Hemispheric Energy Balance and Tropical Precipitation Shifts: The Impacts of Forcing Location

热带地区的降雨量变化很大，该地区的许多人口容易受到水资源紧张的影响。最近的研究已经确定，远程辐射加热和冷却，例如火山和人为气溶胶的冷却效应，是与热带辐合区（ITCZ;赤道附近海洋上的狭窄高降水区）相关的热带降水南北转移的驱动因素。这种变化与20世纪下半叶北非萨赫勒地区的长期干旱有关，也可能对其他地区产生影响。 对远距离加热和冷却作用的研究也解释了为什么太平洋和大西洋ITCZ通常位于赤道以北，以及为什么上一个冰河时代的代理数据显示大西洋ITCZ南移，当时北方大部分地区覆盖着高反射率的冰雪。ITCZ的变化和远程辐射强迫之间的关系被解释在半球间的热量输送：如果一个半球相对于另一个半球冷却，热量将从较暖的半球向较冷的半球输送，和大气能量输送通过赤道将发生通过对流层上层分支的Hadley环流。 但是，哈德利环流的翻转运动要求，当能量在上层以一个方向传输时，水分必须在表面附近以另一个方向传输。 因此，湿气从较冷的半球穿越赤道到达较暖的半球，导致ITCZ降水向较暖的半球转移（在今天的气候中，北方半球平均比南半球冷，因此ITCZ位于赤道以北）。本项目继续了PI对辐射强迫、半球间热量输送、ITCZ降水的南北移动。 一个关键的问题是，该理论假设跨赤道热传输将发生在大气中，该理论是使用大气模型耦合到“平板”海洋模型，不允许海洋传输的变化。 初步证据表明，在海洋而不是在大气中发生输送的程度取决于辐射强迫发生的位置，例如，南半球中高纬度地区对辐射强迫的热输送似乎主要发生在海洋中，而北大西洋的辐射强迫在大气中产生的输送比在海洋中产生的输送更多。 研究人员使用共同体地球系统模式（CESM）进行实验，以了解大气和海洋之间热传输分配的依赖性，并确定这种分配如何取决于辐射强迫应用的区域。这里追求的其他问题包括在ITCZ响应的差异时，发生在陆地而不是海洋表面的辐射强迫，并在响应的差异取决于大小和符号（加热与冷却）的辐射forcing.Shifts在热带降水是非常实际的，以及科学的兴趣，鉴于该地区的人口众多和脆弱性。预测热带降水未来变化的尝试依赖于气候模式的输出，但这种指导受到不同模式之间模拟降水变化的巨大差异的阻碍。 因此，理解降水变化的理论框架是非常可取的。 除了研究本身对社会产生的更广泛影响外，PI还开展了各种推广活动，包括为K-12年级和西雅图科学中心等当地场所制作教育视频。 此外，该项目还支持和指导一名研究生和一名博士后研究助理，从而为该研究领域的未来劳动力提供支持。

Rainfall in the tropics can be highly variable, and much of the region's large population is vulnerable to water stress. Recent research has identified remote radiative heating and cooling, for example the cooling effect volcanic and anthropogenic aerosols, as a driver of north-south shifts in tropical precipitation associated with the intertropical convergence zones (ITCZs; narrow regions of high preciptation over the oceans near the equator). Such shifts are implicated in the prolonged drought in the Sahel region of North Africa during the latter half of the 20th century, and could be consequential for other regions as well. Research on the role of remote heating and cooling has also produced an explanation for why the Pacific and Atlantic ITCZs are typically north of the equator, and for the southward shift of the Atlantic ITCZ shown in proxy data from the last ice age, when much of the Northern Hemisphere was covered with highly reflective ice and snow. The relationship between ITCZ shifts and remote radiative forcing is explained in terms of inter-hemispheric heat transport: if one hemisphere is cooled relative to the other heat will be transported from the warmer hemisphere toward the colder hemisphere, and atmospheric energy transport across the equator will take place through the upper-tropospheric branch of the Hadley circulation. But the overturning motion of the Hadley cell requires that as energy is transported in one direction at upper levels moisture must be transported the other way near the surface. Thus moisture crosses the equator from the colder hemisphere to the warmer hemisphere, resulting in a shift of ITCZ precipitation toward the warmer hemisphere (in today's climate the Northern Hemisphere is colder on average than the Southern Hemisphere, thus the ITCZ is north of the equator).This project continues the PIs' exploration of the relationship between radiative forcing, inter-hemispheric heat transport, and north-south shifts of ITCZ precipitation. One key issue is that the theory assumes that cross-equatorial heat transport will occur in the atmosphere, and the theory was developed using atmospheric models coupled to "slab" ocean models which do not allow changes in ocean transport. Preliminary evidence suggests that the extent to which the transport occurs in the ocean rather than the atmosphere depends on where the radiative forcing occurs, for example heat transport in response to radiative forcing in the middle and high latitudes of the Southern Hemisphere appears to occur primarily in the ocean, while radiative forcing in the North Atlantic yields more transport in the atmosphere than in the ocean. The PIs perform experiments with the Community Earth System Model (CESM) to understand the dependence of the partitioning of heat transport between the atmosphere and ocean and to determine how this partitioning depends on the region in which the radiative forcing is applied. Other issues pursued here include differences in ITCZ response when the radiative forcing occurs over land rather than ocean surfaces, and differences in the response depending on the magnitude and sign (heating versus cooling) of the radiative forcing.Shifts in tropical precipitation are of great practical as well as scientific interest given the region's large population and vulnerability. Attempts to anticipate future changes in tropical precipitation rely on output from climate models, but such guidance is hampered by large differences in simulated precipitation change from one model to another. A theoretical framework for understanding precipitation change is thus highly desirable. Beyond the societal broader impacts of the research itself, the PIs perform a variety of outreach activities, including educational videos for grades K-12 and at local venues such as the Seattle Science Center. In addition, the project supports and mentors a graduate student and a postdoctoral research associate, thereby providing for the furture workforce in this research area.