Seminar

Abstract

We analyze the sources for spread in the response of the Northern Hemisphere wintertime stratospheric polar vortex (SPV) to global warming in Climate Model Intercomparison Project Phase 5 (CMIP5) and Phase 6 (CMIP6) model projections. About half of the intermodel spread in SPV projections by CMIP6 models, but less than a third in CMIP5 models, can be attributed to the intermodel spread in stationary planetary wave driving. In CMIP6, SPV weakening is mostly driven by increased upward wave flux from the troposphere, while SPV strengthening is associated with increased equatorward wave propagation away from the polar stratosphere. We test hypothesized factors contributing to changes in the upward and equatorward planetary wave fluxes and show that an across-model regression using projected global warming rates, strengthening of the subtropical jet and basic state lower stratospheric wind biases as predictors can explain nearly the same fraction in the CMIP6 SPV spread as the planetary wave driving (r = 0.67). The dependence of the SPV spread on the model biases in the basic state winds offers a possible emergent constraint; however, a large uncertainty prevents a substantial reduction of the projected SPV spread. The lack of this dependence in CMIP5 further calls for better understanding of underlying causes. Our results improve understanding of projected SPV uncertainty; however, further narrowing of the uncertainty remains challenging.

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Alexey Karpechko is a research professor in the Meteorological Research Unit at the Finnish Meteorological Institute (FMI), Finland. Dr Karpechko graduated from the Russian Hydrometeorological University, Russia, in 1996 and earned a PhD degree in Meteorology from the University of Helsinki in 2007. He's been working at FMI since 2002 and also worked as a research scientist at Polar Geophysical Institute, Russia (1997-2002) and as a postdoc researcher at the University of East Anglia, UK (2007-2009). He is primarily working on the large-scale atmospheric dynamics, stratosphere processes, and stratosphere-troposphere dynamical coupling with a focus on stratospheric role in seasonal and sub-seasonal predictability. He served as a Scientific Steering Group member of the WCRP/SPARC core project in 2015-2017 and was a Lead Author of the 2018 UNEP/WMO Ozone Assessment Reports in 2018.