Understanding How Latent Heat Release and Cloud Radiative Effects Shape Jet Variability and Jet Shifts Under Climate Change

University Of California-San Diego Scripps Inst Of Oceanography, La Jolla CA

Investigators

Abstract

Jets flying over the middle latitudes of either the Northern or Southern Hemisphere commonly encounter eastward winds with speeds over 100 miles per hour, substantially reducing flight times going east. The presence of such strong flow in the upper troposphere exerts a controlling influence on middle latitude weather systems and even subtle changes in the jet streams can be consequential for surface weather. Simulations of climate change due to greenhouse warming typically show shift of the jet streams toward higher latitudes in both hemispheres, and observations show indications that poleward shifting has already occurred. The poleward shift is similar to the unforced natural variability of the longitudinally averaged jet streams, in which north-south shifting is the most prominent mode. Work under this award considers the role of moisture in the natural north-south shifting of the jet streams and their poleward movement in response to greenhouse gas increases. Moisture can affect the mean jet position through the heating that occurs when it condenses to form cloud droplets, and the clouds cause additional heating and cooling through cloud radiative effects (CREs), in particular cooling by reflecting sunlight back to space and warming by the trapping of outgoing longwave radiation. Recent work suggests that CREs play a strong role in the poleward drift of jets found in climate change simulations, but the exact mechanism has not yet been determined and little work has been done on the role of CREs in natural variability. The role of latent heating in the poleward drift is also relatively unexamined, and two recent studies disagree as to whether latent heating reduces or increases the persistence of naturally occurring jet displacements. The research uses a suite of simplified models, in which moist effects can be added in a controlled manner, to study the influence of latent heating and CREs on north-south jet fluctuations and forced poleward drift. A further resource for the work is the archive of model simulations produced for the Clouds On-Off Klimate Intercomparison Experiment (COOKIE), in which comprehensive climate models are integrated with and without CREs. The work has societal relevance given the effects of jet shifts on surface weather, including their role in determining the frequency and severity of extreme events associated with frontal storms and blocking events. The project provides support and training to a graduate student and postdoctoral research associate, thereby developing the scientific workforce in this research area. The project also recruits an undergraduate student each summer through the Scripps Undergraduate Research Fellowship (SURF) program. In addition, the award supports high school science education through the purchase and use of rotating tanks from the DIYnamics Project, which are used for classroom demonstrations of jet streams, cyclones, and other features of the atmospheric circulation. 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.