Mechanisms of Climate Variability and Change

这项工作解决了决定平均气候、气候变异性和大气环流对温室气体引起的全球变暖的反应的基本物理机制。许多研究都集中在云辐射效应和大气环流的相互作用上。需要解决的一个问题是，在赤道西太平洋，云的辐射效应引起的大气加热和相关的水蒸气与该区域的能量输出之间存在密切联系。PI假设，云和水汽异常对大气的辐射加热在水平能量输出中起决定性作用，部分原因是对流层上层的云辐射加热增加了对输出的需求。研究进一步考虑了对流层上层云辐射加热将热带辐合带(ITCZ)限制在赤道附近的一条窄带上的可能影响。考虑到气候模式中持续存在的双重ITCZ偏差，云辐射效应在限制ITCZ方面的作用将是令人感兴趣的，因为这些模式在赤道两侧错误地产生了ITCZ。进一步的研究考虑了云辐射效应在确定大气环流对全球变暖反应的关键特征方面的作用，包括哈德利环流的扩大(伴随着亚热带干燥地区的扩大)和模式模拟中常见的中纬度急流的极移。需要解决的一个相关问题是南大洋上空短波云辐射反馈中的偶极子，在低纬度地区反馈为负(因此有助于抵消温室效应)，但在高纬度地区为正反馈。PI和他的合作者之前的工作表明，高纬度地区的正反馈是由于云层变亮，因为更温暖的温度有利于云层中的液滴而不是冰粒。要讨论的另外两个专题是在赤道太平洋部分产生类似厄尔尼诺现象的机制，以及自然变化对南极洲周围海冰覆盖增加的趋势和北极海冰减少的贡献。这项工作是通过观测分析和各种数值模式模拟进行的，例如，考察了在云对非气候相互比较实验中进行的实验，以了解云辐射效应在能量输出和ITCZ限制中的作用。该项目具有更广泛的社会影响，因为更好地了解气候系统对于预测和应对气候多变和变化可能产生的后果具有价值。此外，该项目还资助了三名研究生，从而促进了下一代气候科学研究人员的发展。国际和平协会还通过各种场所与公众和利益攸关方进行接触。

This work addresses basic physical mechanisms that determine mean climate, climate variability and the atmospheric circulation response to greenhouse gas-induced global warming. Much of the research focuses on the interaction of cloud radiative effects and atmospheric circulation. One issue to be addressed is the close association over the western equatorial Pacific between atmospheric heating due to the radiative effects of clouds and associated water vapor and energy export from the region. The PI hypothesizes that the radiative heating of the atmosphere by clouds and vapor anomalies plays a deciding role in horizontal energy export, in part because cloud radiative heating in the upper troposphere increases the requirement for export. The research further considers the possible effect of upper-tropospheric cloud radiative heating in confining the intertropical convergence zone (ITCZ) to a narrow strip near the equator. A role for cloud radiative effects in confining the ITCZ would be of interest given the persistent double-ITCZ bias in climate models, in which models incorrectly produce an ITCZ on either side of the equator. Further research considers the role of cloud radiative effects in determining key features of the atmospheric circulation response to global warming, including the expansion of the Hadley cell (accompanied by an expansion of the dry regions in the subtropics) and the poleward shift of the midlatitude jet streams commonly found in model simulations. A related issue to be addressed is the dipole in shortwave cloud radiative feedback over the Southern Ocean, where the feedback is negative in lower latitudes (where it thus serves to counteract greenhouse warming) but positive in higher latitudes. Previous work by the PI and his collaborators suggests that the positive feedback at higher latitudes is due to cloud brightening as warmer temperatures favor liquid droplets over ice particles in clouds. Two other topics to be addressed are the mechanisms which produce El Nino-like conditions in the equatorial Pacific sector, and the contribution of natural variability to the increasing sea ice cover trend around Antarctica and the decline of sea ice in the Arctic. The work is conducted through analysis of observations and a variety of numerical model simulations, for example the experiments conducted in the Clouds On Off Climate Intercomparison Experiment are examined to understand the role of cloud radiative effects in energy export and ITCZ confinement. The project has societal broader impacts due to the value of improved understanding of the climate system for anticipating and responding to the likely consequences of climate variability and change. In addition, the project supports three graduate students, thus promoting the development of the next generation of climate science researchers. The PI also engages in outreach to the public and stakeholders through a variety of venues.