An Intraseasonal Oscillation Composite Life Cycle in the NCAR CCM3.6 with Modified Convection

Abstract

The NCAR CCM3.6 with microphysics of clouds with relaxed Arakawa–Schubert convection produces an intraseasonal oscillation that is highly dependent on lower-tropospheric moistening by surface convergence. Model intraseasonal convection is most highly correlated with surface convergence at zero lag, causing enhanced convection to be associated with 850-mb easterly anomalies, where surface convergence is strongest. The tendency for surface convergence to maximize within 850-mb easterly anomalies is consistent with meridional frictional convergence into equatorial surface pressure troughs associated with planetary-scale tropical wave circulations. Anomalous vertical advection associated with meridional surface convergence influences model convection by moistening the lower troposphere. Observed Madden–Julian oscillation (MJO) convection and lower-tropospheric specific humidity are also significantly correlated with surface convergence, although correlations are weaker than in the model, and convergence leads convection anomalies. Observed MJO enhanced convection tends to fall closer to the point of maximum convergence in the 850-mb equatorial zonal wind anomaly field. Although surface convergence appears important for both observed and model intraseasonal convection, the significant differences between observed and modeled intraseasonal variability suggest that interactions between convection and the large-scale circulation in the model are not completely realistic. The wind-induced surface heat exchange (WISHE) mechanism cannot explain the preference for model intraseasonal enhanced convection to coincide with 850-mb easterly anomalies. When the effects of WISHE are removed by fixing the surface wind speed in the calculation of surface latent heat fluxes, the phase relationship between model intraseasonal wind and convection anomalies does not change. Removing WISHE may produce a more robust model intraseasonal oscillation, however. Model intraseasonal oscillation circulation features are better defined, and spectral power in the MJO band is more prominent when WISHE is removed.