Recent studies suggest that frequency of active phases of the Madden-Julian Oscillation (MJO) over the Maritime Continent and western Pacific, that is, the MJO Phases 4-6 defined by the real-time multivariate MJO (RMM) index, has increased in recent winters. A robust positive trend in MJO Phase 4-6 days during 1979-2015 winters is confirmed in this study. Our analyses, however, suggest that this trend could be exaggerated due to the blended low-frequency variability signals in the RMM. When the winter RMM is reconstructed using anomalous fields after removing their winter mean instead of the previous 120-day mean as for the original RMM, the robust trend in MJO Phase 4-6 days can no longer be detected. Therefore, cautions need to be exercised when applying the RMM for studies on the low-frequency variability and climate trend in MJO activity and using the derived MJO variability to interpret associated changes in climate systems.

The Madden–Julian oscillation (MJO) is the major mode of intra‐seasonal variability in the Tropics, yet researches about factors related to this oscillation have not been well understood, particularly on an inter‐annual timescale. As a crucial index of the thermal condition of the plateau, the Tibetan Plateau snow cover (TPSC) has attracted great attention with its potential climatic effects. In this paper, observational evidences present that the inter‐annual variation in the wintertime TPSC can exert influence on the convection variability associated with the MJO. For the excessive TPSC winters, the MJO convection tends to be stronger in the Indian Ocean (phases 2–3), while it is more vigorous over the western Pacific (phases 6–7) during the reduced TPSC winters. Furthermore, the South Asia High (SAH) is found to be the key system in the potential physical mechanism. A reduced (excessive) TPSC can excite upper‐level anomalous anticyclone (cyclone) over the Tibetan Plateau and eastern China, which is in favour of westwards extension (eastwards withdrawal) and enhancing (weakening) of the SAH over tropical oceans. Through an anomalous tropical zonal‐vertical circulation, the intensified ascending (descending) motions over the tropical western Pacific may subsequently depress (enhances) the convection over the Indian Ocean. Over the Indian Ocean (phases 2–3) and the western Pacific (phases 6–7), the zonal‐vertical circulation associated with the MJO is exactly superposed with the inter‐annual anomalous vertical circulation linked with the TPSC, so the according changes of MJO convection are most significant in these two categorized phases.

Previous observations suggested a connection between the Madden–Julian Oscillation (MJO) and the North Atlantic Oscillation (NAO), yet few of them contrasted the influences between the MJO and the NAO on extreme weather and climate events. In this study, it is found that the impacts of the MJO and the NAO on winter cold wave amplitude (CWA) over China can be distinctive. Time‐lagged correlation analysis indicates that the response of the CWA to the MJO is characterized by a significant anomalous centre over the Tibetan Plateau and its adjacent region, whereas that to the NAO is mainly over western and northeastern China. The ECMWF model from the Sub‐seasonal to Seasonal (S2S) project can approximately reproduce the relationship between CWA and these indices at intraseasonal time‐scale. The MJO‐associated convective activities along the Equator can modulate the local Hadley circulation. When the convection centre of the MJO is located over the Maritime Continent, it can trigger subsidence over the Tibetan Plateau and its adjacent region, warm the surface and consequently decrease the CWA. When descending flows associated with the MJO prevail over the Maritime Continent due to the eastward propagation of the MJO, the situations tend to be opposite. The intraseasonal variations of the NAO will induce eastward‐propagating Rossby wave trains, which usually favour the significantly positive (negative) pressure, temperature and consequently negative (positive) CWA anomalies over western and northeastern China. Thus, the MJO and the NAO might provide two critical predictability sources for sub‐seasonal forecasts of the extreme temperature events.

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.