Assessment of Gross Moist Stability and its Relationship to the MJO in Reanalysis Data

This study uses the ERA-Interim and Climate Forecast System (CFS) 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 timescales. 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. However, this relationship does not hold using reanalysis data. On seasonal and inter-to-decadal timescales, MJO convective activity is greater in regions of higher NGMS, indicating a positive correlation between MJO convective activity and the NGMS. The results suggest that the negative correlation between MJO convective activity and the NGMS on seasonal to longer timescales suggested by previous theoretical and modeling studies does not occur in the atmosphere, at least as represented by the reanalysis. On the other hand, the same reanalysis data suggest that the NGMS is still a useful indicator of whether the atmospheric state is favorable for MJO convection on intraseasonal timescales. As shown by previous modeling and observational studies, on intraseasonal timescales, 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. We suggest that the relationship between MJO convective activity and the NGMS is consistent with the theoretical expectation only on intraseasonal and perhaps shorter timescales. MJO events therefore tend to develop during intraseasonal declines in NGMS superimposed on background positive NGMS. Our results therefore indicate that the MJO does not evolve as a result of instability intrinsic to its environment, but that the instability evolves as part of the MJO itself.