MJO representation in the Community Earth System Model 2 Large Ensemble: detection, seasonality and amplitude

The Madden-Julian Oscillation (MJO) is the leading mode of tropical intraseasonal variability in the atmosphere and impacts weather and climate, both locally in the tropics as well as globally through tropical–extratropical interactions. Various metrics have been used to classify the intensity and character of the MJO. One popular metric is the Real-time Multivariate MJO (RMM) index, which is based on an empirical orthogonal function (EOF) of zonal winds at 850 hPa and 200 hPa and outgoing longwave radiation (OLR). To date, many climate models have struggled to reproduce a realistic MJO RMM signal, due to issues associated with model tropical convective schemes and failure to capture the eastward propagation of the MJO circulation. In this study, we assess the seasonality and amplitude of the MJO in the CESM2 Large Ensemble version 2 (CESM2-LE) experiment. We reproduce the RMM index from model output to explore the utility of OLR versus precipitation in MJO representation. Overall, we find that the model climatologies of MJO look similar to those present in the NCEP–NCAR reanalysis (for winds) and NOAA polar-orbiting satellites (for OLR). Both OLR and precipitation are found to be good indicators of MJO activity. When considering MJO amplitude over four categories (inactive, RMM  <  1.0; active, RMM  >  1.0; very active, RMM  >  1.5; and extremely active, RMM  >  2.5) the CESM2-LE model tends produce more inactive and fewer very active MJO events. Dynamically, MJO events in the CESM2-LE model are slower to propagate and weaker than observed. This is further demonstrated by model tendencies for strong MJO events to transition to weaker events faster than observed, particularly over the Indian Ocean and Maritime Continent.