The Madden-Julian Oscillation and Mean Easterly Winds

A recent analytical model of the Madden-Julian Oscillation (MJO), the WISHE-moisture mode theory, successfully models all the main characteristics of the MJO, namely, that it is a planetary, unstable, and eastward propagating tropical mode. The main assumption of the WISHE-moisture mode theory is that an interplay between the global mean easterly zonal winds and moisture leads to the propagation and destabilization of the MJO. We investigate this theory by building a climatological MJO using reanalysis, OAFlux surface latent heat fluxes, and outgoing longwave radiation data and compare it to the WISHE-moisture mode. The necessary condition for the WISHE-moisture mode theory-easterly global mean zonal winds-is always present in the tropics. Comparing the zonal wind, moisture, and moisture tendency anomalies to the WISHE-moisture mode, we find that the WISHE-moisture mode theory is in agreement with the reanalysis-derived climatological MJO. Reanalysis and OAFlux surface latent heat fluxes are in agreement with the WISHE-moisture mode theory. Although meridional surface winds seem to contribute negatively to the surface latent heat flux anomaly, we show that their effect is secondary in the region of enhanced surface latent heat fluxes which moisten the boundary layer and lead to the propagation of the MJO.