Nonlinear Dynamics of Daily-weekly Boreal Spring InterTropical Convergence Zone (ITCZ) Shifts over the Eastern Pacific Ocean

教科书上的世界风模式图表显示，赤道周围有一个狭窄的带，南北半球的信风在那里交汇，造成上升运动、深云和大雨。这张简单的静态图片作为一个强有力的概念框架，解释了热带辐合带（ITCZ）的存在，以及它与全球大气环流和地球气候能量平衡的联系。但是现实世界的ITCZ不是静态的，虽然从静态图像发展出来的理论可以解释其南北季节性迁移和伴随冰河期的长期变化，但它们可能缺乏解释ITCZ短期变异性所需的关键成分。在这里进行的工作主要集中在最近对北半球春季东太平洋的研究中确定的短期ITCZ波动动态（Haffke et al 2016）。这些波动包括从北半球到南半球的移动和分裂事件，其中单个ITCZ被一对辐合带所取代，其中一对辐合带位于赤道两侧。这种变化和分裂发生在几天的时间里，该项目试图根据大气边界层（ABL，大气的最低一公里左右）的风的变化来解释它们。首席研究员（PI）假设，这些波动是由影响ITCZ区域的大尺度压力梯度的相对细微变化驱动的。气压梯度导致赤道附近ABL风的南北分量变化，这些风对水汽和经向动量的非线性平流导致ITCZ快速移动或分裂。通过观测分析、纵向平均ABL动量方程和理想化的计算机模拟，对这一假设进行了检验和探索。这项工作的更广泛影响包括通过爱荷华州立大学的教师研究经验（RET）计划开展的教育活动，目标是初中和高中科学教师。来自爱荷华州中部学校的教师参加了为期六周的暑期项目相关研究，还参加了一系列关于科学教学教学法的研讨会和定期讨论小组。在项目后期，项目负责人和他所在部门的学生参观了项目教师所在的学校，参加了全天的天气和气候科学活动。这些活动包括演讲和实践活动，其中一些活动使用旋转水箱来展示天气系统的动态。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Textbook diagrams of the world's wind patterns show a narrow band around the equator where the tradewinds of the Northern and Southern Hemispheres flow together, causing rising motions, deep clouds, and heavy rain. This simple, static picture has served as a powerful conceptual framework to explain the existence of the band, know as the Intertropical Convergence Zone (ITCZ), and its connection to the global atmospheric circulation and the energy balance of earth's climate. But the real-world ITCZ is not static, and while theories developed from the static picture can account for its north-south seasonal migration and longer-term shifts accompanying ice ages, they may lack key ingredients needed to explain short-term ITCZ variability.Work performed here focuses specifically on the dynamics of short-term ITCZ fluctuations identified in a recent study of the Eastern Pacific in Northern Hemisphere Spring (Haffke et al 2016). The fluctuations include shifts from the Northern to the Southern Hemisphere and splitting events in which a single ITCZ is replaced by a pair of convergence zones, one on either side of the equator. The shifts and splits take place over the course of a few days and the project seeks to account for them in terms of changes in the winds of the atmospheric boundary layer (ABL, the lowest kilometer or so of the atmosphere). The Principal Investigator (PI) hypothesizes that these fluctuations are driven by relatively subtle changes in the large-scale pressure gradients affecting the region of the ITCZ. The pressure gradients lead to changes in the north-south component of the ABL winds near the equator, and the nonlinear advection of moisture and meridional momentum by these winds causes rapid ITCZ shifting or splitting. This hypothesis is tested and explored through a combination of observational analysis, focused on the longitudinally averaged ABL momentum equation, and idealized computer simulations.The broader impacts of the work include educational activity conducted through Iowa State University's Research Experiences for Teachers (RET) program targeted at middle and high school science teachers. Teachers from central Iowa schools participate in research related to this project over six summer weeks and also participate in a seminar series and regular discussion group on the pedagogy of science teaching. Later in the project the PI and students from his department visit the schools of the teachers in the program for all-day events devoted to the science of weather and climate. The events include presentations and hands-on activities, some using rotating tanks to demonstrate the dynamics of weather systems.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.