Dynamical coupling between the troposphere and stratosphere in Earth System Models

Abstract.
Dynamic coupling between the extratropical tropospheric and stratospheric circulations during
boreal winter and austral spring are primarily mediated on short timescales via vertically
propagating waves generated in the troposphere and their interaction with the stratospheric flow,
but can also be modulated by phenomena that vary on sub-seasonal to decadal timescales. As a
result, stratosphere-troposphere coupling processes need to be taken into account to fully
understand, for example, the occurrence of extreme climate events linked to shifts in the
extratropical storm tracks; climate variability on seasonal time scales such as teleconnections
related to the El Niño-Southern Oscillation (ENSO) and stratospheric Quasi-Biennial Oscillation
(QBO); and the response of the climate system to natural (e.g., solar, and volcanic aerosols) and
anthropogenic (greenhouse gas increase, changes in ozone depleting substances) forcings.
Yet climate models (including those part of CMIP5/6) often lack a proper representation of
stratospheric processes and realistic dynamic stratosphere-troposphere interactions, which can be
related to model-specific configurations and parameterized (e.g., small-scale waves) or absent
(e.g., interactive stratospheric chemistry) processes. These biases are tied to further model biases
in the positions and variability of the tropospheric jets and their connection to extreme events.
We propose to develop process-oriented diagnostics (PODs) to measure the fidelity of key
dynamical stratosphere-troposphere coupling processes and to inform actionable pathways to
future model improvement. Our specific objectives of the proposed project are as follows:
1. Develop PODs that systematically evaluate the two-way stratosphere-troposphere coupling
processes in Earth System Models, and make these PODs publicly available through
implementation in the Model Diagnostics Task Force (MDTF) framework,
2. Apply PODS to CMIP6 simulations (both pre-industrial control and historical) and
simulations of relevant CMIP-Endorsed Model Intercomparison Projects (MIPs) to
diagnose and benchmark how different model configurations impact the representation of
two-way stratosphere-troposphere coupling processes.
This Type 1 proposed project addresses the MAPP competition “Process-Oriented Diagnostics
for Climate Model Improvement and Applications” by developing PODs to benchmark process-
level deficiencies in stratosphere-troposphere coupling in Earth System models. This work would
close a gap in the Model Diagnostics Task Force framework, which currently does not include
PODs that examine these processes and their linkages to related climate phenomena. The
stratosphere-troposphere coupling processes we seek to address are relevant to the Climate
Program Office’s high-priority climate risk areas because they are closely connected with surface
climate extremes driven by changes to the storm tracks and/or teleconnections; and to NOAA’s
mission to advance our understanding of the Earth’s climate system and to use this knowledge to
advance resilience of our Nation. We anticipate our diagnostics and their evaluation to have broad
impacts for better understanding uncertainties in model simulations associated with large-scale
atmospheric dynamics, and for improved representation of coupling processes linked to climate
extremes.