Future MJO Change and Its Impact on Extreme Precipitation and Temperature Over the Western US in CMIP6

March 14, 2025

A new paper entitled “Future MJO change and its impact on extreme precipitation and temperature over the western US in CMIP6” was recently published in the Journal of Geophysical Research: Atmospheres and authored by CW3E researcher Jiabao Wang, Mike DeFlorio (CW3E), Hyemi Kim (Ewha Womans University, Republic of Korea), Kristen Guirguis (CW3E), and Alexander Gershunov (CW3E). As part of CW3E’s 2025-2029 Strategic Plan, CW3E seeks to leverage novel observations, modeling, and innovations in forecast verification to advance the understanding of extreme weather in the West and their “forecasts of opportunity” where subseasonal prediction skill is relatively high to inform current and future resource and risk management. This study (Wang et al. 2025) analyzed 23 Coupled Model Intercomparison Project Phase 6 (CMIP6) models and showed systematic changes in multiple Madden-Julian oscillation (MJO) characteristics in the future climate, which are primarily contributed by the steepening of the mean meridional moisture gradient over the Indo-Pacific warm pool in a warming climate, and revealed the potential changes in MJO impacts on extreme precipitation and temperature over the western US. This research was sponsored by the California Department of Water Resources Atmospheric River Program.

The multi-model mean of the CMIP6 models shows a ∼17% increase in precipitation amplitude, an ∼11%–14% increase in circulation amplitude, a ∼9% increase in propagation speed, a ∼2-day decrease in period, and a ∼5° eastward extension of the MJO in the future climate (2065-2100) compared to current climate (1979-2014). Analysis of the lower tropospheric moisture budget suggests the dominant role of an increased meridional advection of mean moisture caused by the steepening of mean moisture gradient over the Indo-Pacific warm pool in a warming climate in the majority of models. The stronger anticyclonic gyres to the east of the MJO convection center along with the enhanced moisture gradient favor an enhanced export of moisture away from the Equator and a local moistening at the flanks of the MJO convection center. The above change in the moisture advection leads to a more substantial positive moisture tendency to the east of MJO convection and hence an enhanced eastward MJO propagation with strengthened amplitude and faster speed (Fig. 1).

Previous studies have shown a significant linkage between MJO characteristics (e.g., location, amplitude, propagation speed) and extratropical circulations and extreme weather (e.g., Liang et al. 2022; Wang et al. 2024). In this study, we examined how the downscaled CMIP6 models capture the observed MJO-extreme relationships and how those relationships may change in the future. In the current climate, wet extremes over California (CA) significantly decrease during MJO phases 2&3 when the enhanced convection is over the Indian Ocean and increase when the MJO is located over the western Pacific in phases 6&7. Model projections suggest that changes in CA precipitation extremes tend to be stronger and more substantial in the future climate in response to MJO activity. On the other hand, MJO phases 1&8 with convection over the Western Hemisphere and Africa generally correspond to an overall decrease in warm spells over the western US, while MJO phases 4&5 with convection over the Maritime Continent are associated with an overall increase. Unlike the wet extremes, MJO-associated changes in warm spells tend to be weaker over most of the western US in the future climate (Fig. 2).

Figure 1. (a)-(b) Multi-model-mean (MMM) filtered precipitation anomalies (shading) and 850-hPa to 700-hPa vertically integrated moisture tendency anomalies (contour, 0.3×10^-6kgm^-2s^-1 interval) in historical and future climates, respectively. (c) Difference between (b) and (a), which represents the future change in MJO precipitation and moisture tendency (0.2×10^-6kgm^-2s^-1 interval). Dots and hatch in (c) denote the significant difference in precipitation and moisture tendency, respectively, at the 0.05 significance level. The red box denotes the region east to the Maritime Continent (MC, day -8 to day 2, 120°E-160°E) where significant changes in moisture tendency are found. (Right top) Filtered 850-hPa to 700-hPa integrated moisture budget anomalies (unit: 10^-6kgm^-2s^-1) averaged over the red box in (c). The residual includes other terms such as precipitation, fluxes, and vertical advection, which is the difference between moisture tendency and horizontal advection. (Right bottom) Future changes in the meridional gradient of mean moisture where dots indicate the difference between future and historical runs at the 0.05 significance level. (Adapted from Figs. 3, 5, 6 in Wang et al. 2025)

The findings in this study indicate a high likelihood of changes in MJO-extreme relationships in the future climate. Given that MJO is considered the dominant source for subseasonal predictability with a time range from 2 weeks to 2 months and this time range currently has much lower skills than the weather and seasonal timescales, this study suggests that the MJO may still provide very useful information for predicting precipitation at the subseasonal timescale in the future but may be less useful in predicting temperature as shown by most CMIP models.

Figure 2. Area-averaged wet extreme and warm spell frequency response over Northern California (CA), Central CA, and Southern CA in historical (blue line) and future (red line) periods after each MJO phase combination. Shading indicates the multi-model standard deviation. (Fig. 10 in Wang et al. 2025)

Wang, J., DeFlorio, M. J., Kim, H., Guirguis, K., & Gershunov, A. (2025). Future MJO change and its impact on extreme precipitation and temperature over the western US in CMIP6. Journal of Geophysical Research: Atmospheres, 130(5), e2024JD042123. https://doi.org/10.1029/2024JD042123

Wang, J., DeFlorio, M. J., Gershunov, A., Guirguis, K., Delle Monache, L., & Ralph, F. M. (2024). Association of western US compound hydrometeorological extremes with Madden-Julian oscillation and ENSO interaction. Communications Earth & Environment, 5, 314. https://doi.org/10.1038/s43247-024-01449-w

Liang, S., Wang, D., Ziegler, A. D., Li, L. Z., & Zeng, Z. (2022). Madden–Julian Oscillation-induced extreme rainfalls constrained by global warming mitigation. npj Climate and Atmospheric Science, 5(1), 67. https://doi.org/10.1038/s41612-022-00291-1