Collaborative Research: Tropical Forcing of Hydrological Variations Over North America during the Last Millennium

该项目将调查一种假设，即持续的北美干旱是由热带地区持续的拉尼娜条件或厄尔尼诺-南方涛动变化的减少和厄尔尼诺现象的缺失所造成的。干旱的机制将通过对仪器观测时期（1856年以后）的观测和模式模拟的详细分析来确定。这些知识将用于理解过去一千年和全新世中期的干旱，试图将古水文变化归因于热带太平洋珊瑚记录的厄尔尼诺-南方涛动的变化。将利用珊瑚化石记录估计过去1100年和全新世中期的热带太平洋海面温度，然后用于强迫气候模式的整体整合。伴生整体将进一步包括太阳辐照度变化和火山活动的外部强迫，以及全新世中期的轨道变化。对这些整体的比较将突出外部强迫的直接影响和厄尔尼诺-南方涛动的变化对北美降水的相对重要性。模拟的水文变化将与从树木年轮记录（库克干旱地图集）以及湖沼学、地貌学和历史证据推断的水文变化进行比较。将使用一个中间耦合模式和一个耦合的全球环流模式来研究过去一千年热带太平洋变化的原因，这两个模式将受到太阳辐照度、火山活动和轨道构型变化的影响。这些模拟将对中世纪暖期和小冰期蒙德极小期进行，以确定中间模式模拟的厄尔尼诺-南方涛动的变化是否可以在更完整的模式中重现，并检查耦合动力学对全球其他地区的影响。这项研究预计将提高对过去几千年干旱机制的理解，改善在水资源和农业规划决策中所依据的知识基础，以期对未来持续干旱的风险提供更可靠的评估。

This project will investigate the hypothesis that persistent North American droughts are forced by persistent La Nina conditions in the tropics or by reduced variance of the El Nino-Southern Oscillation and an absence of El Ninos. The mechanisms of drought are to be worked out by detailed analysis of observations and model simulations of the period of instrumental observations (post 1856). This knowledge will be applied to understanding drought over the last millennium and in the mid-Holocene in an attempt to attribute the paleohydrological variations to changes in the El Nino-Southern Oscillation recorded by tropical Pacific corals. Tropical Pacific sea surface temperatures will be estimated for periods within the last 1100 years and for the mid-Holocene using fossil coral records and then used to force ensemble integrations of a climate model. Companion ensembles will further include external forcing by altered solar irradiance and volcanism and, for the mid-Holocene, orbital changes. Comparison of the ensembles will highlight the relative importance for North American precipitation of the direct impact of external forcings and the changes in the El Nino-Southern Oscillation. Simulated hydrological variations will be compared to those inferred from tree ring records (the Cook Drought Atlas) and limnological, geomorphological and historical evidence.The causes of the tropical Pacific changes over the last millennium will be investigated using an intermediate coupled model and a coupled global circulation model which will be subjected to changes in solar irradiance, volcanism and orbital configuration. These simulations will be conducted for the Medieval Warm Period and the Maunder Minimum period of the Little Ice Age, to ascertain if the changes in the El Nino-Southern Oscillation simulated by the intermediate model are reproducible in a more complete model, and to examine the impacts of coupled dynamics in other regions of the globe. This research is expected to improve the understanding of the mechanisms of drought over the last few millennia, improving the knowledge base upon which decisions in water resource and agricultural planning are made, with a view to offering a more reliable assessment of the future risks of persistent drought.