Long-term, Ground-based Observations Challenge Conventional MJO Evolution Structure in the Tropical Western Pacific

The evolution of MJO-associated moist static energy (MSE) has been investigated extensively that, for the most part, were confined to the open ocean or large islands, and relied primarily on reanalysis data or model simulations. Here, we present a systematic analysis of the intraseasonal evolution in MSE in the tropical western Pacific (TWP) using long-term, high-quality, ground-based observations from two Department of Energy – Atmospheric Radiation Measurement (DOE-ARM) sites at Manus and Nauru – both of which are located close to the equator. We use 14 years (2000 to 2014) of bias-corrected radiosonde data to estimate vertical profiles of MSE during MJO propagation across the western Pacific. Instead of the conventional real-time multivariate MJO (RMM) index, we use large-scale precipitation tracking (LPT) to accurately locate the MJO precipitation centroid. This approach allows us to develop composites of MSE vertical structure relative to the distance between the ARM sites and the MJO. Preliminary results suggest that composite MSE evolution based on all MJO events aligns with conventional expectations. However, significant differences emerge for MJO events propagating closer to the ARM sites compared to those further away, with distant MJO events deviating from the typical pattern (amplitude and phase) of MJO evolution. The results are expected to help understand slow versus fast MJO evolution and evaluate reanalysis products.