The Madden-Julian Oscillation (MJO), the dominant mode of intraseasonal variability in the tropical atmosphere, is characterized by large-scale deep convection coupled with planetary-scale circulation. MJO events generally originate over the Indian Ocean and propagate eastward into the western Pacific. While the convective component weakens near the dateline, the circulation component continues eastward, completing a cycle within 30—90 days. Because of its strong influence on global weather and climate anomalies, understanding MJO variability is essential for improving extended-range forecasts.
MJO activity exhibits interdecadal variability in intensity, propagation speed, and spatial extent. Previous studies have primarily focused on intensity and propagation, while the period characteristics of MJO circulation and their interdecadal variations remain less well documented. Variability in MJO period length are is particularly relevant, as prolonged periods are associated with climate extremes such as southern China heatwaves, Meiyu rainfall in the Yangtze River basin, and Coastal Niño events.
The Atlantic Multidecadal Oscillation (AMO) is a leading mode of interdecadal climate variability that modulates ENSO, western Pacific tropical cyclones, and the Asian monsoon. Recent evidence suggests that AMO may influence MJO convective propagation by altering background winds and low-level moisture over the tropical Pacific. However, whether AMO also regulates the periodic characteristics of MJO circulation has not been systematically examined.
This study investigates the role of AMO in modulating the interdecadal variability of MJO circulation periods using ERA-20C reanalysis and ERSSTv5 sea surface temperature data. The analysis addresses three questions: 1) whether AMO induces interdecadal variations in MJO circulation period characteristics, 2) the physical processes responsible for these variations, and 3) the mechanism through which AMO exerts its influence.
The results show that AMO significantly modulates MJO circulation periods. During positive AMO phases, the mean circulation period shortens to approximately 45 days, whereas during negative phases it lengthens to about 60 days, with a correlation coefficient of -0.77. This variability is mainly associated with changes in propagation speed over the Western Hemisphere: accelerated propagation during positive phases results in shorter circulation periods, while decelerated propagation during negative phases produces longer periods. The modulation of propagation speed is linked to changes in the zonal sea surface temperature gradient between the tropical central-eastern Pacific and the tropical Atlantic. Positive AMO phases feature cold anomalies in the tropical central-eastern Pacific and warm anomalies in the tropical Atlantic, which enhance the zonal gradient and favor faster propagation.
Although the circulation period varies with AMO phase, the convective period remains stable, with power spectra consistently peaking near 60 days within the 30—90-day band. This indicates that the coupling between MJO convection and circulation is modulated on interdecadal timescales.
These findings demonstrate a trans-basin linkage between AMO and the interdecadal variability of MJO circulation periodicity, highlighting the role of AMO in regulating intraseasonal variability through large-scale ocean-atmosphere interactions.