ENSO Modulation of MJO Event Variability in Observations, GCMs, and the Skeleton Model

Current generations of global climate models (GCMs) often struggle with simulating many of the observed features of the Madden-Julian oscillation (MJO), including producing sufficient variance at intraseasonal timescales and its eastward propagation and maintenance. As such, sensitivities of the MJO to the shape and strength of the tropical warm pool remain largely unknown, considering that recent observational studies also contain conflicting results pertaining to MJO event length and propagation speed for different phases of the El Niño-Southern Oscillation (ENSO) and Indian Ocean dipole (IOD). Understanding the statistical influence of natural climate variations in the large-scale tropical forcing may help lead towards improved simulations of the MJO in global climate and weather forecast models.

This study examines differences in MJO event climatology as a function of ENSO phase in observations and numerical models. Specifically, we examine those models with good MJO simulations from the recent MJO Task Force and GEWEX Atmosphere System Study (GASS) intercomparison project, in addition to the low-order MJO “skeleton model” developed by Majda and Stechmann (2009, 2011). Preliminary observational results indicate that MJO events are shorter-lived during El Niño, in agreement with previous work. There is also a relative increase in the number of individually occurring MJOs compared to neutral or La Niña conditions. The frequency of MJOs and corresponding event lengths are less sensitive to tropical sea surface temperatures (SSTs) in the low-order model, though the skeleton model demonstrates additional eastward propagation with enhanced MJO variance over the central Pacific when using El Niño SSTs. Similar results are found when forcing the skeleton model with ENSO-varying profiles of radiative cooling and low-level moistening, again consistent with observations. Time permitting, changes to the simulated and observed MJO climatology as a function of the IOD are also discussed.