Changes in Madden-Julian Oscillation Precipitation and Wind Variance Under Global Warming

The Madden-Julian oscillation (MJO) is the leading mode of tropical intraseasonal variability, having profound impacts on many weather and climate phenomena across the tropics and extratropics. Previous studies using a limited number of models have suggested complex changes in MJO activity in a warmer climate. While most studies have argued that MJO precipitation amplitude will increase in a future warmer climate, others note that this is not necessarily the case for MJO wind variability. This distinction is important since MJO wind fluctuations are responsible for producing remote impacts on extreme weather through teleconnections. In this study, we examine projected changes of MJO precipitation and wind variance at the end of the 21st century in Representative Concentration Pathway 8.5 using the multimodel Coupled Model Intercomparison Project phase 5 data set. Under global warming, most models show an increase in MJO band precipitation variance, while wind variability decreases. The discrepancy between MJO precipitation and wind variance changes under global warming is shown to be due to increases in tropical static stability in a warmer climate. The multimodel mean shows a 20% increase in both the 500-hPa vertical tropical dry static energy gradient and the ratio of intraseasonal precipitation to 500hPa omega fluctuations, consistent with scaling by weak temperature gradient theory. These results imply that tropical static stability increases may weaken the MJO's ability to influence extreme events in future warmer climate by weakening wind teleconnections, even though MJO precipitation amplitude may increase. Plain Language Summary The Madden-Julian oscillation has profound impacts on weather and climate phenomena over the tropical and extratropical regions, including a modulation of atmospheric rivers, hurricanes, and atmospheric blocking. We can currently exploit knowledge of the MJO to predict such phenomena several weeks in advance. Therefore, understanding how the MJO may change under global warming is important to understanding its future impacts and for prediction. In this manuscript, we show how MJO precipitation and wind variance change at the end of the 21st Century under global warming (RCP8.5) using the CMIP5 multi-model dataset. We derive the interesting result that global warming enhances MJO precipitation amplitude while weakening the MJO circulation. This result is important, as MJO teleconnections to other parts of the world are mediated by the circulation response, and our results suggest that MJO impacts on atmospheric rivers, hurricanes, and other extreme events may become less predictable in a future warmer climate due to the weakened circulation. We also physically explain our results by relating weaker circulations to the increase in the vertical dry static energy gradient in a warmer climate.