Changes in a modeled MJO with idealized global warming

Abstract

This study estimates how the Madden-Julian oscillation (MJO) will change with uniform global warming of 2 and 4 K at the Earth surface using an aqua-planet version of the NCAR CAM2 implemented with the Tiedtke convection scheme. Solar insolation is specified at the vernal equinox with a diurnal cycle. Thirty-year integrations are carried out for each case and the last 20-year’s results are used for analysis. For the warmer cases, the modeled MJO’s eastward propagation remains dominant at zonal wave numbers 1–4, and notable increase occurs in variance, power spectra, and the number of prominent MJO events. The convective heating is enhanced more in upper troposphere, and the MJO power spectra increase more on 20–30 days than on 30–60 days. In all cases, composite life cycles of prominent MJO events show that the anomalous surface latent heat flux lags precipitation by about 90° in phase, characterizing the nonlinear wind induced surface heat exchange (WISHE) to destabilize the MJO. Interacting with a warmer surface in the 4 K case, perturbations of zonal wind and temperature at bottom model level contribute to the nonlinear WISHE coherently with the latent heat flux. Meanwhile anomalous boundary layer convergence leads precipitation by some 45° in phase, indicating the frictional moisture convergence to maintain the enhanced MJO.