Factors Regulating the Multidecadal Changes in MJO Amplitude over the Twentieth Century

Abstract

This study examined multi-decadal changes in the amplitude of the boreal-winter Madden-Julian Oscillation (MJO) over the 20th century using two century-long reanalysis datasets (20CR and ERA-20C). Both revealed reasonable MJO variability as other state-of-the-art reanalysis datasets. We detected pronounced multi-decadal variations along with an increasing trend in MJO amplitude during the period 1900–2009 in both datasets, although this linear trend was less significant in the reconstructed MJO index proposed by Oliver and Thompson. The two 20th century reanalysis datasets and the Oliver-Thompson MJO index consistently showed the intensified amplitude of MJO precipitation and circulation in the later decades (1970–1999) compared to the earlier decades (1920–1949). The most significant enhancement of MJO precipitation in the later decades appeared over the western Pacific warm pool. To understand the mechanisms controlling the changes in western-Pacific MJO precipitation amplitude over the 20th century, we diagnosed the moisture budget equation. The enhanced MJO precipitation variability in the later decades mainly came from increased moisture associated with a strengthened low-level convergence anomaly working on background mean moisture [−(𝑞̅∇ ∙ 𝑉′)]. Further diagnosis showed that the effect of anomalous circulation (∇ ∙ 𝑉′) change on the MJO precipitation amplitude change over the 20th century was about an order larger than that of mean moisture (𝑞̅) change, different from the mechanisms (i.e., increased gradient of 𝑞̅) responsible for the intensified MJO precipitation amplitude under future warmer climate. The enhanced MJO circulation anomalies during 1970–1999 may be caused by an enhanced diabatic heating anomaly, offset partly by the increased mean static stability.

Emergence of Madden-Julian oscillation precipitation and wind amplitude changes in a warming climate

“…Jenney et al (2021) added that multi-model uncertainty, which may come from the uncertainty in the parameterized moist physics in the model as mentioned by Rushley et al (2019), also contributes to the uncertainty in future detection of the MJO characteristics 19,20 . In addition to the model results above, observational evidence also suggest that detecting MJO changes is complicated by interannual-to-multidecadal climate variations in the past few decades 16,21 .…”

Decadal Variability of the Extratropical Response to the Madden–Julian Oscillation

“…The MJO varies on interannual (Chen et al., 2016; Hsu & Xiao, 2017; Kessler, 2001) and decadal (Fu et al., 2020; Jones & Carvalho, 2006; Wu et al., 2021) time scales. The extratropical response to the MJO is dependent on the background state of the atmosphere (Henderson et al., 2017), which can also vary on interannual and decadal time scales.…”

Decadal variability of the extratropical response to the Madden-Julian Oscillation

“…The MJO varies on interannual (Chen et al, 2016;Hsu & Xiao, 2017;Kessler, 2001) and decadal (Fu et al, 2020;Jones & Carvalho, 2006;Wu et al, 2021) time scales. The extratropical response to the MJO is dependent on the background state of the atmosphere (Henderson et al, 2017), which can also vary on interannual and decadal time scales.…”