature weakened during Heinrich events, in particular Heinrich event 1 that preceded the last deglaciation. In this study, the consequences for a weaker Split Jet on the Southern ocean circulation and biogeochemistry will be investigated through idealized simulations using the ocean component of the Community Earth System Model version 1 with a simple ocean biogeochemistry module. Focus will be placed on understanding changes to the ocean surface temperature and frontal locations, changes to the upwelling and carbon fluxes. These simulations will be compared and contrasted against simulations where annular-mode like westerly changes is imposed. Through comparison between these simulations with available ocean paleoproxies, an assessment can be made as to which westerly configuration was more likely. If the Split Jet hypothesis turns out to be more correct, it has strong implications for our conceptualization and understanding of Southern ocean climate and circulation changes.

南大洋环流在调节地球气候，特别是连接深海和地表方面发挥着突出的作用。这种联系对地球的碳循环特别重要，因为从深海涌出的水将深海储存的碳带到地表，最终以二氧化碳的形式进入大气层。上升流的增加被认为是上一次冰川消融期间大气二氧化碳浓度最初上升的原因，这是由于地表西风带的变化以及由此导致的风力驱动的上升流的变化。该项目通过使用最先进的气候模型，研究了西风带在这段时间内的变化如何影响南大洋环流和碳循环的最新假说。这一假说的预测将与主导当前思维的西风变化的“环状模式”观点的预测形成对比，并与现有的古生物观测结果进行比较。这项研究为每一种假说提供了具体的检验，这一过程将有助于我们加深对南半球西风如何影响海洋气候和碳循环的理解。该项目还将加强两名美国科学家与一名台湾海外合作者之间的合作。以前的研究大多假设古西风变化是纬向一致的，类似于今天主导南半球西风带年际变化的所谓“环状模式”行为。然而，现实中没有足够的古替代数据来证实这一解释，在最近的一项研究中，其中一位PI提出了另一种假设，即通过南太平洋分裂急流的调制来实现西风变化。分裂急流是南太平洋的一个区域特征，西风带的核心在澳大利亚周围一分为二，这种分裂延伸到整个太平洋。根据动力学推理和与古代理记录的比较，PI认为在海因里希事件期间，特别是在最后一次冰川消融之前的海因里希事件1期间，分裂急流特征减弱。在这项研究中，将通过使用共同体地球系统模型第1版的海洋部分和一个简单的海洋生物地球化学模块，通过理想化的模拟来研究较弱的分裂急流对南大洋环流和生物地球化学的影响。重点将放在了解海洋表面温度和锋面位置的变化、上升流和碳通量的变化上。这些模拟将与实施类似西风变化的环形模式的模拟进行比较和对比。通过将这些模拟与现有的海洋古气候指标进行比较，可以评估哪种西风配置更有可能。如果分裂急流假设被证明是更正确的，它将对我们对南方海洋气候和环流变化的概念化和理解有很大的影响。