Role of the Boundary Layer Moisture Asymmetry in Causing the Eastward Propagation of the Madden–Julian Oscillation*

Abstract

The moisture budget associated with the eastward-propagating Madden-Julian oscillation (MJO) was diagnosed using 1979-2001 40-yr ECMWF Re-Analysis (ERA-40) data. A marked zonal asymmetry of the moisture relative to the MJO convection appears in the planetary boundary layer (PBL, below 700 hPa), creating a potentially more unstable stratification to the east of the MJO convection and favoring the eastward propagation of MJO. The PBL-integrated moisture budget diagnosis indicates that the vertical advection of moisture dominates the low-level moistening ahead of the convection. A further diagnosis indicates that the leading term in the vertical moisture advection is the advection of the background moisture by the MJO ascending flow associated with PBL convergence. The cause of the zonally asymmetric PBL convergence is further examined. It is found that heating-induced free-atmospheric wave dynamics account for 75%-90% of the total PBL convergence, while the warm SST anomaly induced by air-sea interaction contributes 10%-25% of the total PBL convergence.The horizontal moisture advection also plays a role in contributing to the PBL moistening ahead of the MJO convection. The leading term in the moisture advection is the advection across the background moisture gradient by the MJO flow. In the western Indian Ocean, Maritime Continent, and western Pacific, the meridional moisture advection by the MJO northerly flow dominates, while in the eastern Indian Ocean the zonal moisture advection is greater. The contribution of the moisture advection by synoptic eddies is in general small; it has a negative effect over the tropical Indian Ocean and western Pacific and becomes positive in the Maritime Continent region.

A trio-interaction theory for Madden–Julian oscillation

“…In both observation (Benedict and Randall 2007;Hsu and Li 2012) and model simulation , the BL frictional convergence has been found to be important for the eastward propagation of the MJO by strongly coupling with the shallow/congestus clouds, resulting in a vertically rearward-tilted structure of the MJO. To validate the critical role of the BL moisture convergence in the trio-interaction theory and to diagnose numerical models' problems, we suggest focusing on the zonal structural asymmetry in the lower troposphere generated by the BL moisture feedback, including the phase leading of the BL moisture convergence to major convection (by about 4-5 days) and the associated lower troposphere moistening (increase in equivalent potential temperature), destabilization (increase of the convective instability), the 700 hPa diabatic heating and the generation of MJO available potential energy.…”

“…This theory regards the moisture (or moist static energy) as the first order important. It emphasizes moisture-convection feedback (Woolnough et al 2001;Grabowski and Moncrieff 2004) and moisture transport (Maloney 2009;Maloney et al 2010;Andersen and Kuang 2012;Hsu and Li 2012;Kim et al 2014;Pritchard and Bretherton 2014;Liu and Wang 2016a, b). A simple empirical model was proposed Maloney 2012, 2013), which is further implemented by Adames and Kim (2016).…”

“…As a result, the frictional organization of convective heating can couple the Kelvin and Rossby waves together with convective heating. The BL moisture convergence can also accumulate moist static energy and increase convective instability to the east of the major convection (Hsu and Li 2012), leading to eastward propagation of the MJO.…”

A general theoretical framework for understanding essential dynamics of Madden–Julian oscillation

“…As a result, the frictional organization of convective heating can couple the Kelvin and Rossby waves together. The BL moisture convergence can also accumulate moist static energy and increase convective instability to the east of the major convection (Hsu and Li 2012), leading to eastward propagation of the MJO. The model describes the moisture feedback and trio-interaction among convective heating, low-frequency Kelvin and Rossby waves and the BL friction convergence (FC) (Fig.…”

“…In addition to theoretical modeling, various processes and mechanisms have been identified or speculated to play significant roles in MJO dynamics based on numerical model experiences and observations, including shallow convection-BL circulation interaction (Johnson et al 1999;Kikuchi and Takayabu 2004;; stratiform cloud-wave interaction (Mapes 2000;Khouider and Majda 2006;Kuang 2008;Fu and Wang 2009;Seo and Wang 2010;Holloway et al 2013); cloud-radiation interaction (Lee et al 2001;Raymond 2001;Bony and Emanuel 2005;Andersen and Kuang 2012); moisture-convection feedback (Woolnough et al 2001;Grabowski and Moncrieff 2004;Benedict et al 2014), and moisture transport (Maloney 2009;Maloney et al 2010;Andersen and Kuang 2012;Hsu and Li 2012;Kim et al 2014;Pritchard and Bretherton 2014). More details were reviewed by Zhang (2005) and Wang (2005Wang ( , 2012.…”