Transient Response of MJO Precipitation and Circulation to Greenhouse Gas Forcing

Abstract

Recent studies have shown that Madden‐Julian oscillation (MJO) precipitation anomaly amplitude tends to increase while associated circulations weaken at the end of 21st century in Coupled Model Intercomparison Project phase 5 models under Representative Concentration Pathway 8.5. Transient changes of MJO characteristics earlier in the 21st century have received less attention. In this study, changes of MJO precipitation and circulation amplitude during these interim time periods under Representative Concentration Pathway 8.5 are examined in Coupled Model Intercomparison Project phase 5 models. Multimodel mean changes in MJO precipitation and circulation amplitude are not individually detectable in the early and middle 21st century relative to the historical period (1986–2005). However, robust multimodel mean decreases in the ratio of MJO wind to precipitation anomalies occur even early in the 21st century. This decreased ratio is explained by increasingly large tropical static stability as the climate warms, which under weak temperature gradient balance mandates that a diabatic heating anomaly is balanced by an increasingly weaker circulation anomaly. These results suggest the robustness of weak temperature gradient theory for explaining MJO dynamics, not only in an equilibrium climate but also in the transient response.

Investigating Recent Changes in MJO Precipitation and Circulation in Multiple Reanalyses

“…The present paper provides a preliminary assessment of MJO activity changes in precipitation and vertical velocity over the past four decades that include both anthropogenic forcing and natural variability and uses a century‐long data set to assess recent changes in the context of natural variability over the longer record. Our results based on observations support those previously derived from climate models (e.g., Bui & Maloney, 2019) suggesting that decreases in MJO ω 400 / P occur as surface temperatures warm due to anthropogenic forcing. Nevertheless, discrepancies between results from ERA5 and MERRA‐2 leave lingering questions about the degree to which changes to the MJO can be explained by WTG theory, including the assumption that Q 1 has no vertical structural changes in response to climate warming.…”

“…Specifically, the occurrence of an MJO event is defined as when the magnitude of the outgoing longwave radiation‐based MJO index (OMI; downloaded from NOAA PSL website; see Kiladis et al, 2014, for definition) exceeds 1.0. Note that we split our analysis into 19‐year periods, and so OMI is normalized within each time period (as in Bui & Maloney, 2019) to reflect possible changes in variance of outgoing longwave radiation fields. Boreal winter (November to April) MJO composites for each of its eight phases are then generated for 30‐ to 90‐day bandpass filtered variables as is commonly done in the MJO literature (e.g., Kiladis et al, 2014).…”

“…If it is further assumed that precipitation is proportional to Q 1 in MJO convective regions, and that the vertical structure of Q 1 is not changed (Maloney & Xie, 2013), it follows that at a given level, where P is the surface precipitation rate and Δ denotes the relative change from a reference state to a new state. Bui and Maloney (2019) examined GCM simulations forced by Representative Concentration Pathway 8.5 (RCP8.5) in a subset of models participating in the Coupled Model Intercomparison Project 5 (CMIP5) that simulated realistic MJOs. While the amplitude changes of MJO precipitation and vertical velocity were individually not detectable until 2080, the ratio of MJO vertical velocity to precipitation amplitude showed detectable decreases as early as 2021–2040.…”

Fifty Years of Research on the Madden‐Julian Oscillation: Recent Progress, Challenges, and Perspectives

“…Before averaging across the warm pool, amplitude is defined at each location as the root mean squared anomaly across all eight composite phases of the MJO defined according to Wheeler and Hendon (2004). Figure is reproduced courtesy of the American Geophysical Union from Bui and Maloney (2019b).…”

Changes to the Madden‐Julian Oscillation in Coupled and Uncoupled Aquaplanet Simulations With 4xCO2

“…Bui and Maloney (2019a) reported a reduction in MJO precipitation amplitude in the early and middle 21st century in several CMIP5 models in RCP8.5 relative to the historical period, which they hypothesized might have a component due to the direct response of the MJO to greenhouse gas forcing rather than to changes in the thermodynamic environment that lag the forcing. Bui and Maloney (2019b) later analyzed 11 CMIP5 simulations including multiple ensemble members from one model to show that individual changes in MJO precipitation and circulation amplitude in RCP8.5 are not detectable until the last decades of the 21st century, casting doubt on the early 21st century findings of Bui and Maloney (2019a). Despite inability to detect individual changes of MJO precipitation and circulation amplitude, decreases in the ratio of MJO circulation to precipitation anomaly amplitude were detectable as early as the first half of the 21st century, consistent with increased tropical static stability produced by global mean temperature warming (e.g., Maloney & Xie, 2013).…”