Frictional convergence and the Madden-Julian oscillation

Abstract

Frictional surface convergence is shown to be important to the Madden-Julian oscillation (MJO) of the tropical troposphere using both observations and an atmospheric general circulation model (GCM). An observed composite MJO lifecycle is created using the first two empirical orthogonal functions of the NCEP 850 mb equatorial zonal wind as an index. Lower tropospheric water vapor over the Indian and western Pacific Oceans is significantly correlated with 1000 mb convergence during an MJO lifecycle. Frictional convergence may help to precondition the atmosphere for strong MJO convection by moistening the lower troposphere.Several convection schemes were substituted into the NCAR CCM3 GCM to improve model intraseasonal variability. The McRAS convection scheme, that includes a parameterization of unsaturated convective downdrafts and a relative humidity threshold for convective initiation, produces much improved intraseasonal variability over the default CCM3 convection scheme of Zhang and McFarlane. Intraseasonal variability is decreased for higher relative humidity thresholds with McRAS, contrary to the results of Wang and Schlesinger (1999). Removal of the unsaturated convective downdraft and rain evaporation parameterization greatly diminishes intraseasonal variability in zonal winds and precipitation. An excessively dry tropical troposphere is most likely responsible for the degradation of intraseasonal variability.Low-level MJO specific humidity anomalies in the model are strongly correlated with surface convergence. This strong coupling with surface convergence causes model MJO convection anomalies to be associated with 850 mb easterly perturbations. Where the eastward propagating model MJO signal is strong, low-level specific humidity anomalies precede those at middle and upper levels. Surface convergence affects model convection by moistening the lower troposphere, suggesting that surface convergence during an MJO lifecycle may help to precondition the atmosphere for intense MJO convection. Meridional convergence onto the equator contributes most of the anomalous convergence during an MJO lifecycle in both the model and observations. The wind-induced surface heat exchange (WISHE) mechanism of Emanuel (1987) and Neelin and Yu (1994) does not significantly contribute to the MJO-like mode in the CCM3 with McRAS.