Momentum Budget Analysis of Westerly Wind Events Associated with the Madden-Julian Oscillation during DYNAMO

The Dynamics of the Madden-Julian Oscillation (DYNAMO) field campaign was conducted over the Indian Ocean (IO) from October 2011 to February 2012 to investigate the initiation of the Madden-Julian Oscillation (MJO). Three MJOs accompanying one or more westerly wind events (WWEs) occurred in late October, late November, and late December 2011, respectively. Momentum budget analysis is conducted in this study to understand the contributions of the dynamical processes involved in the wind evolution associated with the MJO active phases over the IO during DYNAMO using European Centre for Medium-Range Weather Forecasts (ECMWF) analysis. This analysis shows that westerly acceleration at lower levels associated with the MJO active phase generally appears to be maintained by the pressure gradient force (PGF), which is partly canceled by meridional advection of the zonal wind. Westerly acceleration in the mid-troposphere is mostly attributable to vertical advection. In addition, the MJO in late November (MJO2), accompanied by two different WWEs (WWE1, WWE2) spaced a few days apart, is further diagnosed. Unlike other WWEs during DYNAMO, horizontal advection is more responsible for the westerly acceleration in the lower troposphere for the WWE2 than the PGF. Interactions between the MJO2 convective envelope and convectively coupled waves (CCWs) have been further analyzed to illuminate the dynamical contribution of these synoptic scale equatorial waves to the WWEs during MJO2. We suggest that differences in the developing processes among WWEs can be attributed to the different types of CCWs.The Dynamics of the Madden-Julian Oscillation (DYNAMO) field campaign was conducted over the Indian Ocean (IO) from October 2011 to February 2012 to investigate the initiation of the Madden-Julian Oscillation (MJO). Three MJOs accompanying one or more westerly wind events (WWEs) occurred in late October, late November, and late December 2011, respectively. Momentum budget analysis is conducted in this study to understand the contributions of the dynamical processes involved in the wind evolution associated with the MJO active phases over the IO during DYNAMO using European Centre for Medium-Range Weather Forecasts (ECMWF) analysis. This analysis shows that westerly acceleration at lower levels associated with the MJO active phase generally appears to be maintained by the pressure gradient force (PGF), which is partly canceled by meridional advection of the zonal wind. Westerly acceleration in the mid-troposphere is mostly attributable to vertical advection. In addition, the MJO in late November (MJO2), accompanied by two different WWEs (WWE1, WWE2) spaced a few days apart, is further diagnosed. Unlike other WWEs during DYNAMO, horizontal advection is more responsible for the westerly acceleration in the lower troposphere for the WWE2 than the PGF. Interactions between the MJO2 convective envelope and convectively coupled waves (CCWs) have been further analyzed to illuminate the dynamical contribution of these synoptic scale equatorial waves to the WWEs during MJO2. We suggest that differences in the developing processes among WWEs can be attributed to the different types of CCWs.