An Assessment of Madden-Julian Oscillation Impacts on Weather Using Different MJO Indices

During the boreal winter of 2015-2016, a convective envelope associated with a Madden-Julian Oscillation (MJO) event propagated well to the east of its usual domain over the Pacific Ocean due to an exceptionally strong El Niño. Subsequently, California received an increase in rainfall, which raises the question of whether this MJO was related to precipitation in California or other meteorological variables over North America and beyond. More broadly, it motivates the question of the role of El Niño in MJO propagation and resulting downstream impacts. Since the discovery of the MJO, several well-known indices have been used to identify an MJO event and the date of initiation. However, these dates can vary considerably between indices depending on whether the index focuses on convection or circulation. A goal of this research is to investigate to what extent the downstream weather impacts in the tropics and extratropics is dependent on the index utilized to define the MJO. In this study, the OLR-based MJO Index (OMI) and the Real-time Multivariate Index (RMM) are compared to determine which index better represents precipitation and surface temperature impacts for the continental United States. In addition, this study assesses the role of the MJO during El Niño versus La Niña and “neutral” years in affecting teleconnections associated with intraseasonal tropical convection.

In order to analyze the role of the MJO in mid-latitude and tropical teleconnections during different ENSO phases, a 120-day high pass filter was utilized to remove interannual variability, and focus on the intraseasonal signals of each season. This filtering was applied to OLR, streamfunction, precipitation and surface temperature. Both the OMI and RMM indices were then used to determine the phase of active MJO events during 1979-2016. The data were grouped into El Niño, La Niña, and neutral years, and composited by MJO phase for each group. As expected, teleconnections vary depending on where the latent heat associated with the MJO is located in the tropics. It was determined that the signals are contingent on the index used and these differences can be clearly seen in the circulation field. Specifically, unlike RMM, the OMI index is consistent with the December 2015 MJO event, as precipitation is evident in California during later phases of the MJO for each ENSO phase when anomalous convection is located over the central Pacific. In addition, the circulation perturbations are remarkably similar for each phase of the MJO regardless of the ENSO state, which can also explain the similarity in the temperature fields seen across each sample.