Collaborative Research: Dynamical Mechanisms for Midlatitude-Arctic Interactions and Associated Weather Extremes in a Warming Climate

Persistent weather systems in winter cause cold spells, heavy rainfall, or snow. These weather extremes are closely related to the location and intensity of midlatitude jet streams, which blow from west to east and often shift to the north and south. As Earth’s climate gets warmer, the jet streams are affected by the tug-of-war between the rapid surface warming over the Arctic and the enhanced upper-tropospheric warming in the tropics. The investigators will examine several interrelated dynamical mechanisms for midlatitude-Arctic interactions and the associated weather extremes. The first theme is the linkage between the planetary waves in the stratosphere and the risk of cold events. This task will test the hypothesis that extreme wave events in the stratospheric polar vortex serve as a source of intraseasonal predictability for cold events over North America. This linkage between the stratosphere and surface temperature will be analyzed in observations and climate models. Secondly, the investigators will examine a hypothesis that a weaker jet stream under rapid Arctic warming would lead to more frequent weather extremes in mid-latitudes. This task will perform idealized numerical simulations to examine the effect of jet speed or jet structure on the north and south shifts of a jet and thus attempt to reconcile different views on the changes in midlatitude circulation waviness in response to climate warming. The third task is on atmospheric rivers, the narrow intense moisture transport associated with midlatitude cyclones. Atmospheric rivers dominate the poleward moisture flux in the middle and high latitudes, which contribute to Arctic warming. Intense moisture transport can be influenced by either wind speed or temperature, and the two factors will be studied in an atmospheric transport model of water vapor and cloud tracers.Midlatitude weather extremes have profound socioeconomic impacts. Predictive skills from extreme stratospheric events could improve the sub-seasonal forecast of winter cold spells over North America, with important implications for energy consumption and transportation. A better understanding of extreme cold events in a warming climate can foster better communications of global warming science between the scientific community and the public. Improved predictions of the intense moisture transport by midlatitude cyclones into the Arctic help better adapt to the changing Arctic. Besides supporting an early career scientist at Penn State University, the proposal will train two graduate student researchers at UCLA in the analysis of observations and climate models. Several UCLA undergraduates will be trained in statistical analysis of climate data, particularly for STEM majors and underrepresented minorities. The idealized models developed in this study will be made available on a GitHub website with sample python scripts.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.