RAPID: Detectability of Changes in the Walker Circulation in Response to Global Warming

这是由一封亲爱的同事来信(NSF 11-006)资助的16个快速反应(RAPID)项目之一，该信鼓励对为政府间气候变化专门委员会第五次评估报告(IPCC AR5)准备的气候模型模拟进行诊断分析。在这些项目中进行的研究有望导致更详细的模式相互比较，更好地理解稳健的模式行为，以及更好地理解和量化未来气候模拟中的不确定性。Walker环流是一种横跨赤道太平洋的东西倾覆环流，与西太平洋和海洋大陆的低海平面气压(SLP)和赤道东太平洋的高SLP有关。Walker环流强度的变化是热带气候变化的主要形式，与降雨量和其他气候变量的大范围波动有关，全球变暖迫使Walker环流的长期变化可能会产生同样的结果。本文研究了20世纪为AR5进行的气候模拟中Walker环流对全球变暖的响应，并将模型模拟与可用观测进行了比较。以前的工作表明，模式模拟与20世纪赤道太平洋东西向海平面气压(SLP)梯度的小幅减小基本一致，这表明Walker环流略有减弱。但仍有几个问题需要解决：1)Walker环流的减弱取决于全球平均降水对变暖的敏感性，对于这一点，气候模式和最近过去的观测之间并不一致；2)尚不清楚所观测到的变暖的特征是类似厄尔尼诺的还是类似拉尼娜的，这对赤道东西向SLP趋势有影响；3)SLP梯度的观测变化可能是与太平洋盆地强烈的低频变率有关的虚假趋势。为解决这些问题，海洋观测中心将对20世纪期间下列变量的线性趋势进行模式观测比较：东西SLP梯度、热带平均降水量、热带平均水汽含量、东西海表面温度梯度、温跃层倾斜和赤道太平洋的热层结。在可能的情况下，将使用同一模型的多个模拟来评估自然变化对20世纪趋势的潜在贡献。该项目的更广泛影响在于它支持IPCC AR5，该项目旨在向世界各地的决策者提供关于气候变化及其后果的信息。Walker环流变率在热带气候变率中起主导作用，影响全球人口稠密和气候敏感地区的降雨量和其他气候变量的分布。全球变暖导致的沃克细胞的变化可能会产生同样的影响。因此，这里进行的关于Walker环流中观察到的变化在多大程度上构成自然可变性或气候变暖引起的变化，以及气候模型模拟在多大程度上能够准确地描绘产生这些变化的基本机制的研究，可能会对关注热带气候变化影响的决策者产生重要影响。

This is one of 16 Rapid Response (RAPID) projects funded as the result of a Dear Colleague Letter (NSF 11-006) encouraging diagnostic analyses of climate model simulations prepared for the Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC AR5). Research conducted in these projects is expected to lead to more detailed model intercomparisons, better understanding of robust model behaviors, and better understanding and quantification of uncertainty in future climate simulations.The Walker circulation is an east-west overturning circulation that spans the equatorial Pacific Ocean, associated with low sea level pressure (SLP) over the western Pacific and the Maritime Continent and higher SLP in the eastern equatorial Pacific. Variability of the strength of the Walker cell is a dominant form of climate variability in the tropics, associated with large-scale fluctuations in rainfall and other climatic variables, and secular change in the Walker cell forced by global warming would likely have the same consequences.Work performed here examines the response of the Walker circulation to global warming in 20th century climate simulations performed for the AR5, and compares model simulations with available observations. Previous work has shown that model simulations are generally consistent with the observed small reduction of the east-west sea level pressure (SLP) gradient across the equatorial pacific over the 20th century, which indicates a slight weakening of the Walker circulation. But several issues remain to be addressed: 1) the weakening of the Walker circulation depends on the sensitivity of global mean precipitation to warming, for which there is no agreement between climate models and observations over the recent past; 2) it is not clear whether the signature of the observed warming is El Nino-like or La Nina-like, which has implications for the east-west equatorial SLP trend; 3) it is possible that the observed changes in the SLP gradient are a spurious trend associated with the strong low frequency variability of the Pacific basin. To address these issues, the PI would conduct a model-observation comparison of linear trends in the following variables during the 20th century: the east-west SLP gradient, tropical-mean precipitation, tropical-mean water vapor content, the east-west sea surface temperature gradient, the tilt of the thermocline, and the thermal stratification of the equatorial Pacific Ocean. Where possible, multiple simulations from the same model would be used to assess the potential contribution of natural variability to the 20th-century trends.The broader impact of the project lies in its support of the IPCC AR5, which is intended to provide information on climate change and its consequences to decision makers worldwide. Walker cell variability plays a dominant role in the variability of tropical climate, affecting the distribution of rainfall and other climatic variables over heavily populated and climatically sensitive regions worldwide. Changes in the Walker cell due to global warming would likely have the same effects. Research conducted here regarding the extent to which the observed changes in the Walker cell constitute natural variability or warming-induced change, and the extent to which climate model simulations can accurately portray the fundamental mechanisms producing the changes, could thus have important implications for decision makers concerned with the implications of tropical climate change.