Isla Simpson, National Center for Atmospheric Research (NCAR), Boulder, CO
Future projections in the large-scale atmospheric circulation response to climate change: uncertainties, implications and progress
Accurate future projections of the climate system are hindered by a number of sources of uncertainty: forcing uncertainty, internal variability, and model structural uncertainty. Accurately projecting forced changes in the large scale atmospheric circulation is key to projecting future regional climate and hydroclimate but many uncertainties in how the atmospheric circulation will evolve in the future still exist. This talk will review the projections of the large-scale atmospheric circulation as simulated by numerical Earth System Models that participated in the Coupled Model Intercomparison Projects 3, 5 and 6 i.e., model generations which span from the early 2000's to present day. The extent to which atmospheric circulation projections and their uncertainties have evolved as Earth System Models have increased in complexity will be assessed and the scope for narrowing down uncertainties using a method known as emergent constraints will be discussed. Emergent constraints are statistical relationship, across model ensembles, between an aspect of the present day climate and an aspect of future projected climate change. If they are robust and can be understood, then they offer the potential to narrow down the uncertainties resulting from inter-model differences. It will be assessed whether various previously proposed emergent constraints on jet stream position, atmospheric stationary wave responses and changes in the stratospheric polar vortex have stood the test of time and can be used to effectively narrow down projection uncertainties in these features.