Decadal Regime Shifts in the Pacific Ocean: Mechanisms, Hydroclimatic Imprints, and Predictability

20年间太平洋海表温度（SST）差异的地图可能会显示一个相对温暖的海表温度（SST）的楔形区域，从北美和南美海岸向西延伸并向赤道方向延伸，覆盖了热带太平洋的大部分地区，两侧是两个半球高纬度的冷海表温度。这种模式和它的负值，一个横跨热带太平洋的较冷海温的楔形，占太平洋年代际海温变率的很大一部分，称为PDV。PDV模式是有趣的，因为它与世界各地长时间的干燥和潮湿条件有关。特别是在美国西部和中部地区，在过去的几十年里，温暖的太平洋楔有利于干旱，而寒冷的楔有利于干旱。在南美洲也发现了暖与湿、冷与干的关联，这是可以预期的，因为楔形的赤道对称，并且强加PDV模式的模式模拟可以重现美洲的湿与干条件。PDV模式引起了相当大的关注，但我们对它的理解受到海温观测记录的长度的限制，这些记录可以追溯到大约一个半世纪，因此只捕捉到少数事件。该奖项下的工作扩展了我们对PDV模式及其与大陆水文气候的联系的理解，利用基于树木年轮和其他代理数据的过去千年的气候重建。这些包括北美干旱地图集（北美干旱地图集，或NADA），季风亚洲（MADA），南美（SADA），欧洲和北非（旧世界干旱地图集）等，以及古流体动力学数据同化（PHYDA）和最后一千年再分析（LMR）产品。这些基于代理的数据集得到了古气候模式比对项目（PMIP4）和群落地球系统模式最后千年集合（CESM LME）的模拟分析的补充。需要解决的一个问题是，PDV模式的行为在多大程度上以其暖相和冷相之间的突然“政权转变”为特征。突变是20世纪PDV模式的一个显著特征，但如果考察更长的时期，渐变可能更为常见。另一个问题是，在前几个世纪，PDV变暖和变冷是否像20世纪那样明显，以及PDV对大陆水文气候的影响是否同样明显。研究还考虑了PDV模式可预测的程度，这是一个通过附加模式模拟（包括来自年代际气候预测项目的初始化后播模拟）来追求的问题。由于长期降雨和干旱对社会和生态的影响，这项工作具有更广泛的影响。考虑到PDV对北美和南美的类似影响，该项目通过邀请来自阿根廷和智利的科学家，并在几所南美大学召开为期一周的研讨会，为在线听众提供远程参与，建立了与南美的研究联系。该项目还设有一个网站，用于发布结果。项目参与者开展各种教育和推广活动，包括在美国自然历史博物馆、哥伦比亚青年峰会和当地公立学校进行演讲。此外，该项目支持一名研究生，从而支持该研究领域的劳动力发展。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

A map of the difference in Pacific sea surface temperature (SST) between two decades might show a wedge-shaped region of relatively warm sea surface temperatures (SSTs) extending west and toward the equator from the coasts of North and South America, covering much of the tropical Pacific and flanked by cold SSTs in the higher latitudes of both hemispheres. This pattern and its negative, with a wedge of colder SSTs spanning the tropical Pacific, account for a large fraction of Pacific decadal SST variability, referred to as PDV. The PDV pattern is of interest due to its association with prolonged periods of drier and wetter conditions around the world. In particular more precipitation is found over the western and central US in decades featuring the warm Pacific wedge while the cold wedge favors drought. The association of warmer with wetter and cooler with drier is also found in South America, as might be expected given the equatorial symmetry of the wedge, and model simulations in which the PDV pattern is imposed can reproduce the wetter and drier conditions over the Americas. The PDV pattern has attracted considerable attention but our understanding of it is limited by the length of the observational record for SSTs, which goes back roughly a century and a half and thus only captures a handful of events.Work under this award extends our understanding of the PDV pattern and its connections to continental hydroclimate using climate reconstructions for the last millennium based on tree rings and other proxy data. These include drought atlases for North America (The North American Drought Atlas, or NADA), Monsoon Asia (MADA), South American (SADA), and Europe and North Africa (the Old World Drought Atlas), among others, as well as the Paleo-Hydrodynamics Data Assimilation (PHYDA) and Last Millennium Reanalysis (LMR) products. These proxy-based datasets are complemented by analysis of simulations from the Paleoclimate Model Intercomparision Project (PMIP4) and the Community Earth System Model Last Millennium Ensemble (CESM LME). One question to be addressed is the extent to which the behavior of the PDV pattern is characterized by abrupt "regime shifts" between its warm and cold phases. Abrupt shifts are a notable feature of the PDV pattern in the 20th century but gradual transitions might be more common if a longer period is examined. Another question is whether the PDV warming and cooling were as pronounced in earlier centuries as in the 20th century, and whether PDV impacts on continental hydroclimate were as pronounced. The research also considers the extent to which the PDV pattern is predictable, a question which is pursued through additional model simulations including initialized hindcast simulations from the Decadal Climate Prediction Project.The work has broader impacts due to the societal and ecological impacts of prolonged pluvials and droughs. Given the similar PDV impacts over North and South America the project builds research connections to South America by hosting scientists from Argentina and Chile and convening a week-long workshop offering remote participation to a an online audience at several South American universities. The project also hosts a website for the dissemination of results. Project participants conduct a variety of education and outreach activities including presentations at the American Museum of Natural History, the Columbia Youth Summit, and local public schools. In addition, the project supports a graduate student, thereby supporting workforce development in this research area.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.