SGER: Understanding Tropical-Subtropical Forcing and Predictability of Long-Term North American Drought in Coupled Models

这是气候变率和可预测性计划（CLIVAR）试点项目DRICOMP下的一笔拨款，该项目是干旱耦合模式项目的一部分，重点是对干旱机制进行初步探索，因为它们在全球气候模型的输出中有所体现，并试图评估这些模型在模拟干旱方面的可靠性。越来越多的文献表明，厄尔尼诺-南方涛动（ENSO）是全球干旱变率的一个基本驱动因素，而类似拉尼娜的条件在导致北美干旱的环流异常中起着关键作用。海洋-大气耦合系统如何在赤道太平洋维持持续的凉爽条件，这是一个重要但鲜为人知的过程，对理解和预测北美长期干旱具有重要意义。本项目将初步尝试回答以下四个问题：(1)在最先进的耦合气候模式中，赤道太平洋的持续冷期是如何表现的；(2)这些条件是通过什么物理机制导致北美持续干旱的；(3)对这些问题的理解如何有助于延长北美长期干旱的预测提前时间；(4)可预测性的机制和前景如何依赖于转型气候的本质？将采取双管齐下的方法：首先，根据其海洋成分的细节确定的一系列耦合模式将用于描述和解释赤道太平洋持续凉爽条件的表征；其次，北美长期干旱变率（根据帕尔默干旱严重指数- PDSI）及其与赤道太平洋的关系，包括它如何依赖于人为强迫，将在一整套气候模式比较项目3 （CMIP3）气候模式中进行分析。通过确定远强迫场本身（即赤道太平洋海温的低频演变）是如何被调制的，以及该信号与PDSI之间的一致性，将探讨延长预期的可预测性问题。这些研究的更广泛的影响在于其对改善北美干旱预测的潜在贡献，以及对主要研究人员（研究生）的支持。

This is a grant under a Climate Variability and Predictability (CLIVAR) Program pilot project called DRICOMP, for the Drought in Coupled Models Project, which focuses on making initial explorations into the mechanisms of drought as they are represented in the output of global climate models and on attempting to assess the reliability of these models in simulating drought.A growing body of literature indicates that El Nino - Southern Oscillation (ENSO) is a fundamental driver of global drought variability and that La Nina-like conditions play a key role in the circulation anomalies leading to North American drought. An important yet poorly understood process with strong implications for understanding and predicting long-term North American drought is how the coupled ocean-atmosphere system maintains persistent cool conditions in the equatorial Pacific Ocean.This project will make an initial attempt to answer four questions: (1) how are persistent cool episodes in the equatorial Pacific Ocean represented in state-of-the-art coupled climate models, (2) through what physical mechanisms do such conditions lead to persistent North American drought, (3) how can an understanding of such questions serve to extend the predictive lead time for long-term North American drought, and (4) how do the mechanisms and prospects for predictability depend on the essence of a climate in transition?A two-pronged approach will be taken: first, a selection of coupled models as determined from the details of their ocean component will be used to characterize and explain the representation of persistent cool equatorial Pacific conditions; secondly, long-term North American drought variability (in terms of the Palmer drought severity index - PDSI) and its relationships with the equatorial Pacific, including how that appears to depend on anthropogenic forcing, will be analyzed in the full suite of Climate Model Intercomparison Project 3 (CMIP3) climate models. The problem of predictability at extended lead times will be explored by ascertaining how the remote forcing field itself (i.e., low-frequency evolution of equatorial Pacific SST) is modulated, and the coherence between that signal and the PDSI.Broader impacts of these are in its potential contribution to improved prediction of North American drought at extended leads and in supporting the principal investigator, who is a graduate student.