EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Do CMIP5 emergent constraints on the large scale atmospheric circulation work to constrain CMIP6 projections?

Isla Simpson1, Fances Davenport2, Abdullah Al Fahad3, and Flavio Lehner1,4
Isla Simpson et al.
  • 1National Center for Atmospheric Research, Climate and Global Dynamics Laboratory, Boulder, United States of America (
  • 2Department of Earth System Science, Stanford University, USA
  • 3Atmospheric, Oceanic and Earth Sciences, George Mason University, USA
  • 4Institute for Atmospheric and Climate Science, ETH Zurich, Switzerland

Accurate future projections of the climate system are hindered by a number of sources of uncertainty: forcing uncertainty, internal variability and model structural uncertainty. An ``Emergent constraint'' is a technique that has been devised to reduce projection uncertainties arising from the model structural component. It consists of a statistical relationship (across a model ensemble) between a model’s representation of some aspect of the present day climate and its future projected climate change. This relationship can then be used to imply the future projected change, given the observed value of that present-day aspect. However, in order for the emergent constraint to be considered robust it must: (a) be accompanied by a physical mechanism and (b) be robust to out-of-sample testing.


In prior Coupled Model Intercomparison Projects (CMIP), in particular CMIP5, a number of emergent constraints on the large scale atmospheric circulation were proposed, with implications for regional hydroclimate change. These include: (1) a relationship between a model’s climatological jet latitude and its future projected poleward shift in the Southern Hemisphere; (2) a relationship between a model’s future projected wintertime circulation and hydroclimate change over North America and its climatological representation of stationary waves in the North Pacific; and (3) a relationship between a model’s future projected precipitation change over California and its representation of the relationship between ENSO and California precipitation. Constraints (2) and (3) actually imply opposite constraints on California precipitation changes for the real world, which speaks to the need for a deeper understanding of these emergent constraints and a comprehensive assessment of their robustness.


While the CMIP6 archive does not represent a true ``out-of-sample’’ test of CMIP5 emergent constraints, it does provide us with a new dataset composed of new and/or more advanced models in which to assess their robustness. This presentation will review the proposed emergent constraints on the large-scale atmospheric circulation and assess whether or not they are robust across both the CMIP5 and CMIP6 ensembles. Their potential for constraining regional hydroclimate projections will also be discussed.


How to cite: Simpson, I., Davenport, F., Al Fahad, A., and Lehner, F.: Do CMIP5 emergent constraints on the large scale atmospheric circulation work to constrain CMIP6 projections?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4001,, 2020

This abstract will not be presented.