Asymmetric Modulation of MJO by Two Types of ENSO

Abstract The modulation of Madden–Julian oscillation (MJO) by El Niño–Southern Oscillation (ENSO) has been widely investigated, yet the mechanisms—particularly the combined influences of ENSO phase (El Niño/La Niña) and type (eastern Pacific/central Pacific)—remain elusive. Here, we unravel that differences in MJO characteristics (convective intensity, propagation, and structure) between El Niño and La Niña are asymmetric across two types of ENSO. These asymmetries are interpreted through established MJO frameworks, involving vertical wind shear, moisture–convection feedback, and structure–propagation nexus. Due to greater interphase differences in low-frequency background winds and humidity, eastern Pacific (EP) ENSO exerts stronger modulation on MJO intensity and northward propagation than central Pacific (CP) ENSO. Weaker MJO modulation of CP ENSO also stems from its distinct regulations of low- and high-frequency MJO components. Moreover, MJO eastward propagation is consistently faster during EP ENSO than during CP ENSO, primarily attributable to stronger premoistening and larger front Walker cell zonal scale leading MJO deep convection, independent of ENSO phases. The MJO propagation over the Maritime Continent also exhibits asymmetry: While MJO events propagate coherently into the central Pacific during both EP ENSO phases, they are frequently blocked during CP La Niña but not during CP El Niño. To explain further these propagation asymmetries, we propose five diagnostics based on winds, convection, stratiform heating, and humidity across intraseasonal and low-frequency time scales. Variations in the strength and behavior of these diagnostics under different ENSO backgrounds effectively explain the diversity of MJO propagation. This study advances toward a more comprehensive understanding of how diverse ENSOs modulate the MJO. Significance Statement The Madden–Julian oscillation (MJO) is the dominant mode of 20–100-day fluctuations in tropical winds and clouds and features significant year-to-year variations in its intensity and propagation. Among numerous factors that regulate MJO behaviors, El Niño–Southern Oscillation (ENSO) has been examined extensively in previous studies. Nevertheless, how the MJO varies across different phases and types of ENSO remains elusive, especially whether there exist asymmetric El Niño–La Niña differences in the MJO under two types of ENSO has received much limited attention. Using observation and reanalysis products, we have explored these issues comprehensively by diagnosing the convective intensity, propagation, and structure of MJO. Our findings promote the development of comprehensive paradigms for diverse ENSO-modulated MJOs.