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

热带地区的降雨量可能变化很大，该地区的大量人口很容易受到水分胁迫的影响。最近的研究确定，远距离辐射加热和冷却，例如火山和人为气溶胶的冷却效应，是热带辐合带(热带辐合区；赤道附近海洋上空降水较高的狭窄区域)热带降水南北变化的驱动因素。这种变化与北非萨赫勒地区在20世纪后半叶的长期干旱有关，也可能对其他地区产生影响。对远程加热和冷却作用的研究也解释了为什么太平洋和大西洋的ITCZ通常位于赤道以北，以及上一次冰期的替代数据显示大西洋ITCZ向南移动的原因，当时北半球的大部分地区被高反射率的冰雪覆盖。从半球间热量输送的角度解释了ITCZ位移与远程辐射强迫之间的关系：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输送到较冷的半球，而大气能量将通过Hadley环流的对流层上层分支输送到赤道。但哈德利环流的翻转运动要求，当能量在高层向一个方向传输时，水汽必须以另一种方式传输到接近地表的地方。因此，水汽从较冷的半球越过赤道到较暖的半球，导致ITCZ降水向较暖的半球移动(在今天的气候中，北半球平均比南半球冷，因此ITCZ位于赤道以北)。本项目继续PIS对辐射强迫、半球间热量输送和ITCZ降水南北移动之间的关系的探索。一个关键问题是，该理论假设将在大气中发生跨赤道的热传输，该理论是利用大气模型与不允许海洋传输变化的“板条”海洋模型相结合而发展起来的。初步证据表明，辐射强迫发生在海洋而不是大气中的程度取决于辐射强迫发生的地方，例如，南半球中高纬度地区响应辐射强迫的热量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比在海洋中产生更多的输送。PIS利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量输送分配的相关性，并确定这种分配如何取决于应用辐射强迫的区域。这里探讨的其他问题包括，当辐射强迫发生在陆地而不是海洋表面时，ITCZ反应的差异，以及反应的差异取决于辐射强迫的大小和符号(加热和冷却)。鉴于该地区人口众多和脆弱，热带降水的变化具有极大的实际意义和科学意义。预测未来热带降水变化的尝试依赖于气候模型的输出，但这种指导受到不同模型之间模拟降水变化的巨大差异的阻碍。因此，一个了解降水变化的理论框架是非常可取的。除了研究本身的更广泛的社会影响之外，PI还开展了各种外展活动，包括为K-12年级和在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，从而为这一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.