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

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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.