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: 1537105
• 负责人: Katsumi Matsumoto
• 金额: $ 6.28万
• 依托单位: University of Minnesota-Twin Cities
• 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 2017-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1537105&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.

南大洋的环流在调节地球气候方面发挥着重要作用，特别是将深海与地表联系起来。这种联系对地球的碳循环尤其重要，因为来自深海的上升流将储存在深海的碳带到地表，最终以二氧化碳的形式进入大气。上升流的增加被认为在最后一次消冰期间引起了大气中二氧化碳浓度的最初上升，这是由地表西风带的变化和由此引起的风驱动的上升流的变化引起的。该项目通过使用最先进的气候模型，研究了最近的一个假设，即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。这一假设的预测将与主导当前思想的西风变化的“环状模式”观点进行对比，并与现有的古代用观测结果进行比较。研究提供了具体的试验；这一过程将有助于我们进一步了解南半球西风带是如何影响海洋气候和碳循环的。​以前的研究在很大程度上假设古西风带的变化是纬向均匀的，类似于今天主导南半球西风带年复一年变化的所谓“环状模式”行为。然而，实际上没有足够的古代用数据来证实这一解释，在最近的一项研究中，其中一位pi提出了通过南太平洋分裂射流调制西风变化的另一种假设。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分裂成两个，这个分裂延伸到整个太平洋。基于动力学推理和与古代用记录的比较，PI认为分裂射流特征在海因里希事件期间减弱，特别是在最后一次消冰期之前的海因里希事件1。本研究将利用群落地球系统模式第1版的海洋组件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分裂射流减弱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。这些模拟将与施加环状模式如西风变化的模拟进行比较和对比。通过将模拟结果与现有的海洋古代用资料进行比较，可以评估哪一种配置更可能是西风配置。如果分裂射流假说被证明是更正确的，它将对我们对南大洋气候和环流变化的概念化和理解产生重大影响。