Variable Resolution Modeling of the Large-Scale Atmospheric Circulation Response to North Atlantic Sea-Surface Temperature (SST) Anomalies

大气环流对下伏海表面温度(SST)变化的反应程度是气候动力学的中心议题。在热带地区，大气环流对海温变化的响应很容易从厄尔尼诺事件的演变中看出。但在中高纬度地区，环流变化与下垫面海温变化之间没有明显的对应关系。然而，最近的研究表明，与墨西哥湾流蜿蜒有关的海温变化影响了北半球大部分地区冬季大气环流十年至十年的变化。观测证据是提示性的，但不是决定性的，因为环流变化固有的噪声和较短的观测记录不允许明确的归属。气候模型模拟提供了一种绕过观测记录短小的方法，以及设计受控实验的能力，而使用中等分辨率(比如每经纬度一个网格点)的模型可以产生非常长的模拟。但这类模拟通常没有显示出环流对墨西哥湾流SST变化的响应，最近的实验表明，需要更高的分辨率来捕捉导致环流响应的物理和动力学。不幸的是，使用如此高分辨率的全球模拟计算过于密集，无法进行深入实验。该奖项的研究使用可变分辨率(VR)模型考虑了墨西哥湾流SST变化的影响，在该模型中，世界大部分地区的网格间距为1度，但北极和北大西洋北部的分辨率提高到1/8度，即网格点之间约12公里。该模式是社区大气模式(CAM)的一个版本，是社区地球系统模式(CESM)的大气成分模式。初步结果表明，在代表墨西哥湾流波动的强制SST变化的强迫下，半球环流有一个强大的响应，其空间模式类似于北大西洋涛动。进一步的工作是将VR-CAM仿真与较低分辨率的CAM仿真进行比较，以确定高分辨率捕获的物理和动力学。一种假设是，需要更高的分辨率来捕捉海温变化对锋面天气系统穿越墨西哥湾流时上升运动的影响。该项目的另一个目标是使用机器学习来开发在VR模拟中捕获的小尺度效应的参数化，以便能够以更粗略且计算强度较小的模型分辨率来近似地表示它们。由于希望更好地理解北半球冬季环流的十年至十年的变化，这项工作具有社会意义。最近的研究表明，十年环流的可变性比基于气候模型模拟的预期更可预测，这表明模型可能缺少成功进行长期预测所需的关键因素。因此，在这里开展的工作可能有助于开发能够进行具有重大社会价值的长期预测的模型。此外，该项目还有助于VR-CAM的开发，将向世界各地的CESM用户和开发人员提供。首席研究人员通过华盛顿大学气候变化计划以及西雅图太平洋科学中心的科学传播奖学金计划向公众进行宣传。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。

The extent to which the atmospheric circulation responds to changes in the underlying sea surface temperature (SST) is a central topic in climate dynamics. In the tropics the response of atmospheric circulation to SST variability is easily seen in the evolution of El Nino events. But in the middle and high latitudes there is no comparably strong and unambiguous attribution of circulation change to variations in underlying SST. Nevertheless recent research suggests that SST variability associated with the meandering of the Gulf Stream affects the decade-to-decade variability of the wintertime atmospheric circulation over much of the Northern Hemisphere. The observational evidence is suggestive but not conclusive, as the inherent noisiness of circulation variability and the short length of the observational record do not permit definitive attribution. Climate model simulations offer a way around the shortness of the observed record, along with the ability to design controlled experiments, and models using moderate resolution (say one gridpoint per degree of latitude and longitude) can generate very long simulations. But such simulations have not generally shown a circulation response to Gulf Stream SST variations, and recent experiments suggest that much higher resolutions are necessary to capture the physics and dynamics leading to the circulation response. Unfortunately global simulations using such high resolutions are too computationally intensive to permit in-depth experimentation.Research under this award considers the effects of Gulf Stream SST variations using a variable resolution (VR) model in which the grid spacing over most of the world is one degree but resolution over the Arctic and northern North Atlantic is increased to 1/8th degree, or about 12km between gridpoints. The model is a version of the Community Atmosphere Model (CAM), the atmospheric component model of the Community Earth System Model (CESM). Preliminary results show a robust hemispheric circulation response, with a spatial pattern similar to the North Atlantic Oscillation, forced by an imposed SST change representative of a Gulf Stream fluctuation. Further work compares the VR-CAM simulations with CAM simulations at lower resolution to identify the physics and dynamics captured by the high resolution. One hypothesis is that higher resolution is necessary to capture the influence of SST change on the rising motions associated with frontal weather systems as they cross the Gulf Stream. A further goal of the project is to use machine learning to develop parameterizations of the small-scale effects captured in the VR simulations so that they can be represented approximately at coarser and less computationally intensive model resolutions.The work has societal relevance due to the desirability of better understanding of decade-to-decade changes in the wintertime circulation over the Northern Hemisphere. Recent work suggests that the decadal circulation variabililty is more predictable than would be expected based on climate model simulations, suggesting that models may be missing key ingredients needed for successful long-range predictions. The work performed here may thus contribute to the development of models capable of long-range predictions with substantial societal value. In addition, the project contributes to the development of VR-CAM, which will be made available to the worldwide community of CESM users and developers. The Principal Investigators conduct outreach to the public through the University of Washington Program on Climate Change, as well as the Science Communication Fellowship Program at the Pacific Science Center in Seattle.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.