Warm Pliocene mid-latitude upwelling sites, with implications to future southwestern North America aridity under climate change

上新世早期(530万年前)是最近的自然发生的暖期。它的二氧化碳浓度估计为400ppm，与目前由于人为排放而达到的水平相似。上新世的气候经常被认为是21世纪气候的过去类比。今天，寒冷的深水在加州沿海的海洋上升流系统中浮出水面。气候重建表明，上新世期间该地区的地表水比现在高出约9摄氏度。其他证据表明，当时北美西南部非常潮湿，与今天形成鲜明对比。这些上新世的重建与政府间气候变化专门委员会(IPCC)对不久的未来气候的预测有很大不同。政府间气候变化专门委员会的预测表明，美国西南部的部分地区将变得更干燥，而不是更潮湿。IPCC对加州上升流系统的预测非常不确定。这项拟议的研究将检验对上新世观测到的可能的解释。这将有助于更好地了解中纬度沿岸上升流系统未来可能发生的变化。这样的系统对渔业至关重要。这项研究还将使我们更好地理解我们在气候变暖的情况下预测北美西南部干旱的能力。该项目更广泛的影响包括对研究生、做学期和暑期研究项目的本科生以及高中生暑期实习生的培训。更广泛的影响活动还将包括通过一门关于气候变化科学的公开课向STEM大学生进行宣传。计划中的工作将审查可能解释这些过去气候观测结果的拟议反馈。在这种反馈中，较弱的上升流导致上升区海面温度(SST)变暖，从而增强邻近陆地上的降水并在那里形成湿地，然后湿地的存在削弱了有利于上升流的沿海风，从而进一步削弱了上升流本身。这项研究的意义源于对一种具有重要社会经济后果的新型反馈的探索。这项工作将研究海洋上升流系统的一些相关基本方面，并使用海洋和大气模型的层次结构，这将提供机会来测试建议的反馈机制并研究其结果。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 early Pliocene (5.3–3 million years ago) was the most recent naturally-occurring warm period. It had an estimated carbon dioxide concentration of 400 ppm, similar to the level reached at present due to anthropogenic emissions. The Pliocene climate is often held up as a past analog for 21st-century climate. Today, cold deep water rises to the surface in the California coastal ocean upwelling system. Climate reconstructions suggest that the surface water in that region during the Pliocene was dramatically warmer than at present—by some 9 degrees C. Other evidence suggests that southwestern North America was very wet at that time, also in contrast to today. Those Pliocene reconstructions differ significantly from Intergovernmental Panel on Climate Change (IPCC) projections of near future climate. The IPCC projections suggest that parts of the southwestern USA will get drier, not wetter. The IPCC projections of the California upwelling system are very uncertain. The proposed research will examine a possible explanation of the Pliocene observations. It will lead to a better understanding of possible future changes to mid-latitude coastal upwelling systems. Such systems are critical to the fishing industry. The study will also lead to a better understanding of our ability to predict the aridity of southwest North America in a warmer climate. The project Broader Impacts include training of a graduate student, undergraduate students doing term and summer research projects, and high school student summer interns. The Broader Impacts activities will also include outreach to STEM college students via an open course on the science of climate change.The planned work will examine a proposed feedback that may explain these past climate observations. In this feedback, weaker upwelling leads to a warmer upwelling-region sea-surface temperature (SST), which enhances precipitation over the adjacent land and creates wetlands there, and then the existence of wetlands weakens the upwelling-favorable coastal winds and, therefore, further weakens the upwelling itself. The significance of the proposed study arises from the exploration of a novel feedback with important socioeconomic consequences. This work will study some related fundamental aspects of ocean upwelling systems and use a hierarchy of ocean and atmospheric models that will provide the opportunity to test the proposed feedback mechanism and study its consequences.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.