The Evolving Role of the Ocean and the Atmosphere in Decadal to Multidecadal Modes of Climate Variability

应对气候变化影响的努力因难以区分主要由人类活动引起的外部强迫的影响和气候系统的自然内部变异性而变得复杂。十年到十年的变化通常以一系列气候模式为特征，如大西洋数十年涛动(AMO)，在这种模式下，赤道以北的大西洋有持续20至40年的变暖和变冷交替事件。气候模式通常被认为是自然变异性，因为它们可以在19世纪及更早的观察中找到，当时工业排放还没有上升。为此，区域气候变化通常被认为是气候模式形式的自然变化和外部强迫变化的叠加，这种变化与气候模式不同，具有不同的空间模式，涉及不同的物理机制。首席研究者对这一假设提出了质疑，并提出了一个更具挑战性的假说：1950年前，气候模式主要是由大气-海洋耦合产生的自然变化，但1950年后，气候模式主要是外部强迫，海洋和大气之间的关系被外部强迫改变。这一假设是由PI最近对20世纪气候的模拟工作所推动的，其中包括气溶胶污染的辐射效应的强迫作用。气溶胶排放的主要来源已从西欧和美国东部转移到中国和南亚，因此对气溶胶辐射强迫变化的响应可能在十年尺度上引起区域气候变化。这一假说还基于这样的观点，即气候系统通过产生内部变化模式的动力学来响应外部强迫，因此对外部强迫的响应相当于对自然内部模式的激励。研究是通过各种气候模型模拟进行的，其中包括利用共同体地球系统模型(CESM)进行的模拟的大型集合(LES)。其中包括没有外部强迫的模拟大涡模拟，以及孤立对气溶胶强迫的响应的单强迫模拟。这项工作还利用了为欧洲海洋环境监测系统开发的更简单配置的层次结构，包括根据AGS-2040073开发的“铅笔海洋”配置，该配置表示海洋中的湍流垂直热传输，但不模拟洋流中水的横向运动。鉴于需要在区域一级应对气候变化影响，这项工作具有社会意义。如果PIs的假设是正确的，将需要对区域气候预测的不确定性估计进行一些修订，目前没有考虑气溶胶排放未来演变的不确定性。这项工作对气候预测工作也有影响，在气候预测工作中，通常假设可以根据大气和海洋的当前状态以及对两者之间耦合的足够准确的表示来预测气候模式(例如，见AGS-2231237)。这项工作还与气候对沿海社区的影响直接相关，特别是考虑到AMO对大西洋飓风活动有重大影响。研究人员与迈阿密气候复原力委员会合作，Resilient305研究协作组将研究结果直接应用于基于社区的决策。该项目还为一名博士后助理、一名研究生和一名本科生提供支持和培训，私人投资机构开展了一系列外联活动，包括迈阿密大学女性科学日，这是针对6年级和7年级女孩的年度活动。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。

Efforts to address climate change impacts are complicated by the difficulty of distinguishing between the effects of external forcing, predominantly caused by human activities, and the natural internal variability of the climate system. Decade-to-decade variations are typically characterized by a set of climate modes such as the Atlantic Multidecadal Oscillation (AMO), in which the Atlantic north of the equator has alternating episodes of warming and cooling that last 20 to 40 years. The climate modes are generally regarded as natural variability because they can be found in observations from the 1800s and earlier, before the rise of industrial emissions. For this reason regional climate change is often viewed as the superposition of natural variability in the form of climate modes and externally forced change which is distinct from the modes, with a different spatial pattern and involving different physical mechanisms.Here the Principal Investigators (PIs) question this assumption and pose a more challenging hypothesis: prior to 1950 climate modes are largely natural variability arising from atmosphere-ocean coupling, but after 1950 the modes are largely externally forced, and the relationship between the ocean and atmosphere is altered by the external forcing. The hypothesis is motivated by the PIs' recent work with simulations of 20th century climate which include forcing by the radiative effects of aerosol pollution. The dominant sources of aerosol emissions have shifted from western Europe and the eastern US to China and southern Asia, thus the response to shifting aerosol radiative forcing could cause regional climate change on the decadal timescale. The hypothesis is also predicated on the idea that the climate system responds to external forcing through the dynamics that generate the internal variability modes, so that the response to external forcing amounts to an excitation of the natural internal modes.The research is conducted with a variety of climate model simulations including large ensembles (LEs) of simulations performed with the Community Earth System Model (CESM). These include LEs of simulations with no external forcing and single-forcing simulations which isolate the response to aerosol forcing. The work also takes advantage of the hierarchy of simpler configurations developed for CESM, including the "pencil ocean" configuration developed under AGS-2040073, which represents turbulent vertical heat transfer in the ocean but does not simulate the lateral movement of water in ocean currents.The work has societal relevance given the need to address climate change impacts at the regional level. If the PIs' hypothesis is correct it will require some revision of uncertainty estimates for regional climate projections, which do not currently take into account uncertainty in the future evolution of aerosol emissions. The work also has implications for climate prediction efforts, in which it is commonly assumed that climate modes can be predicted from the current state of the atmosphere and oceans and a sufficiently accurate representation of the coupling between the two (see for example AGS-2231237). The work is also directly relevant to climate impacts on coastal communities, particularly given that the AMO has a substantial effect on Atlantic hurricane activity. The researchers work with the Miami Climate Resilience Committee and the Resilient305 Research Collaborative provides a direct application of research results to community-based decision making. The project also provides support and training to a postdoctoral associate, a graduate student and an undergraduate, and the PIs perform a number of outreach activities including the University of Miami Women in Science day, an annual event for girls in the 6th and 7th grade.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.