The moisture source sequence for the Madden‐Julian Oscillation as derived from satellite retrievals of HDO and H2O

Abstract

A number of competing theories to explain the initiation mechanism, longevity and propagation characteristics of the Madden‐Julian Oscillation (MJO) have been developed from observational analysis of the tropical climate and minimal dynamical models. Using the isotopic composition of atmospheric moisture from paired satellite retrievals of H2O and HDO from the boundary layer and mid troposphere, we identify the different sources of moisture that feed MJO convection during its life cycle. These fluxes are then associated with specific dynamical processes. The HDO/H2O isotope ratio data show that during the early phase of the MJO, the mid‐troposphere is dominated by moisture evaporated from the ocean surface that was transported vertically undergoing minimal distillation. The contribution from the evaporative source diminishes during early convective activity but reappears during the peak of MJO activity along with an isotopically depleted flux, which is hypothesized to originate from easterly convergence. The contribution of different moisture sources as shown from the HDO/H2O data is consistent with model results where the sustaining of deep convection requires a feedback between convergence, precipitation strength and evaporation. In the wake of an MJO event, the weak vertical isotopic gradient, depletion in boundary layer δD and the uniquely moist and depleted vapor in the mid troposphere all point toward a prominent presence of moisture originated from rainfall re‐evaporation, which confirms the prediction that the transition from convective to stratiform rains is important to the moisture budget of the MJO.