Gross moist stability and the Madden-Julian Oscillation in reanalysis data

This study uses the ERA-Interim and Climate Forecast System reanalysis datasets to examine the relationship between the convective activity of the Madden Julian Oscillation (MJO) and the normalized gross moist stability (NGMS) on various time-scales. Previous studies based on proposed theories for the MJO and the assessments of general circulation models suggest that MJO convection tends to be more active where the NGMS is smaller or effectively negative. This relationship only appears using reanalysis data on some time scales and limited geographical regions. On seasonal time-scales, MJO convective activity shifts with the regions of positive NGMS and net moisture import, indicating positive correlation between MJO convective activity and the NGMS. On interannual to decadal time-scales, stronger MJO convective events tend to occur with a background state of smaller NGMS exclusively over the central Pacific basin, indicating negative correlation. When the environment switches between net moisture export to import, MJO convective activity tends to follow the region of net moisture import, leading to its positive correlation with NGMS. However, when the environment already has net moisture import and positive NGMS, MJO convective activity is negatively correlated with the NGMS and stronger MJO convective events tend to occur with smaller positive NGMS. Therefore, the background NGMS is not a globally applicable metric for MJO convective activity and it needs to be used with caution over such environments where the NGMS can oscillate around zero. On intraseasonal time-scales, as shown by previous modelling and observational studies, the NGMS becomes anomalously negative prior to the onset of MJO enhanced convection and becomes positive as the convection peaks over the Indo-Pacific basin. The results of this study demonstrate that the relationship between MJO convective activity and the NGMS in reanalysis data largely depends on time-scales and geographical region.