Causes of climate extremes-generating ocean states

2013/2014年和2015年初整个北半球的冬季包括加利福尼亚州的长期干旱、北美东北部的极端寒冷和多雪的冬季以及英国的严重洪水。观测分析和建模已将这些不同的极端气候与海面温度（SST）异常联系起来。所引发的热带印度洋和太平洋海温异常与厄尔尼诺或拉尼娜相关的海温异常不同。南印度洋和热带西太平洋海温异常偏暖，海洋大陆和赤道太平洋中部海温异常偏冷。最近的一个类似情况是 90/91 年的冬天。产生极端海洋状态的第二个例子是中赤道太平洋持续多年的寒冷条件。在有海表温度记录的上一个半世纪里，北美西部的极端干旱一直与这种热带太平洋条件有关。 1999 年至 2002 年就是这样的近期状态之一。与厄尔尼诺和拉尼娜事件相比，这两种极端状态的海洋动力学和热力学尚未得到广泛研究。该项目将对导致海洋采用这些海温模式的海洋动力和热力学过程进行详细分析。待研究的海洋状态会在全球范围内产生干旱、洪水、极寒和降雪，特别是在北美和欧洲。这些事件的社会影响是巨大的。改进其预测以及对其发生频率和强度的了解是实现适应和备灾的关键。预测需要理解，拟议的工作将促进对造成海温异常的海洋原因的理解，从而促进海洋强迫天气和极端气候的预测和表征。研究结果将提高我们对此类危险和事件进行建模和预测的能力。此外，这项工作产生的结果将很容易传达给社区，社区可以通过首席研究员密切参与 NOAA 干旱工作组和国家综合干旱信息系统工作组来推进预测、社会规划和适应工作。创建的数据将通过哥伦比亚大学数据库国际研究所提供，并作为研究资源提供给整个社区。该项目的重点是分析三个海洋再分析中包含的上层海洋的动量和热量收支：欧洲中期天气预报中心的海洋再分析（1958年至今）、估计海洋环流和气候联盟（第4版，1993年至今）和地球物理流体动力学实验室的集合气候数据同化系统（1960 年至今）。将使用标准数值程序评估平均流和瞬态涡流对表面通量、垂直混合和动态海洋热收敛的贡献，以评估极端生成状态。动力和混合贡献将与上层海洋热含量的变化以及海流和垂直运动的变化有关，而这些变化又与风应力强迫有关。在一系列假设的指导下，将在数据准确性和样本量限制允许的范围内确定产生极端海洋状态的原因。将检查海洋再分析和表面通量的长期趋势以及气候模型模拟的整体，以寻找人类驱动的气候变化正在改变极端海洋状态的可能性及其物理原因的证据。该项目将对与两个重要的极端海温异常相关的海洋状态的海洋学和海洋大气原因进行全面评估。这项工作主要以观测为基础，将确定近期气候和极端天气海洋原因的物理原理。

The winters of 2013/14 and early 2015 across the Northern hemisphere included chronic drought in California, extreme cold and snowy winters in northeast North America and severe floods in the U.K. Observational analyses and modeling have linked these disparate climate extremes to sea surface temperature (SST) anomalies. The SST anomalies in the tropical Indian and Pacific Oceans that have been invoked are distinct from those associated with El Nino or La Nina. The SST anomalies were warm in the Southern Indian Ocean and Western Tropical Pacific and neutral to cool around the maritime continent and central Equatorial Pacific Ocean. A recent analog to that state was the winter of 1990/91. A second example of an extremes-generating ocean state is that of persistent, multiyear cold conditions in the Central Equatorial Pacific Ocean. For the last century and a half for which SST records exists, extreme drought in western North America has been associated with such Tropical Pacific Ocean conditions. 1999 to 2002 was one such recent state. In contrast to El Nino and La Nina events, the ocean dynamics and thermodynamics of these two extremes-generating states have not been extensively studied. This project will conduct a detailed analysis of the ocean dynamic and thermodynamic processes that cause the oceans to adopt these SST patterns. The ocean states to be studied generate droughts, floods, extreme cold and snow across the globe, particularly in North America and Europe. The societal impacts of these events have been large. Improving their prediction and the understanding of their frequency of occurrence and intensity are key to enabling adaptation and disaster preparedness. Prediction requires understanding and the proposed work will advance understanding of the oceanic causes of the SST anomalies responsible for them, thus enabling advances in prediction and characterization of ocean-forced weather and climate extremes. The results of the study will improve our capability to model and forecast such hazards and events. Further, results generated in this work will readily be communicated to the communities that can advance efforts for prediction, societal planning and adaptation via lead investigator's close involvement in NOAA's Drought task force and the National Integrated Drought Information System's Task Force. The data created will be served via the International Research Institute of Columbia University's Data Library and be available as a research resource to the entire community.The project is centered on the analysis of the momentum and heat budgets of the upper oceans contained within three ocean reanalyses: the European Center for Medium Range Weather Forecasts' Ocean Reanalysis (1958 to present), the consortium for Estimating the Circulation and Climate of the Ocean (Version 4, 1993 to present) and the Geophysical Fluid Dynamics Laboratory's Ensemble Climate Data Assimilation system (1960 to present). The contributions of surface fluxes, vertical mixing and dynamical ocean heat convergence by the mean flow and transient eddies will be evaluated for the extremes generating states using standard numerical procedures. Dynamical and mixing contributions will be related to changes in upper ocean heat content and to changes in currents and vertical motion which in turn will be related to wind stress forcing. Guided by a series of hypotheses, the causes of the extremes-generating ocean states will be determined to the extent allowed by data accuracy and sample size limitation. Long term trends in the ocean reanalyses and surface fluxes, as well as ensembles of climate model simulations, will be examined for any evidence that human-driven climate change is altering the probability of extremes-generating ocean states and the physical reasons why. The project will provide a comprehensive assessment of the oceanographic and ocean-atmosphere causes of the ocean states associated with two important extremes-generating SST anomalies. This work, largely observationally-based, will determine the physics underlying oceanic causes of recent climate and weather extremes.