Contrasting the energy recharge-discharge cycle between propagating and eastward-decaying Madden–Julian Oscillation events

Abstract

In this study, we contrast the distinct energy recharge-discharge cycle between propagating and eastward-decaying Madden–Julian Oscillation (MJO) events during the boreal winter season (November–April) from 1979 to 2018 using the moist static energy (MSE) budget and gross moist stability (GMS) plane analyses. A total of 41 MJO events are selected during the 40-year period, including 16 strong propagating (SP), 13 weak propagating (WP) and 12 eastward-decaying (ED) types of MJOs. The column-integrated MSE budget shows that, depending on the phases, vertical and horizontal advection terms take turns leading the role in discharging or recharging the MSE. In the phase when the net flux heating reaches the maximum, vertical advection plays the lead role in discharging the MSE. In the subsequent phases when the net flux heating gradually weakens, horizontal advection becomes the dominant factor in discharging the MSE. A similar situation occurs during the recharge phases to balance the net flux cooling. The SP MJO exhibits a stronger energy recharge-discharge cycle compared to the WP and ED MJOs, and the contrast significantly enlarges from the Indian Ocean to the Maritime Continent. The GMS plane analysis further reveals that four different types of convection attribute to the recharge-discharge process. During the wet phases, the top-heavy ascending motion and negative shallow convection jointly stabilize the atmosphere by exporting the MSE; while during the dry phases, the top-heavy descending motion and the positive shallow convection together destabilize the atmosphere through importing the MSE. Moreover, the recharge (discharge) phase leads the amplifying (decaying) phase by a quarter cycle is essential to maintain a robust energy recharge-discharge cycle to drive the eastward propagating MJOs.

Appendix A

1.1 Estimating phase speeds of SP, WP and ED MJO events

Figure 

Fig. 8

a–c Hovmöller diagrams of OLR (shadings, W m⁻²) and U850 (contours, m s⁻¹) anomalies over the Indo-Pacific warm pool from day − 20 to day 40 associated with the composited SP (left), WP (center) and ED (right) MJO events, respectively. Data are averaged over the latitude belt of 15° S–5° N. The solid and dash contours represent positive and negative values, respectively. The magenta dash lines denote the linearly regressed phase lines of MJO. The OLR anomalies significant at the 95% confidence level are stippled

8a–c illustrates the Hovmöller diagrams of OLR and U850 anomalies composited respectively over the SP, WP and ED MJO events. The enhanced convection (negative OLR anomalies) is generally accompanied by low-level westerly wind anomalies. All three types of MJO events show a clear eastward propagation of convection and circulation over the Indian Ocean (60° E–90° E). We note that while the convection intensity associated with the SP and WP MJOs slightly weakens over the Maritime Continent (100° E–135° E), it tends to rejuvenate over the western Pacific warm pool (135° E–160° E). By contrast, the convection intensity of the ED MJO decays quickly prior to approaching the Maritime Continent. Following Zhang and Ling (2017), the regions with OLR anomalies less than – 10 W m⁻² are selected to obtain the linearly regressed phase lines (denoted by the magenta dash lines) for various types of MJOs. As shown in Fig. 8, a stronger MJO convection event is accompanied by a slower phase speed. Specifically, the SP and WP MJOs propagate eastward across the Indo-Pacific warm pool (60° E–180°) at a speed of about 3.88 m s⁻¹ and 5.90 m s⁻¹, respectively, whereas the ED MJO propagates eastward only over the Indian Ocean (60° E–105° E) with a much faster speed of about 8.01 m s⁻¹.