Mechanisms of Climate Variability and Change

这项工作涉及确定平均气候、气候变率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。 大部分的研究集中在云辐射效应和大气环流的相互作用。 需要解决的一个问题是，在赤道西太平洋，由于云的辐射效应而引起的大气加热与相关的水蒸气和该区域的能量输出之间存在着密切的联系。 PI假设云和蒸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部的云辐射加热增加了输出的需求。 研究进一步考虑了对流层上层云辐射加热在将热带辐合带（ITCZ）限制在赤道附近的一个狭窄带中的可能影响。 云辐射效应在限制ITCZ中的作用将是令人感兴趣的，因为气候模型中存在持续的双ITCZ偏差，其中模型错误地在赤道两侧产生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的反应的关键特征方面的作用，包括哈德利环流圈的扩大（伴随着亚热带干旱地区的扩大）和在模型模拟中常见的中纬度急流向极移动。 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，在低纬度地区反馈是负的（因此有助于抵消温室效应），但在高纬度地区是正的。 PI和他的合作者以前的工作表明，高纬度地区的正反馈是由于云变亮，因为温暖的温度有利于云中的液滴而不是冰颗粒。 另外两个要讨论的专题是在赤道太平洋地区产生类似厄尔尼诺现象的机制，以及自然变异对南极洲周围海冰覆盖面增加趋势和北极海冰减少的影响。 这项工作是通过分析观测和各种数值模式模拟进行的，例如，在云开闭气候相互比较实验中进行的实验被检查，以了解云辐射效应在能量输出和ITCZ约束中的作用。 该项目具有更广泛的社会影响，因为它有助于更好地了解气候系统，以预测和应对气候变异性和变化的可能后果。 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。 PI还通过各种渠道与公众和利益攸关方进行外联。

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.