Tropical-Extratropical Interactions in a Hierarchy of Model Complexity

中高纬度地区的大多数天气系统（“热带外”）起源于热带外，并由热带外过程驱动，特别是沿着锋的温度对比和迫使风在高点和低点周围循环的科里奥利力。 但是，热带外天气系统可以通过各种途径与热带天气和气候发生强烈的相互作用。 一个重要的例子是大气河流（AR），其中中纬度系统的南风产生从热带延伸到中纬度的狭窄的水汽细丝，在那里它们在沿海地区（如加州）产生大雨和洪水。 另一个例子是马登-朱利安振荡（MJO），这是一种热带降水和大气环流的大型模式，形成于印度洋，并缓慢向东传播到热带太平洋中部。 MJO对中纬度天气有多种影响，包括美国西海岸沿着AR的增加和导致热浪和冷空气爆发的急流的蜿蜒。 在较长时间尺度上，MJO和热带-外热带相互作用的一个关键问题是海气耦合的作用，因为由于风驱动的蒸发，海表面温度（SST）和次表层海洋热储存的变化的重要性目前是一个悬而未决的问题。以及称为纬向模态的较慢的变化，其中中纬度的变化导致SST异常，并在一两年内传播到热带。PI开发并使用不同复杂程度的数值模型配置层次，其中最复杂的配置用于逼真地模拟感兴趣的现象。 逐步简化的模型版本用于降低模拟的复杂性，以便PI可以分解出不必要的细节，使他们能够以最简单和最容易理解的形式分析潜在的动态。 例如，初步工作表明，可以在“水行星”模型中产生AR，在该模型中，大陆被海洋表面取代，SST被近似为纬度的简单函数（赤道附近温暖，朝向两极逐渐冷却）。 尽管缺乏大陆、季节性和真实的SST，模拟产生了可信的AR，这表明AR的基本动力学最好在简单的模型中进行研究，这些模型缺乏这些复杂性。层次结构中的模型是社区大气模型版本5（CAM 5）的版本，PI将在其中添加一个具有“超参数化”对流的水行星版本，更适合模拟MJO，和两个新的海洋混合层的代表。 一个版本增加了一个简化的冷却率计算和松弛到一个特定的温度廓线，一个配置，这是足以复制冷尾流背后的热带气旋。 另一个基本上是来自并行海洋计划的完整物理混合层模型，在每个海洋表面网格点上以独立的列实现。 模拟与简单和更复杂的混合层之间的比较被用来了解MJO和对流模式的物理决定的海洋和上覆大气之间的热量和水分交换的依赖性。这项工作具有科学更广泛的影响，通过模型层次的发展。 该项目开发的模型版本将作为社区地球系统模型（CESM）的一部分提供给研究界，该模型由国家大气研究中心主持。 它们是使用CESM代码库（包括CAM 5）开发更简单模型的持续努力的一部分，并将得到文档和参考模拟的支持。 更广泛地说，热带-外热带相互作用具有社会和科学意义，因为它们对美国极端天气的影响。

Most weather systems in middle and high latitudes (the "extratropics") originate in the extratropics and are driven by extratropical processes, in particular temperature contrasts along fronts and the Coriolis force that compels winds to circulate around highs and lows. But extratropical weather systems can interact strongly with tropical weather and climate through a variety of pathways. One consequential example is atmospheric rivers (ARs), in which southerly winds in midlatitude systems produce narrow filaments of moisture extending from the tropics to the midlatitudes, where they produce heavy rain and flooding in regions such as coastal California. Another example is the Madden-Julian Oscillation (MJO), a large pattern of tropical precipitation and atmospheric circulation that forms in the Indian Ocean and propagates slowly eastward into the central tropical Pacific. The MJO has a variety of effects on midlatitude weather, including an increase in ARs along the US west coast and meanders of the jet stream that cause heat waves and cold air outbreaks. A key question for the MJO and for tropical-extratropical interactions on longer timescales is the role of air-sea coupling, as the importance of changes in sea surface temperature (SST) and subsurface ocean heat storage due to wind-driven evaporation is currently an open question.This project attempts to isolate the fundamental dynamics which drive the MJO, ARs, and slower variations known as meridional modes in which variability in the midlatitudes causes SST anomalies which propagate into the tropics over a year or two. The PIs develop and use a hierarchy of numerical model configurations at varying levels of complexity, in which the most complex configuration is used to realistically simulate the phenomena of interest. Progressively simpler model versions are used to reduce the complexity of the simulation so that the PIs can factor out non-essential details, allowing them to analyze the underlying dynamics in their simplest and most comprehensible form. For example, preliminary work shows that ARs can be produced in an "aquaplanet" model, in which the continents are replaced by ocean surface and SSTs are approximated as a simple function of latitude (warm near the equator and gradually cooling towards the poles). The simulation produces credible ARs despite the lack of continents, seasonality, and realistic SSTs, suggesting that the fundamental dynamics of ARs are best examined in simple models which lack these complexities.Models in the hierarchy are versions of the Community Atmosphere Model version 5 (CAM5), to which the PIs will add an aquaplanet version with "superparameterized" convection that is more appropriate for simulating the MJO, and two new representations of the ocean mixed layer. One version adds a simplified cooling rate calculation and relaxation to a specified temperature profile, a configuration which is sufficient to reproduce cold wakes behind tropical cyclones. The other is essentially the full physics mixed layer model from the Parallel Ocean Program, implemented in stand-alone columns at each ocean surface gridpoint. Comparisons between simulations with simpler and more complex mixed layers are used to understand the dependence of the MJO and meridional mode on the physics determining heat and moisture exchange between the ocean and overlying atmosphere.The work has scientific broader impacts through the development of the model hierarchy. The model versions developed in this project will be made available to the research community as part of the Community Earth System Model (CESM), which is hosted by the National Center for Atmospheric Research. They are part of an ongoing effort to developer simpler models using the CESM code base (which includes CAM5), and will be supported with documentation and reference simulations. More broadly, tropical-extratropical interactions are of societal as well as scientific interest due to their influence on US weather extremes.