MJO Representation in the Community Earth System Model 2 Large Ensemble: Detection, Seasonality and Amplitude

The Madden-Julian Oscillation represents the leading mode of tropical intraseasonal variability in the atmosphere impacting weather and climate both locally in the tropics, as well as globally through tropical–extratropical interactions. Various metrics have been used classify the intensity and location of the MJO, such as the Wheeler Hendon Real-time Multivariate MJO (RMM) index, which is based on empirical orthogonal function (EOF) analyses 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 signal in the tropics, due to issues associated with model tropical convective schemes and failure to capture the eastward propagation of the MJO signal.

In this study, we assess the seasonality and amplitude of MJO representation in the CESM2 Large Ensemble version 2 (LENS2) experiment by reproducing the Wheeler Hendon RMM index using model output. Using an ensemble approach, we explore the utility of OLR versus precipitation in MJO detection from model output. Overall, we find that the model climatologies of MJO look similar to those present in the NCEP–NCAR (for winds) and NOAA polar-orbiting series of satellites (for OLR) reanalysis products, and that both metrics, OLR and precipitation, are good indicators of MJO activity. When considering MJO amplitude (inactive RMM <1.0), active RMM >= 1.0, very active RMM>= 1.5 and extremely active RMM >= 2.5) the LENS2 model tended to produce more inactive and very active MJO events. Spatially, regions of greatest disparities included the Indian Ocean and Maritime continent sectors.