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

由于难以区分主要由人类活动造成的外部强迫的影响和气候系统内部的自然变异性，应对气候变化影响的努力变得更加复杂。 十年到十年的变化通常以一系列气候模式为特征，例如大西洋年代际振荡（AMO），其中赤道以北的大西洋有持续20至40年的交替变暖和变冷事件。气候模态通常被认为是自然变率，因为它们可以在19世纪和更早的观测中发现，在工业排放增加之前。 因此，区域气候变化通常被视为气候模态形式的自然变率和外部强迫变化的叠加，外部强迫变化不同于模态，具有不同的空间格局，涉及不同的物理机制。在这里，首席研究员（PI）质疑这一假设，并提出一个更具挑战性的假设：1950年以前的气候模态主要是由海气耦合引起的自然变率，而1950年以后的气候模态主要是外部强迫的，海洋和大气之间的关系被外部强迫改变。这一假设的动机是PI最近对20世纪世纪气候的模拟工作，其中包括气溶胶污染的辐射效应。 气溶胶排放的主要来源已经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的响应可能导致区域气候在十年尺度上的变化。 该假说还预测的想法，气候系统响应外部强迫通过动力学，产生内部变率模式，使外部强迫的响应相当于激发的自然internalmodes.The研究进行了各种气候模式模拟，包括大型合奏（LE）的模拟与社区地球系统模式（CESM）。 这些包括LE模拟没有外部强迫和单一强迫模拟隔离气溶胶强迫的响应。 这项工作还利用了为CESM开发的较简单配置的层次结构，包括根据AGS-2040073开发的"铅笔海洋"配置，该配置代表海洋中的湍流垂直热传递，但不模拟洋流中水的横向运动。 如果PI的假设是正确的，它将需要对区域气候预测的不确定性估计进行一些修订，目前没有考虑到气溶胶排放未来演变的不确定性。 这项工作也对气候预测工作有影响，在气候预测工作中，人们通常认为，气候模式可以从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（例如，见AGS-2231237）。 这项工作也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弹性委员会和Resilient305研究合作提供了一个直接应用的研究成果，以社区为基础的决策。 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，PI开展了一些外联活动，包括迈阿密大学妇女科学日，该奖项反映了NSF的法定使命，并通过使用基金会的智力价值和更广泛的影响审查进行评估，被认为值得支持的搜索.

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.