Mechanisms driving MJO teleconnection changes with warming in CMIP6

Abstract

Abstract. Teleconnections from the Madden-Julian Oscillation (MJO) are a key source of predictability of weather on the extended time scale of about 10–40 days. The MJO teleconnection is sensitive to a number of factors, including the mean state dry static stability, the mean flow, and the propagation and intensity characteristics of the MJO itself, which are traditionally difficult to separate across models. Each of these factors may evolve in response to increasing greenhouse gas emissions, which will impact MJO teleconnections and potentially impact potential predictability on extended time scales. Current state-of-the-art climate models do not agree on how MJO teleconnections will change in a future climate. Here, we use results from the Coupled Model Intercomparison Project Phase 6 (CMIP6) historical and SSP585 experiments in concert with a linear baroclinic model to separate and investigate alternate mechanisms explaining why and how MJO teleconnections over the North Pacific and North America will change in a future climate, and to identify key sources of inter-model uncertainty. We find that decreases to the MJO teleconnection due to increases in tropical dry static stability alone are robust, and that uncertainty in mean state winds are a key driver of uncertainty in future MJO teleconnections. We find no systematic relationship between changes in Rossby wave excitation and the MJO teleconnection. However, we find that models that predict increases (decreases) in the stationary Rossby wave number over the gulf of Alaska also predict stronger (weaker) teleconnections over North America. Uncertainty in future changes to the MJO's intensity, eastward propagation speed, and eastward propagation extent are other important sources of uncertainty in future MJO teleconnections, although to a lesser degree than uncertainty in the mean state. The overall outlook is a reduction of the boreal winter MJO teleconnection across the vast majority of CMIP6 models, especially over the North Pacific, but with some nuance over North America due to larger sensitivity to expansion of the MJO's eastward extent.

Storylines for Future Projections of Precipitation Over New Zealand in CMIP6 Models

“…For example, Changes to the Rossby wave sources in the model clusters was not directly investigated here as only a small number of models had the required high‐frequency output available. However, other Northern Hemisphere‐focused studies have found that in terms of modifying the strength and pathways of existing teleconnections, the changes in the background flow (as examined here) play a much larger role than direct changes to the Rossby wave source (Wang et al., 2022), but these also contribute more strongly to inter‐model uncertainty (Jenney et al., 2021). As mentioned above, changes in La Niña frequency (a major driver of change and variability in the Rossby wave source), and which would be expected to increase high pressure to the east of New Zealand, were not found to be robust to suggest that changes in the background zonal flow is likely the more dominant driver.…”