Influence of the model lid on the simulated tropospheric climate

Many studies have shown that the stratosphere can influence the tropospheric climate, however many climate models do not have a well resolved stratosphere. We examine here the influence of a raised model lid and better resolved stratosphere on the changes in the simulation of mean climate and climate variability in the Community Earth System Model (CESM).

We examine the frequency and dynamics of sudden stratospheric warmings (SSWs) and show that the coupling between the stratosphere and troposphere is different in the CESM with a higher model top as compared to the CESM with a lower model top. In particular, even though both models show similar downward extension of the Northern Annual Mode (NAM) from the stratosphere to the troposphere, the persistence time in the model with lower top is substantially shorter.  This is in agreement with the previous CMIP5 comparison between low-top model and high-top models, and highlights the lack of stratospheric dynamical variability when the model lid is not high enough.

As the El Nino Northern Oscillation is also impacted by the change in the model top, we also examine how stratospheric changes influence the ocean atmosphere coupling.