Atmospheric Mechanisms for MJO Decay Over the Maritime Continent

Abstract

Eastward propagating Madden‐Julian oscillation (MJO) events that develop in the Indian Ocean from November to April are separated into events whose convective anomalies do and do not propagate across the Maritime Continent (MC). Propagating (P) events are divided into strong (sP) and weak (wP) subsets based on the initial amplitudes of their convective anomalies. Eastward decaying (ED) MJO events have initial amplitudes similar to those of wP events. Roughly half of all MJO events encounter westward propagating transient dry precursor (TDP) signals over the MC. These TDPs, which are external to the MJO circulation system, overwhelm the weak, eastward propagating moist anomaly in ED events, leading to MJO termination. In wP events, the MJO moist anomaly is stronger than the TDP dry anomaly, and the MJO propagates beyond the MC. ED MJO events that do not encounter TDPs decay because of insufficient moistening in the southern MC region. The background states of TDP‐ and non‐TDP‐affected MJO events resemble La Niña and El Niño conditions, respectively. The regional regulation of TDP activity and background moisture gradients by El Niño–Southern Oscillation can both be understood in terms of the zonal shift and the meridional expansion or contraction of Warm Pool moisture in response to El Niño–Southern Oscillation sea surface temperature anomalies. Our results demonstrate that modulation of TDPs by the background state moisture can affect the MJO in a manner that reinforces its propagation or decay via the interaction of MJO circulation anomalies and background moisture gradients.

Distinct Propagation Characteristics of Intraseasonal Variability Over the Tropical West Pacific

“…The mean state conditions favorable for the propagation of the WPIM are strongest mainly east of the Indian Ocean, which may help explain why the WPIM weakens significantly before reaching the Indian Ocean. A preliminary analysis suggests that ENSO may play a significant role in the interannual variability of intraseasonal convective events, in agreement with DeMott et al ().…”

“…The spatial structures revealed in Figure resemble the salient structure of El Niño‐Southern Oscillation (ENSO) extremes, for example, with years of strong MJO propagation having a more El Niño‐like large‐scale structure and years of strong WPIM propagation having a more La Niña‐like large‐scale structure. DeMott et al () also showed that the large‐scale background state conditions for strong MJO propagating events versus MJO events terminating at the Maritime Continent have similar structures. Therefore, it is possible that the interannual variability we are detecting related to strong MJO and WPIM years is impacted by the strength of ENSO.…”

“…DeMott et al () found similar dry westward propagating signals (10–30‐day timescales) that cause many MJO events to dissipate near the Maritime Continent, and they discuss how the activity of these dry signals is highly modulated by interannual (La Niña versus El Niño) variations in background moisture gradients over the WP. The results in DeMott et al () could be relevant to the interannual variability seen in Figure for years with strong eastward propagation and years with strong westward propagation.…”

Possible Role of the Diurnal Cycle in Land Convection in the Barrier Effect on the MJO by the Maritime Continent

“…Indeed, stronger MJO events have higher chances to survive the barrier effect than weaker ones. But all strong MJO events do not survive the barrier effect and a few weak ones do (Chen & Wang, 2018;DeMott et al, 2018;Kim et al, 2016;Zhang & Ling, 2017). Strength of the MJO alone is neither necessary nor sufficient to overcome the barrier effect.…”

“…The reasons for the MC barrier effect on MJO propagation are not well understood. Proposed mechanisms for the barrier effect include reduced surface fluxes by the islands (Maloney & Sobel, 2004;Sobel et al, 2010), distorted low-level circulations by topography (Hsu & Lee, 2005;Inness & Slingo, 2006;Tan et al, 2018;Wu & Hsu, 2009), and specific large-scale patterns of moisture and circulations (DeMott et al, 2018;Feng et al, 2015;Kim et al, 2014).…”