Collaborative Research: Response of the upper tropical Pacific Ocean to greenhouse gas forcing in observations and models

热带太平洋是一个公认的全球气候异常的驱动因素，也是进入大气的<s:1>𝑂2通量的最大海洋来源。它对温室气体（GHGs）上升的反应将强烈影响未来的全球气候极端事件和碳循环。气候模式模拟了过去几十年该地区的变暖，这在观测记录中是没有的。目前还没有一个明确的解释来解释这种差异，或者它对未来的预测意味着什么。这项工作将对热带太平洋对温室气体驱动的热力学和动力强迫的响应进行独特的详细分析，重点是与观测到的和再分析的海洋环流和温度变化进行比较。虽然很多研究都集中在热带大气对温室气体上升的反应上，但对热带海洋的研究要少得多。因此，在辐射强迫气候变化的背景下，该项目将在理解热带大气-海洋耦合系统的海洋方面取得重要的智力进步。确定热带太平洋对温室气体上升的反应对于预测全球区域气候的变化和影响大气的气候-碳反馈至关重要。这项工作将促进对减缓和适应气候变化至关重要的知识。这项工作将支持两名早期职业研究人员（博士后和研究生）和一名女教员在理解气候系统的一个关键组成部分如何对人为强迫作出反应方面取得根本性进展。首席PI很好地融入了研究北美不断变化的干旱风险及其社会影响的社区，其中赤道太平洋是一个关键驱动因素。在全球变暖期间，自1960年左右以来，伴随着温室气体的急剧增加，观测结果显示赤道太平洋冷舌几乎没有变暖，甚至没有变冷。相比之下，连续耦合模式比对项目（CMIPs）中的模式模拟冷舌变暖。有人认为，这种差异是由于自然界内部的强烈变化，而不是对温室气体的反应。然而，各种各样的研究都表明，CMIP模型不太可能与观测结果相匹配，尽管它们在不太可能的程度上存在分歧。另一方面，假设温室气体的上升导致纬向海温梯度、Walker环流和贸易、动态浅化温跃层和上升流增强的冷却。有人认为，由于在模拟热带太平洋大气-海洋系统时存在偏差，模式的反应在很大程度上是相反的。该项目将研究热带太平洋上层的温度、洋流和热结构如何对温室气体上升作出反应。这是对迄今为止的观测和模型的深入调查，可以对模型进行评估。这项工作是围绕解决以下问题的假设来组织的：1)在确定热带太平洋对温室气体上升的反应中，热力学过程和动力学过程的相对作用是什么？2)气候模式的偏置（过度冷舌、南部辐合带过度发达、东部副热带层云和上升流区过于温暖）是否会导致上升流和俯冲区海温响应的偏置以及进入赤道潜流的运输路径的偏置？模式偏差是否影响热带太平洋海温对温室气体强迫的响应？主要目标是更好地了解海洋在应对温室气体上升方面的积极作用。模型是这项研究的主要工具。为了应对温室气体的增加，这项工作将研究：在副热带俯冲后，地表增加的热量是如何混合下来并在内部输送的；风应力的变化如何影响海流、上升流、热结构和海温；平均风应力如何影响赤道和亚热带之间的内部通道，从而影响冷舌上涌的水的温度。将利用欧洲中期天气预报中心的海洋模型对响应进行分解，再分析以进行验证。实验将以不同的组合方式施加模拟场和观测场，温室气体引起的加热，并重新分析和CMIP6平均和异常风应力。热收支、示踪剂、解释性人工智能方法和因果通路分析将揭示海洋对观测和CMIP6风应力趋势的响应机制，这如何依赖于平均海洋状态的扰动，以及被动和动态海洋过程在确定热带太平洋海温趋势中的相对作用。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The tropical Pacific Ocean is a well-established driver of global climate anomalies and the largest oceanic source of 𝐶𝑂2 flux into the atmosphere. Its response to rising greenhouse gases (GHGs) will strongly influence future global climate means and extremes and the carbon cycle. Climate models simulate a warming in this region for the last several decades, which is not seen in the observational record. There is not a clear explanation of this difference or what it means for future projections. This work will be a uniquely detailed analysis of the tropical Pacific Ocean response to GHG-driven thermodynamic and dynamic forcing focusing on comparisons to observed and reanalyzed changes in ocean circulation and temperature. While much research has focused on the tropical atmosphere’s response to rising GHGs there has been much less work on the tropical oceans. This project will therefore by an important intellectual advance in understanding, in the context of radiatively-forced climate change, the ocean side of the coupled tropical atmosphere-ocean system. Determining how the tropical Pacific responds to rising GHGs is critical to projecting changes in regional climates worldwide and climate-carbon feedbacks that influence atmospheric 𝐶𝑂2. The work will advance knowledge important to climate change mitigation and adaptation. The work will support two early-career researchers (postdoctoral and graduate) and a female faculty member to make fundamental advances in understanding how a key component of the climate system responds to anthropogenic forcing. The lead PI is well integrated into the community researching evolving drought risk over North America, for which the equatorial Pacific is a key driver, and its societal impacts. Results will be communicated there to identify errors and narrow uncertainties regarding near-term hydroclimate projections under GHG-induced changeAmidst the global warming accompanying the sharp rise in GHGs since circa 1960 observations show little warming or even cooling in the equatorial Pacific cold tongue. In contrast, models within successive Coupled Model Intercomparison Projects (CMIPs) simulate warming of the cold tongue. It has been argued that this discrepancy is due to strong internal variations in nature rather than a response to GHGs. Yet various studies all show it is unlikely that CMIP models can match the observations, though they differ on just how unlikely. On the other hand, it has been hypothesized that rising GHGs lead to strengthening of the zonal SST gradient, Walker circulation and trades, a dynamically-shoaled thermocline and enhanced cooling by upwelling. Models, it is argued, have a largely opposite response due to biases in simulating the tropical Pacific atmosphere-ocean system. This project will address how the temperature, currents and thermal structure of the upper tropical Pacific Ocean respond to rising GHGs. It is a deep investigation with observations and models of the period to date for which models can be evaluated. The work is organized around hypotheses that address the questions: 1) what are the relative roles of thermodynamic and dynamic processes in determining the tropical Pacific Ocean response to rising GHGs? 2) do biases in climate models (excessive cold tongue, overdeveloped southern convergence zone, too-warm eastern subtropical stratus cloud and upwelling regions) lead to biased SST responses in upwelling and subducting regions and biased transport pathways into the Equatorial Undercurrent? 3) do model biases influence the response of tropical Pacific SSTs to GHG forcing? The main goal is to better understand the active role of nature’s ocean in the response to rising GHGs. Models are primary tools for this investigation. In response to rising GHGs the work will examine: how heat added at the surface is mixed down and transported in the interior after subduction in the subtropics; how changes in wind stress impact currents, upwelling, thermal structure and SST; how mean wind stress influences interior pathways between the equator and the subtropics and, hence, the temperature of water upwelling in the cold tongue. The responses will be decomposed with experiments with ocean models using European Center for Medium Range Weather Forecasts reanalyses for validation. The experiments will impose, in various combinations, modeled and observed fields, GHG-induced heating, and reanalyzed and CMIP6 mean and anomalous wind stresses. Heat budgets, tracers, Explanatory Artificial Intelligence methods and causal pathway analysis will reveal the mechanisms of ocean responses to observed and CMIP6 wind stress trends, how this depends on the mean ocean state being perturbed, and the relative roles of passive and dynamical ocean processes in determining trends in tropical Pacific SSTs.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.