Model simulations suggest that Madden-Julian Oscillation (MJO) activity changes under the anthropogenic climate change background. However, satellite observations, which provide information of MJO convection activity, are not available before the 1970s, hindering research on the historical long-term variability of MJO. This study aims at extending the data length of MJO indices that include both MJO circulation and convection features, such as the widely used Real-Multivariate MJO (RMM) index, to the pre-satellite era. This paper introduces a new MJO index construction method, of which the outgoing longwave radiation (OLR) input is derived from upper-level geopotential, and names it as the Geopotential-based Multivariate MJO (GMM) index. The GMM index is derived over 1902–2008 and compared with the filtered version of RMM (FMM) index during 1981–2008. The GMM index is shown to (1) have the same climatological properties as the FMM index, (2) be statistically highly correlated to the FMM index, and (3) be able to indicate MJO activities and its convection features in the pre-satellite era. The overall bivariate correlation between FMM index and GMM index based on ERA-20C ranges from 0.959 to 0.968 in different phases. Evaluation results confirm the validity of the proposed MJO index construction method, which could capture MJO convection activity in the pre-satellite era and can be applied to all MJO indices that require input of OLR. This study provides an alternative way that overcomes the difficulty of historical MJO studies, and will be beneficial to our understanding of the long-term change of MJO.
