Zonal‐Scale of the Madden‐Julian Oscillation and Its Propagation Speed on the Interannual Time‐Scale

Abstract

While no significant long-term trend in the propagation speed of the Madden-Julian Oscillation (MJO) in boreal winter is found during the past decades, pronounced year-to-year variability of the MJO phase speed is illustrated by analyzing a century-long record data set. During the winters when fast MJO propagation is observed, the MJO exhibits a much larger zonal-scale than that during the winters with slow propagation. A broader extension in MJO circulation effectively induces stronger and broader lower-tropospheric moistening (drying) to the east (west) of MJO through horizontal moisture advection, prompting a faster MJO phase speed. The larger MJO zonal-scale during the fast MJO propagation winters is coincident with anomalously increased background sea surface temperatures and precipitable water over both the western Indian Ocean and central/eastern Pacific, reminiscent of an expansion of the Indo-Pacific warm pool. A fundamental question remains open regarding the key processes that determine the zonal-scale of MJO organization. Plain Language Summary The Madden-Julian Oscillation (MJO), the most prominent climate mode in Tropics with a prevailing period of 30-60 days, has tremendous influences on global weather extremes. Our present-day numeric climate and weather prediction models, however, remain struggling in simulating the MJO, particularly its slow eastward propagation from the Indian Ocean to the western Pacific. Meanwhile, a knowledge gap remains in understanding the essential physics in regulating this oscillation form. In this study, we examine the year-to-year variability of the MJO propagation speed by analyzing a century-long historical data set to verify the key processes for MJO propagation in the context of existing theories. While no significant climate trend in the MJO propagation speed is found during the past decades, one interesting finding is that the MJO exhibits a significantly larger zonal-scale during the winters with faster propagation speed. More effective moistening by the broadly extended anomalous easterlies to the east of the MJO is found to be critical for the faster eastward propagation. These winters with faster MJO propagation are often characterized by an expansion of the Indo-Pacific warm pool on both its eastern and western edges. A fundamental question is raised by this study regarding the intrinsic spatial-scale of the MJO organization and how it is affected by the environment in the real world. LYU ET AL.

Identifying the Drivers of Caribbean Severe Weather Impacts

“…The anomalous Pacific airflow to the SW Caribbean is analyzed by composite height section on 80 • W (Figure 6a Temporal lag correlations (Figure 6d) indicate a 5-day delay between a surge of tropical Pacific air (80 • W) and severe weather impacts in the central Caribbean (65 • W), yielding a phase speed ~3.5 m/s consistent with MJO [36]. Regressions between W•R and 36-48-day U-wind (Figure 6e) exhibit optimal values in the August-November season, e.g., when MJO have an influence on Caribbean storminess.…”

Westward‐Propagating Disturbances Shape Diverse MJO Propagation

“…Clustering analysis has been recently used to objectively identify diverse MJO propagations (Chen & Wang, 2021; Wang et al., 2021), such as standing, jumping, and slow eastward propagation and fast eastward propagation MJO modes (Wang et al., 2019, hereafter WCL19; Xiang et al., 2021; Wang & Wang, 2023). Although low‐frequency variabilities, such as El Nino–Southern Oscillation (ENSO) and Quasi‐Biennial Oscillation (Lyu et al., 2021; Wei et al., 2023; Xiang et al., 2021), likely control various MJO propagation behaviors, why diversified MJO propagations exist remains elusive.…”