Characterizing the Madden–Julian Oscillation in the western Pacific Ocean from a regional coupled ocean–atmosphere model simulation

Abstract

This study reports on the analysis of the results from a 20 km grid spacing, Regional Coupled ocean–atmosphere Model (RCM) integration over the Western Pacific Warm Pool (WP2). The RCM was integrated over a 20-year period (1986–2005) using reanalysis boundary conditions for the atmosphere and the ocean. This is a first-of-a-kind study with an RCM at 20 km over the WP2. The RCM simulation shows reasonable fidelity of the mean state and of the Madden–Julian Oscillation (MJO). We utilize this successful integration of the RCM to understand a well-known observed feature of MJOs in the WP2 to be of the strongest amplitude during the December–March period of the year. Our analysis of the model integration reveals that the recharge of moist static energy (MSE) prior to peak MJO convection and its discharge during and after the convection explains the MJO in the simulation. The recharge/discharge of the MSE is shown to be largely dictated by horizontal advection, which is stemmed to a small extent by column-integrated radiative heating and surface evaporation. This balance of forces in the evolution of the MSE anomalies and their corresponding variations with sea-surface temperature (SST) anomalies at MJO time-scales in the WP2 is strongest in the December–March period in the RCM simulation.