Geopotential-based Multivariate MJO Index: extending RMM-like indices to pre-satellite era

Model simulations suggest that Madden–Julian oscillation (MJO) activity changes under the anthropogenic climate change background. However, satellite observations, which provide information on MJO convection activity, are not available before 1970s, hindering research long-term historical variability of MJO. This study aims at extending data length indices include both circulation and features, such as widely used Real-Multivariate (RMM) Index, to pre-satellite era. paper introduces a new Index construction method, in outgoing longwave radiation (OLR) input is derived from upper-level geopotential, names it Geopotential-Based Multivariate (GMM) Index. The GMM 1902 2008 based 20th century reanlaysis product, by assuming relationship between OLR geopotential does over time, compared with filtered version RMM (FMM) during 1981–2008 observed precipitation records century. shown (1) have same climatological properties FMM (2) be statistically highly correlated (3) able indicate activities MJO’s features overall bivariate correlation ERA-20C 0.964. Evaluation results confirm validity proposed could capture era can applied all require inputs. provides an alternative way overcomes difficulty studies will beneficial our understanding