Causes of a Lack of Solar-/QBO-MJO Connection in Certain CMIP6 models

A connection between the quasi-biennial oscillation (QBO), the 11-yr solar cycle (SC), and the short-term convective climate oscillation, the Madden-Julian Oscillation (MJO), in boreal winter has been found in observational data, yet is generally lacking in current global climate models (GCMs). Reasons for this are explored by looking at the representation of each step of a proposed mechanism in a selection of models that participated in from the Coupled Model Intercomparison Project 6 (CMIP6). Particular issues are found to be: QBOs that are often westerly biased, a lack of solar signal in the lower stratosphere, insufficient solar or QBO-modulation of extratropical wave activity, too weak reductions in equatorial tropopause static stability in response to extratropical wave forcing, and MJOs that in some cases do not respond to these reductions. Through by-passing many of these deficiencies via data selection, it is demonstrated that effects on the MJO that resemble those found in observations (e.g., strengthening of the MJO following early-winter sudden stratospheric warmings and during easterly QBO winters) can be simulated by a subset of the models. This supports operation of the proposed mechanism, and points to needed model improvements, although additional feedbacks may be needed to reproduce the full modulation. The results of recent simulations using the GISS-E2.2 model are also presented, with particular focus given to whether improvements in different features such as the QBO representation lead to the expected, corresponding improvements in the mechanism representation.