Tropical-Extratropical Interactions in a Hierarchy of Model Complexity

大多数中高纬度地区的天气系统(“温带”)起源于温带外，并由温带过程驱动，特别是锋面上的温度差异和迫使风在高潮和低谷周围循环的科里奥利力。但是温带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。一个相应的例子是大气河流(Ars)，在中纬度系统中的南风产生了从热带延伸到中纬度的细丝湿气，在那里它们在加州沿海等地区产生了暴雨和洪水。另一个例子是马登-朱利安涛动(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，向东缓慢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ARS增加，以及造成热浪和冷空气爆发的急流蜿蜒。对于MJO和较长时间尺度上的热带-温带相互作用来说，一个关键问题是海-气耦合的作用，因为由于风驱动的蒸发导致海洋表面温度(SST)和次表层海洋热量储存的变化的重要性目前是一个悬而未决的问题。该项目试图分离驱动MJO、ARS和较慢变化的基本动力学，在这种变化中，中纬度的变化导致SST异常，并在一到两年内传播到热带地区。PI开发和使用不同复杂程度的数值模型配置的层次结构，其中使用最复杂的配置来真实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可以排除不必要的细节，使它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，ARs可以在“水行星”模型中产生，在该模型中，大陆被海洋表面取代，SSTs近似为纬度的简单函数(赤道附近变暖，向两极逐渐冷却)。尽管没有大陆、季节性和真实的海温，但模拟产生了可信的ARS，这表明ARS的基本动力学最好在缺乏这些复杂性的简单模式中进行研究。层次结构中的模式是社区大气模式版本5(CAM5)的版本，PI将在其中添加一个更适合模拟MJO的水行星版本和两个新的海洋混合层表示。其中一个版本在指定的温度分布中增加了简化的降温速度计算和松弛，这种配置足以重现热带气旋后面的冷尾迹。另一个基本上是来自并行海洋计划的全物理混合层模式，在每个海洋表面GridPoint的独立列中实施。通过对简单混合层和复杂混合层的模拟比较，了解MJO和经向模式对决定海洋和上覆大气之间热量和水汽交换的物理特性的依赖关系。在该项目中开发的模型版本将作为共同体地球系统模型(CESM)的一部分提供给研究界，该模型由国家大气研究中心主办。它们是使用CESM代码库(包括CAM5)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，由于热带-温带相互作用对美国极端天气的影响，它们具有社会和科学意义。

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.