Detangling Long-Term Variability of the Madden-Julian Oscillation Amplitude from Changes in the Observing System

Long-term variability of the Madden-Julian Oscillation (MJO) inferred from tropical in-situ observations are examined during 1940-2023, and compared to that derived from reanalysis products. Previous studies have suggested that the MJO varies at decadal and longer time-scales and that those changes modulate MJO teleconnections and therefore MJO related global impacts. Assessments of long-term MJO variability are challenging because characterizing MJO behavior outside of the satellite era lacks detailed information about its observed state, and/or relies on climate models’ typically poor MJO representation. This study shows that, while tropical sounding observations are spatially and temporally sparse, they are useful to better understand multi-decadal to century MJO amplitude modulations. The analysis presented here indicates an increase in MJO amplitude of about 10% to 20% from 1940 to the mid-1980's followed by a recent slight decrease to present time, where this recent decrease is primarily driven by a weakening of the MJO upper level circulation. The results also suggest that the behavior of the MJO amplitude prior to 1940 is largely uncertain due to insufficient observations. The differences between long-term variability of the MJO derived from (imperfect) reanalysis products with the MJO behavior inferred from (sparse) tropical station observations help us untangle MJO long-term variability that are physical from those related to the observational network changes over the last century.